NAME¶
snitfaq - Snit Frequently Asked Questions
DESCRIPTION¶
OVERVIEW¶
WHAT IS THIS DOCUMENT?¶
This is an atypical FAQ list, in that few of the questions are frequently asked.
Rather, these are the questions I think a newcomer to Snit should be asking.
This file is not a complete reference to Snit, however; that information is in
the
snit man page.
WHAT IS SNIT?¶
Snit is a framework for defining abstract data types and megawidgets in pure
Tcl. The name "Snit" stands for "Snit's Not Incr Tcl",
signifying that Snit takes a different approach to defining objects than does
Incr Tcl, the best known object framework for Tcl. Had I realized that Snit
would become at all popular, I'd probably have chosen something else.
The primary purpose of Snit is to be
object glue--to help you compose
diverse objects from diverse sources into types and megawidgets with clean,
convenient interfaces so that you can more easily build your application.
Snit isn't about theoretical purity or minimalist design; it's about being able
to do powerful things easily and consistently without having to think about
them--so that you can concentrate on building your application.
Snit isn't about implementing thousands of nearly identical carefully-specified
lightweight thingamajigs--not as individual Snit objects. Traditional Tcl
methods will be much faster, and not much more complicated. But Snit
is
about implementing a clean interface to manage a collection of thousands of
nearly identical carefully-specified lightweight thingamajigs (e.g., think of
the text widget and text tags, or the canvas widget and canvas objects). Snit
lets you hide the details of just how those thingamajigs are stored--so that
you can ignore it, and concentrate on building your application.
Snit isn't a way of life, a silver bullet, or the Fountain of Youth. It's just a
way of managing complexity--and of managing some of the complexity of managing
complexity--so that you can concentrate on building your application.
WHAT VERSION OF TCL DOES SNIT REQUIRE?¶
Snit 1.3 requires Tcl 8.3 or later; Snit 2.2 requires Tcl 8.5 or later. See
SNIT VERSIONS for the differences between Snit 1.3 and Snit 2.2.
WHERE CAN I DOWNLOAD SNIT?¶
Snit is part of Tcllib, the standard Tcl library, so you might already have it.
It's also available at the Snit Home Page,
http://www.wjduquette.com/snit.
WHAT ARE SNIT'S GOALS?¶
- •
- A Snit object should be at least as efficient as a
hand-coded Tcl object (see
http://www.wjduquette.com/tcl/objects.html).
- •
- The fact that Snit was used in an object's implementation
should be transparent (and irrelevant) to clients of that object.
- •
- Snit should be able to encapsulate objects from other
sources, particularly Tk widgets.
- •
- Snit megawidgets should be (to the extent possible)
indistinguishable in interface from Tk widgets.
- •
- Snit should be Tclish--that is, rather than trying to
emulate C++, Smalltalk, or anything else, it should try to emulate Tcl
itself.
- •
- It should have a simple, easy-to-use, easy-to-remember
syntax.
HOW IS SNIT DIFFERENT FROM OTHER OO FRAMEWORKS?¶
Snit is unique among Tcl object systems in that it is based not on inheritance
but on delegation. Object systems based on inheritance only allow you to
inherit from classes defined using the same system, and that's a shame. In
Tcl, an object is anything that acts like an object; it shouldn't matter how
the object was implemented. I designed Snit to help me build applications out
of the materials at hand; thus, Snit is designed to be able to incorporate and
build on any object, whether it's a hand-coded object, a Tk widget, an Incr
Tcl object, a BWidget or almost anything else.
Note that you can achieve the effect of inheritance using
COMPONENTS and
DELEGATION--and you can inherit from anything that looks like a Tcl
object.
WHAT CAN I DO WITH SNIT?¶
Using Snit, a programmer can:
- •
- Create abstract data types and Tk megawidgets.
- •
- Define instance variables, type variables, and Tk-style
options.
- •
- Define constructors, destructors, instance methods, type
methods, procs.
- •
- Assemble a type out of component types. Instance methods
and options can be delegated to the component types automatically.
SNIT VERSIONS¶
WHICH VERSION OF SNIT SHOULD I USE?¶
The current Snit distribution includes two versions, Snit 1.3 and Snit 2.2. The
reason that both are included is that Snit 2.2 takes advantage of a number of
new features of Tcl 8.5 to improve run-time efficiency; as a side-effect, the
ugliness of Snit's error messages and stack traces has been reduced
considerably. The cost of using Snit 2.2, of course, is that you must target
Tcl 8.5.
Snit 1.3, on the other hand, lacks Snit 2.2's optimizations, but requires only
Tcl 8.3 and later.
In short, if you're targetting Tcl 8.3 or 8.4 you should use Snit 1.3. If you
can afford to target Tcl 8.5, you should definitely use Snit 2.2. If you will
be targetting both, you can use Snit 1.3 exclusively, or (if your code is
unaffected by the minor incompatibilities between the two versions) you can
use Snit 1.3 for Tcl 8.4 and Snit 2.2 for Tcl 8.5.
HOW DO I SELECT THE VERSION OF SNIT I WANT TO USE?¶
To always use Snit 1.3 (or a later version of Snit 1.x), invoke Snit as follows:
To always use Snit 2.2 (or a later version of Snit 2.x), say this instead:
Note that if you request Snit 2.2 explicitly, your application will halt with
Tcl 8.4, since Snit 2.2 is unavailable for Tcl 8.4.
If you wish your application to always use the latest available version of Snit,
don't specify a version number:
Tcl will find and load the latest version that's available relative to the
version of Tcl being used. In this case, be careful to avoid using any
incompatible features.
HOW ARE SNIT 1.3 AND SNIT 2.2 INCOMPATIBLE?¶
To the extent possible, Snit 2.2 is intended to be a drop-in replacement for
Snit 1.3. Unfortunately, some incompatibilities were inevitable because Snit
2.2 uses Tcl 8.5's new
namespace ensemble mechanism to implement
subcommand dispatch. This approach is much faster than the mechanism used in
Snit 1.3, and also results in much better error messages; however, it also
places new constraints on the implementation.
There are four specific incompatibilities between Snit 1.3 and Snit 2.2.
- •
- Snit 1.3 supports implicit naming of objects. Suppose you
define a new snit::type called dog. You can create instances
of dog in three ways:
dog spot ;# Explicit naming
set obj1 [dog %AUTO%] ;# Automatic naming
set obj2 [dog] ;# Implicit naming
- In Snit 2.2, type commands are defined using the
namespace ensemble mechanism; and namespace ensemble doesn't
allow an ensemble command to be called without a subcommand. In short,
using namespace ensemble there's no way to support implicit naming.
All is not lost, however. If the type has no type methods, then the type
command is a simple command rather than an ensemble, and namespace
ensemble is not used. In this case, implicit naming is still possible.
In short, you can have implicit naming if you're willing to do without type
methods (including the standard type methods, like $type info). To
do so, use the -hastypemethods pragma:
- •
- Hierarchical methods and type methods are implemented
differently in Snit 2.2.
A hierarchical method is an instance method which has subcommands; these
subcommands are themselves methods. The Tk text widget's tag
command and its subcommands are examples of hierarchical methods. You can
implement such subcommands in Snit simply by including multiple words in
the method names:
method {tag configure} {tag args} { ... }
method {tag cget} {tag option} {...}
- Here we've implicitly defined a tag method which has
two subcommands, configure and cget.
In Snit 1.3, hierarchical methods could be called in two ways:
$obj tag cget -myoption ;# The good way
$obj {tag cget} -myoption ;# The weird way
- In the second call, we see that a hierarchical method or
type method is simply one whose name contains multiple words.
In Snit 2.2 this is no longer the case, and the "weird" way of
calling hierarchical methods and type methods no longer works.
- •
- The third incompatibility derives from the second. In Snit
1.3, hierarchical methods were also simply methods whose name contains
multiple words. As a result, $obj info methods returned the full
names of all hierarchical methods. In the example above, the list returned
by $obj info methods would include tag configure and tag
cget but not tag, since tag is defined only implicitly.
In Snit 2.2, hierarchical methods and type methods are no longer simply ones
whose name contains multiple words; in the above example, the list
returned by $obj info methods would include tag but not
tag configure or tag cget.
- •
- The fourth incompatibility is due to a new feature. Snit
2.2 uses the new namespace path command so that a type's code can
call any command defined in the type's parent namespace without
qualification or importation. For example, suppose you have a package
called mypackage which defines a number of commands including a
type, ::mypackage::mytype. Thanks to namespace path, the
type's code can call any of the other commands defined in
::mypackage::.
This is extremely convenient. However, it also means that commands defined
in the parent namespace, ::mypackage:: can block the type's access
to identically named commands in the global namespace. This can lead to
bugs. For example, Tcllib includes a type called ::tie::std::file.
This type's code calls the standard file command. When run with
Snit 2.2, the code broke-- the type's command, ::tie::std::file, is
itself a command in the type's parent namespace, and so instead of calling
the standard file command, the type found itself calling
itself.
ARE THERE OTHER DIFFERENCES BETWEEN SNIT 1.X AND SNIT 2.2?¶
Yes.
- •
- Method dispatch is considerably faster.
- •
- Many error messages and stack traces are cleaner.
- •
- The -simpledispatch pragma is obsolete, and ignored
if present. In Snit 1.x, -simpledispatch substitutes a faster
mechanism for method dispatch, at the cost of losing certain features.
Snit 2.2 method dispatch is faster still in all cases, so
-simpledispatch is no longer needed.
- •
- In Snit 2.2, a type's code (methods, type methods, etc.)
can call commands from the type's parent namespace without qualifying or
importing them, i.e., type ::parentns::mytype's code can call
::parentns::someproc as just someproc.
This is extremely useful when a type is defined as part of a larger package,
and shares a parent namespace with the rest of the package; it means that
the type can call other commands defined by the package without any extra
work.
This feature depends on the new Tcl 8.5 namespace path command, which
is why it hasn't been implemented for V1.x. V1.x code can achieve
something similar by placing
namespace import [namespace parent]::*
- in a type constructor. This is less useful, however, as it
picks up only those commands which have already been exported by the
parent namespace at the time the type is defined.
OBJECTS¶
WHAT IS AN OBJECT?¶
A full description of object-oriented programming is beyond the scope of this
FAQ, obviously. In simple terms, an object is an instance of an abstract data
type--a coherent bundle of code and data. There are many ways to represent
objects in Tcl/Tk; the best known examples are the Tk widgets.
A Tk widget is an object; it is represented by a Tcl command. The object's
methods are subcommands of the Tcl command. The object's properties are
options accessed using the
configure and
cget methods. Snit uses
the same conventions as Tk widgets do.
WHAT IS AN ABSTRACT DATA TYPE?¶
In computer science terms, an abstract data type is a complex data structure
along with a set of operations--a stack, a queue, a binary tree, etc--that is
to say, in modern terms, an object. In systems that include some form of
inheritance the word
class is usually used instead of
abstract data
type, but as Snit doesn't implement inheritance as it's ordinarily
understood the older term seems more appropriate. Sometimes this is called
object-based programming as opposed to object-oriented programming.
Note that you can easily create the effect of inheritance using
COMPONENTS and
DELEGATION.
In Snit, as in Tk, a
type is a command that creates instances -- objects
-- which belong to the type. Most types define some number of
options
which can be set at creation time, and usually can be changed later.
Further, an
instance is also a Tcl command--a command that gives access
to the operations which are defined for that abstract data type.
Conventionally, the operations are defined as subcommands of the instance
command. For example, to insert text into a Tk text widget, you use the text
widget's
insert subcommand:
# Create a text widget and insert some text in it.
text .mytext -width 80 -height 24
.mytext insert end "Howdy!"
In this example,
text is the
type command and
.mytext is
the
instance command.
In Snit, object subcommands are generally called
INSTANCE METHODS.
WHAT KINDS OF ABSTRACT DATA TYPES DOES SNIT PROVIDE?¶
Snit allows you to define three kinds of abstract data type:
- •
- snit::type
- •
- snit::widget
- •
- snit::widgetadaptor
WHAT IS A SNIT::TYPE?¶
A
snit::type is a non-GUI abstract data type, e.g., a stack or a queue.
snit::types are defined using the
snit::type command. For
example, if you were designing a kennel management system for a dog breeder,
you'd need a dog type.
% snit::type dog {
# ...
}
::dog
%
This definition defines a new command (
::dog, in this case) that can be
used to define dog objects.
An instance of a
snit::type can have
INSTANCE METHODS,
INSTANCE
VARIABLES,
OPTIONS, and
COMPONENTS. The type itself can have
TYPE METHODS,
TYPE VARIABLES,
TYPE COMPONENTS, and
PROCS.
WHAT IS A SNIT::WIDGET?, THE SHORT STORY¶
A
snit::widget is a Tk megawidget built using Snit; it is very similar to
a
snit::type. See
WIDGETS.
WHAT IS A SNIT::WIDGETADAPTOR?, THE SHORT STORY¶
A
snit::widgetadaptor uses Snit to wrap an existing widget type (e.g., a
Tk label), modifying its interface to a lesser or greater extent. It is very
similar to a
snit::widget. See
WIDGET ADAPTORS.
HOW DO I CREATE AN INSTANCE OF A SNIT::TYPE?¶
You create an instance of a
snit::type by passing the new instance's name
to the type's create method. In the following example, we create a
dog
object called
spot.
% snit::type dog {
# ....
}
::dog
% dog create spot
::spot
%
In general, the
create method name can be omitted so long as the instance
name doesn't conflict with any defined
TYPE METHODS. (See
TYPE
COMPONENTS for the special case in which this doesn't work.) So the
following example is identical to the previous example:
% snit::type dog {
# ....
}
::dog
% dog spot
::spot
%
This document generally uses the shorter form.
If the
dog type defines
OPTIONS, these can usually be given
defaults at creation time:
% snit::type dog {
option -breed mongrel
option -color brown
method bark {} { return "$self barks." }
}
::dog
% dog create spot -breed dalmation -color spotted
::spot
% spot cget -breed
dalmation
% spot cget -color
spotted
%
Once created, the instance name now names a new Tcl command that is used to
manipulate the object. For example, the following code makes the dog bark:
% spot bark
::spot barks.
%
HOW DO I REFER TO AN OBJECT INDIRECTLY?¶
Some programmers prefer to save the object name in a variable, and reference it
that way. For example,
% snit::type dog { ... }
::dog
% set d [dog spot -breed dalmation -color spotted]
::spot
% $d cget -breed
dalmation
% $d bark
::spot barks.
%
If you prefer this style, you might prefer to have Snit generate the instance's
name automatically.
HOW CAN I GENERATE THE OBJECT NAME AUTOMATICALLY?¶
If you'd like Snit to generate an object name for you, use the
%AUTO%
keyword as the requested name:
% snit::type dog { ... }
::dog
% set d [dog %AUTO%]
::dog2
% $d bark
::dog2 barks.
%
The
%AUTO% keyword can be embedded in a longer string:
% set d [dog obj_%AUTO%]
::obj_dog4
% $d bark
::obj_dog4 barks.
%
CAN TYPES BE RENAMED?¶
Tcl's
rename command renames other commands. It's a common technique in
Tcl to modify an existing command by renaming it and defining a new command
with the original name; the new command usually calls the renamed command.
snit::type commands, however, should never be renamed; to do so breaks
the connection between the type and its objects.
CAN OBJECTS BE RENAMED?¶
Tcl's
rename command renames other commands. It's a common technique in
Tcl to modify an existing command by renaming it and defining a new command
with the original name; the new command usually calls the renamed command.
All Snit objects (including
widgets and
widgetadaptors) can be
renamed, though this flexibility has some consequences:
- •
- In an instance method, the implicit argument self
will always contain the object's current name, so instance methods can
always call other instance methods using $self.
- •
- If the object is renamed, however, then $self's
value will change. Therefore, don't use $self for anything that
will break if $self changes. For example, don't pass a callback
command to another object like this:
.btn configure -command [list $self ButtonPress]
- You'll get an error if .btn calls your command after
your object is renamed.
- •
- Instead, your object should define its callback command
like this:
.btn configure -command [mymethod ButtonPress]
- The mymethod command returns code that will call the
desired method safely; the caller of the callback can add additional
arguments to the end of the command as usual.
- •
- Every object has a private namespace; the name of this
namespace is available in method bodies, etc., as the value of the
implicit argument selfns. This value is constant for the life of
the object. Use $selfns instead of $self if you need a
unique token to identify the object.
- •
- When a snit::widget's instance command is renamed,
its Tk window name remains the same -- and is still extremely important.
Consequently, the Tk window name is available in method bodies as the
value of the implicit argument win. This value is constant for the
life of the object. When creating child windows, it's best to use
$win.child rather than $self.child as the name of the child
window.
HOW DO I DESTROY A SNIT OBJECT?¶
Any Snit object of any type can be destroyed by renaming it to the empty string
using the Tcl
rename command.
Snit megawidgets (i.e., instances of
snit::widget and
snit::widgetadaptor) can be destroyed like any other widget: by using
the Tk
destroy command on the widget or on one of its ancestors in the
window hierarchy.
Every instance of a
snit::type has a
destroy method:
% snit::type dog { ... }
::dog
% dog spot
::spot
% spot bark
::spot barks.
% spot destroy
% spot barks
invalid command name "spot"
%
Finally, every Snit type has a type method called
destroy; calling it
destroys the type and all of its instances:
% snit::type dog { ... }
::dog
% dog spot
::spot
% spot bark
::spot barks.
% dog destroy
% spot bark
invalid command name "spot"
% dog fido
invalid command name "dog"
%
INSTANCE METHODS¶
WHAT IS AN INSTANCE METHOD?¶
An instance method is a procedure associated with a specific object and called
as a subcommand of the object's command. It is given free access to all of the
object's type variables, instance variables, and so forth.
HOW DO I DEFINE AN INSTANCE METHOD?¶
Instance methods are defined in the type definition using the
method
statement. Consider the following code that might be used to add dogs to a
computer simulation:
% snit::type dog {
method bark {} {
return "$self barks."
}
method chase {thing} {
return "$self chases $thing."
}
}
::dog
%
A dog can bark, and it can chase things.
The
method statement looks just like a normal Tcl
proc, except
that it appears in a
snit::type definition. Notice that every instance
method gets an implicit argument called
self; this argument contains
the object's name. (There's more on implicit method arguments below.)
HOW DOES A CLIENT CALL AN INSTANCE METHOD?¶
The method name becomes a subcommand of the object. For example, let's put a
simulated dog through its paces:
% dog spot
::spot
% spot bark
::spot barks.
% spot chase cat
::spot chases cat.
%
HOW DOES AN INSTANCE METHOD CALL ANOTHER INSTANCE METHOD?¶
If method A needs to call method B on the same object, it does so just as a
client does: it calls method B as a subcommand of the object itself, using the
object name stored in the implicit argument
self.
Suppose, for example, that our dogs never chase anything without barking at
them:
% snit::type dog {
method bark {} {
return "$self barks."
}
method chase {thing} {
return "$self chases $thing. [$self bark]"
}
}
::dog
% dog spot
::spot
% spot bark
::spot barks.
% spot chase cat
::spot chases cat. ::spot barks.
%
ARE THERE ANY LIMITATIONS ON INSTANCE METHOD NAMES?¶
Not really, so long as you avoid the standard instance method names:
configure,
configurelist,
cget,
destroy, and
info. Also, method names consisting of multiple words define
hierarchical methods.
WHAT IS A HIERARCHICAL METHOD?¶
An object's methods are subcommands of the object's instance command.
Hierarchical methods allow an object's methods to have subcommands of their
own; and these can in turn have subcommands, and so on. This allows the
programmer to define a tree-shaped command structure, such as is used by many
of the Tk widgets--the subcommands of the Tk
text widget's
tag
method are hierarchical methods.
HOW DO I DEFINE A HIERARCHICAL METHOD?¶
Define methods whose names consist of multiple words. These words define the
hierarchy implicitly. For example, the following code defines a
tag
method with subcommands
cget and
configure:
snit::widget mytext {
method {tag configure} {tag args} { ... }
method {tag cget} {tag option} {...}
}
Note that there is no explicit definition for the
tag method; it is
implicit in the definition of
tag configure and
tag cget. If you
tried to define
tag explicitly in this example, you'd get an error.
HOW DO I CALL HIERARCHICAL METHODS?¶
As subcommands of subcommands.
% mytext .text
.text
% .text tag configure redtext -foreground red -background black
% .text tag cget redtext -foreground
red
%
HOW DO I MAKE AN INSTANCE METHOD PRIVATE?¶
It's often useful to define private methods, that is, instance methods intended
to be called only by other methods of the same object.
Snit doesn't implement any access control on instance methods, so all methods
are
de facto public. Conventionally, though, the names of public
methods begin with a lower-case letter, and the names of private methods begin
with an upper-case letter.
For example, suppose our simulated dogs only bark in response to other stimuli;
they never bark just for fun. So the
bark method becomes
Bark to
indicate that it is private:
% snit::type dog {
# Private by convention: begins with uppercase letter.
method Bark {} {
return "$self barks."
}
method chase {thing} {
return "$self chases $thing. [$self Bark]"
}
}
::dog
% dog fido
::fido
% fido chase cat
::fido chases cat. ::fido barks.
%
ARE THERE ANY LIMITATIONS ON INSTANCE METHOD ARGUMENTS?¶
Method argument lists are defined just like normal Tcl
proc argument
lists; in particular, they can include arguments with default values and the
args argument.
However, every method also has a number of implicit arguments provided by Snit
in addition to those explicitly defined. The names of these implicit arguments
may not used to name explicit arguments.
WHAT IMPLICIT ARGUMENTS ARE PASSED TO EACH INSTANCE METHOD?¶
The arguments implicitly passed to every method are
type,
selfns,
win, and
self.
WHAT IS $TYPE?¶
The implicit argument
type contains the fully qualified name of the
object's type:
% snit::type thing {
method mytype {} {
return $type
}
}
::thing
% thing something
::something
% something mytype
::thing
%
WHAT IS $SELF?¶
The implicit argument
self contains the object's fully qualified name.
If the object's command is renamed, then
$self will change to match in
subsequent calls. Thus, your code should not assume that
$self is
constant unless you know for sure that the object will never be renamed.
% snit::type thing {
method myself {} {
return $self
}
}
::thing
% thing mutt
::mutt
% mutt myself
::mutt
% rename mutt jeff
% jeff myself
::jeff
%
WHAT IS $SELFNS?¶
Each Snit object has a private namespace in which to store its
INSTANCE
VARIABLES and
OPTIONS. The implicit argument
selfns contains
the name of this namespace; its value never changes, and is constant for the
life of the object, even if the object's name changes:
% snit::type thing {
method myNameSpace {} {
return $selfns
}
}
::thing
% thing jeff
::jeff
% jeff myNameSpace
::thing::Snit_inst3
% rename jeff mutt
% mutt myNameSpace
::thing::Snit_inst3
%
The above example reveals how Snit names an instance's private namespace;
however, you should not write code that depends on the specific naming
convention, as it might change in future releases.
WHAT IS $WIN?¶
The implicit argument
win is defined for all Snit methods, though it
really makes sense only for those of
WIDGETS and
WIDGET
ADAPTORS.
$win is simply the original name of the object, whether
it's been renamed or not. For widgets and widgetadaptors, it is also therefore
the name of a Tk window.
When a
snit::widgetadaptor is used to modify the interface of a widget or
megawidget, it must rename the widget's original command and replace it with
its own.
Thus, using
win whenever the Tk window name is called for means that a
snit::widget or
snit::widgetadaptor can be adapted by a
snit::widgetadaptor. See
WIDGETS for more information.
HOW DO I PASS AN INSTANCE METHOD AS A CALLBACK?¶
It depends on the context.
Suppose in my application I have a
dog object named
fido, and I
want
fido to bark when a Tk button called
.bark is pressed. In
this case, I create the callback command in the usual way, using
list:
button .bark -text "Bark!" -command [list fido bark]
In typical Tcl style, we use a callback to hook two independent components
together. But suppose that the
dog object has a graphical interface and
owns the button itself? In this case, the
dog must pass one of its own
instance methods to the button it owns. The obvious thing to do is this:
% snit::widget dog {
constructor {args} {
#...
button $win.barkbtn -text "Bark!" -command [list $self bark]
#...
}
}
::dog
%
(Note that in this example, our
dog becomes a
snit::widget,
because it has GUI behavior. See
WIDGETS for more.) Thus, if we create
a
dog called
.spot, it will create a Tk button called
.spot.barkbtn; when pressed, the button will call
$self bark.
Now, this will work--provided that
.spot is never renamed to something
else. But surely renaming widgets is abnormal? And so it is--unless
.spot is the hull component of a
snit::widgetadaptor. If it is,
then it will be renamed, and
.spot will become the name of the
snit::widgetadaptor object. When the button is pressed, the command
$self bark will be handled by the
snit::widgetadaptor, which
might or might not do the right thing.
There's a safer way to do it, and it looks like this:
% snit::widget dog {
constructor {args} {
#...
button $win.barkbtn -text "Bark!" -command [mymethod bark]
#...
}
}
::dog
%
The command
mymethod takes any number of arguments, and can be used like
list to build up a callback command; the only difference is that
mymethod returns a form of the command that won't change even if the
instance's name changes.
On the other hand, you might prefer to allow a widgetadaptor to override a
method such that your renamed widget will call the widgetadaptor's method
instead of its own. In this case, using
[list $self bark] will do what
you want...but this is a technique which should be used only in carefully
controlled circumstances.
HOW DO I DELEGATE INSTANCE METHODS TO A COMPONENT?¶
See
DELEGATION.
INSTANCE VARIABLES¶
WHAT IS AN INSTANCE VARIABLE?¶
An instance variable is a private variable associated with some particular Snit
object. Instance variables can be scalars or arrays.
HOW IS A SCALAR INSTANCE VARIABLE DEFINED?¶
Scalar instance variables are defined in the type definition using the
variable statement. You can simply name it, or you can initialize it
with a value:
snit::type mytype {
# Define variable "greeting" and initialize it with "Howdy!"
variable greeting "Howdy!"
}
HOW IS AN ARRAY INSTANCE VARIABLE DEFINED?¶
Array instance variables are also defined in the type definition using the
variable command. You can initialize them at the same time by
specifying the
-array option:
snit::type mytype {
# Define array variable "greetings"
variable greetings -array {
formal "Good Evening"
casual "Howdy!"
}
}
WHAT HAPPENS IF I DON'T INITIALIZE AN INSTANCE VARIABLE?¶
Variables do not really exist until they are given values. If you do not
initialize a variable when you define it, then you must be sure to assign a
value to it (in the constructor, say, or in some method) before you reference
it.
ARE THERE ANY LIMITATIONS ON INSTANCE VARIABLE NAMES?¶
Just a few.
First, every Snit object has a built-in instance variable called
options,
which should never be redefined.
Second, all names beginning with "Snit_" are reserved for use by Snit
internal code.
Third, instance variable names containing the namespace delimiter (
::)
are likely to cause great confusion.
DO I NEED TO DECLARE MY INSTANCE VARIABLES IN MY METHODS?¶
No. Once you've defined an instance variable in the type definition, it can be
used in any instance code (instance methods, the constructor, and the
destructor) without declaration. This differs from normal Tcl practice, in
which all non-local variables in a proc need to be declared.
There is a speed penalty to having all instance variables implicitly available
in all instance code. Even though your code need not declare the variables
explicitly, Snit must still declare them, and that takes time. If you have ten
instance variables, a method that uses none of them must still pay the
declaration penalty for all ten. In most cases, the additional runtime cost is
negligible. If extreme cases, you might wish to avoid it; there are two
methods for doing so.
The first is to define a single instance variable, an array, and store all of
your instance data in the array. This way, you're only paying the declaration
penalty for one variable--and you probably need the variable most of the time
anyway. This method breaks down if your instance variables include multiple
arrays; in Tcl 8.5, however, the
dict command might come to your
rescue.
The second method is to declare your instance variables explicitly in your
instance code, while
not including them in the type definition:
snit::type dog {
constructor {} {
variable mood
set mood happy
}
method setmood {newMood} {
variable mood
set mood $newMood
}
method getmood {} {
variable mood
return $mood
}
}
This allows you to ensure that only the required variables are included in each
method, at the cost of longer code and run-time errors when you forget to
declare a variable you need.
HOW DO I PASS AN INSTANCE VARIABLE'S NAME TO ANOTHER OBJECT?¶
In Tk, it's common to pass a widget a variable name; for example, Tk label
widgets have a
-textvariable option which names the variable which will
contain the widget's text. This allows the program to update the label's value
just by assigning a new value to the variable.
If you naively pass the instance variable name to the label widget, you'll be
confused by the result; Tk will assume that the name names a global variable.
Instead, you need to provide a fully-qualified variable name. From within an
instance method or a constructor, you can fully qualify the variable's name
using the
myvar command:
snit::widget mywidget {
variable labeltext ""
constructor {args} {
# ...
label $win.label -textvariable [myvar labeltext]
# ...
}
}
HOW DO I MAKE AN INSTANCE VARIABLE PUBLIC?¶
Practically speaking, you don't. Instead, you'll implement public variables as
OPTIONS. Alternatively, you can write
INSTANCE METHODS to set
and get the variable's value.
OPTIONS¶
WHAT IS AN OPTION?¶
A type's options are the equivalent of what other object-oriented languages
would call public member variables or properties: they are data values which
can be retrieved and (usually) set by the clients of an object.
Snit's implementation of options follows the Tk model fairly exactly, except
that
snit::type objects usually don't interact with
THE TK OPTION
DATABASE;
snit::widget and
snit::widgetadaptor objects, on
the other hand, always do.
HOW DO I DEFINE AN OPTION?¶
Options are defined in the type definition using the
option statement.
Consider the following type, to be used in an application that manages a list
of dogs for a pet store:
snit::type dog {
option -breed -default mongrel
option -color -default brown
option -akc -default 0
option -shots -default 0
}
According to this, a dog has four notable properties: a breed, a color, a flag
that says whether it's pedigreed with the American Kennel Club, and another
flag that says whether it has had its shots. The default dog, evidently, is a
brown mutt.
There are a number of options you can specify when defining an option; if
-default is the only one, you can omit the word
-default as
follows:
snit::type dog {
option -breed mongrel
option -color brown
option -akc 0
option -shots 0
}
If no
-default value is specified, the option's default value will be the
empty string (but see
THE TK OPTION DATABASE).
The Snit man page refers to options like these as "locally defined"
options.
HOW CAN A CLIENT SET OPTIONS AT OBJECT CREATION?¶
The normal convention is that the client may pass any number of options and
their values after the object's name at object creation. For example, the
::dog command defined in the previous answer can now be used to create
individual dogs. Any or all of the options may be set at creation time.
% dog spot -breed beagle -color "mottled" -akc 1 -shots 1
::spot
% dog fido -shots 1
::fido
%
So
::spot is a pedigreed beagle;
::fido is a typical mutt, but his
owners evidently take care of him, because he's had his shots.
Note: If the type defines a constructor, it can specify a different
object-creation syntax. See
CONSTRUCTORS for more information.
HOW CAN A CLIENT RETRIEVE AN OPTION'S VALUE?¶
Retrieve option values using the
cget method:
% spot cget -color
mottled
% fido cget -breed
mongrel
%
HOW CAN A CLIENT SET OPTIONS AFTER OBJECT CREATION?¶
Any number of options may be set at one time using the
configure instance
method. Suppose that closer inspection shows that ::fido is not a brown
mongrel, but rather a rare Arctic Boar Hound of a lovely dun color:
% fido configure -color dun -breed "Arctic Boar Hound"
% fido cget -color
dun
% fido cget -breed
Arctic Boar Hound
Alternatively, the
configurelist method takes a list of options and
values; occasionally this is more convenient:
% set features [list -color dun -breed "Arctic Boar Hound"]
-color dun -breed {Arctic Boar Hound}
% fido configurelist $features
% fido cget -color
dun
% fido cget -breed
Arctic Boar Hound
%
In Tcl 8.5, the
* keyword can be used with
configure in this case:
% set features [list -color dun -breed "Arctic Boar Hound"]
-color dun -breed {Arctic Boar Hound}
% fido configure {*}$features
% fido cget -color
dun
% fido cget -breed
Arctic Boar Hound
%
The results are the same.
HOW SHOULD AN INSTANCE METHOD ACCESS AN OPTION VALUE?¶
There are two ways an instance method can set and retrieve an option's value.
One is to use the
configure and
cget methods, as shown below.
% snit::type dog {
option -weight 10
method gainWeight {} {
set wt [$self cget -weight]
incr wt
$self configure -weight $wt
}
}
::dog
% dog fido
::fido
% fido cget -weight
10
% fido gainWeight
% fido cget -weight
11
%
Alternatively, Snit provides a built-in array instance variable called
options. The indices are the option names; the values are the option
values. The method
gainWeight can thus be rewritten as follows:
method gainWeight {} {
incr options(-weight)
}
As you can see, using the
options variable involves considerably less
typing and is the usual way to do it. But if you use
-configuremethod
or
-cgetmethod (described in the following answers), you might wish to
use the
configure and
cget methods anyway, just so that any
special processing you've implemented is sure to get done. Also, if the option
is delegated to a component then
configure and
cget are the only
way to access it without accessing the component directly. See
DELEGATION for more information.
HOW CAN I MAKE AN OPTION READ-ONLY?¶
Define the option with
-readonly yes.
Suppose you've got an option that determines how instances of your type are
constructed; it must be set at creation time, after which it's constant. For
example, a dog never changes its breed; it might or might not have had its
shots, and if not can have them at a later time.
-breed should be
read-only, but
-shots should not be.
% snit::type dog {
option -breed -default mongrel -readonly yes
option -shots -default no
}
::dog
% dog fido -breed retriever
::fido
% fido configure -shots yes
% fido configure -breed terrier
option -breed can only be set at instance creation
%
HOW CAN I CATCH ACCESSES TO AN OPTION'S VALUE?¶
Define a
-cgetmethod for the option.
WHAT IS A -CGETMETHOD?¶
A
-cgetmethod is a method that's called whenever the related option's
value is queried via the
cget instance method. The handler can compute
the option's value, retrieve it from a database, or do anything else you'd
like it to do.
Here's what the default behavior would look like if written using a
-cgetmethod:
snit::type dog {
option -color -default brown -cgetmethod GetOption
method GetOption {option} {
return $options($option)
}
}
Any instance method can be used, provided that it takes one argument, the name
of the option whose value is to be retrieved.
HOW CAN I CATCH CHANGES TO AN OPTION'S VALUE?¶
Define a
-configuremethod for the option.
A
-configuremethod is a method that's called whenever the related option
is given a new value via the
configure or
configurelist instance
methods. The method can pass the value on to some other object, store it in a
database, or do anything else you'd like it to do.
Here's what the default configuration behavior would look like if written using
a
-configuremethod:
snit::type dog {
option -color -default brown -configuremethod SetOption
method SetOption {option value} {
set options($option) $value
}
}
Any instance method can be used, provided that it takes two arguments, the name
of the option and the new value.
Note that if your method doesn't store the value in the
options array,
the
options array won't get updated.
HOW CAN I VALIDATE AN OPTION'S VALUE?¶
Define a
-validatemethod.
WHAT IS A -VALIDATEMETHOD?¶
A
-validatemethod is a method that's called whenever the related option
is given a new value via the
configure or
configurelist instance
methods. It's the method's responsibility to determine whether the new value
is valid, and throw an error if it isn't. The
-validatemethod, if any,
is called before the value is stored in the
options array; in
particular, it's called before the
-configuremethod, if any.
For example, suppose an option always takes a Boolean value. You can ensure that
the value is in fact a valid Boolean like this:
% snit::type dog {
option -shots -default no -validatemethod BooleanOption
method BooleanOption {option value} {
if {![string is boolean -strict $value]} {
error "expected a boolean value, got \"$value\""
}
}
}
::dog
% dog fido
% fido configure -shots yes
% fido configure -shots NotABooleanValue
expected a boolean value, got "NotABooleanValue"
%
Note that the same
-validatemethod can be used to validate any number of
boolean options.
Any method can be a
-validatemethod provided that it takes two arguments,
the option name and the new option value.
TYPE VARIABLES¶
WHAT IS A TYPE VARIABLE?¶
A type variable is a private variable associated with a Snit type rather than
with a particular instance of the type. In C++ and Java, the term
static
member variable is used for the same notion. Type variables can be scalars
or arrays.
HOW IS A SCALAR TYPE VARIABLE DEFINED?¶
Scalar type variables are defined in the type definition using the
typevariable statement. You can simply name it, or you can initialize
it with a value:
snit::type mytype {
# Define variable "greeting" and initialize it with "Howdy!"
typevariable greeting "Howdy!"
}
Every object of type
mytype now has access to a single variable called
greeting.
HOW IS AN ARRAY-VALUED TYPE VARIABLE DEFINED?¶
Array-valued type variables are also defined using the
typevariable
command; to initialize them, include the
-array option:
snit::type mytype {
# Define typearray variable "greetings"
typevariable greetings -array {
formal "Good Evening"
casual "Howdy!"
}
}
WHAT HAPPENS IF I DON'T INITIALIZE A TYPE VARIABLE?¶
Variables do not really exist until they are given values. If you do not
initialize a variable when you define it, then you must be sure to assign a
value to it (in the type constructor, say) before you reference it.
ARE THERE ANY LIMITATIONS ON TYPE VARIABLE NAMES?¶
Type variable names have the same restrictions as the names of
INSTANCE
VARIABLES do.
DO I NEED TO DECLARE MY TYPE VARIABLES IN MY METHODS?¶
No. Once you've defined a type variable in the type definition, it can be used
in
INSTANCE METHODS or
TYPE METHODS without declaration. This
differs from normal Tcl practice, in which all non-local variables in a proc
need to be declared.
Type variables are subject to the same speed/readability tradeoffs as instance
variables; see
Do I need to declare my instance variables in my
methods?
HOW DO I PASS A TYPE VARIABLE'S NAME TO ANOTHER OBJECT?¶
In Tk, it's common to pass a widget a variable name; for example, Tk label
widgets have a
-textvariable option which names the variable which will
contain the widget's text. This allows the program to update the label's value
just by assigning a new value to the variable.
If you naively pass a type variable name to the label widget, you'll be confused
by the result; Tk will assume that the name names a global variable. Instead,
you need to provide a fully-qualified variable name. From within an instance
method or a constructor, you can fully qualify the type variable's name using
the
mytypevar command:
snit::widget mywidget {
typevariable labeltext ""
constructor {args} {
# ...
label $win.label -textvariable [mytypevar labeltext]
# ...
}
}
HOW DO I MAKE A TYPE VARIABLE PUBLIC?¶
There are two ways to do this. The preferred way is to write a pair of
TYPE
METHODS to set and query the type variable's value.
Type variables are stored in the type's namespace, which has the same name as
the type itself. Thus, you can also publicize the type variable's name in your
documentation so that clients can access it directly. For example,
snit::type mytype {
typevariable myvariable
}
set ::mytype::myvariable "New Value"
TYPE METHODS¶
WHAT IS A TYPE METHOD?¶
A type method is a procedure associated with the type itself rather than with
any specific instance of the type, and called as a subcommand of the type
command.
HOW DO I DEFINE A TYPE METHOD?¶
Type methods are defined in the type definition using the
typemethod
statement:
snit::type dog {
# List of pedigreed dogs
typevariable pedigreed
typemethod pedigreedDogs {} {
return $pedigreed
}
}
Suppose the
dog type maintains a list of the names of the dogs that have
pedigrees. The
pedigreedDogs type method returns this list.
The
typemethod statement looks just like a normal Tcl
proc, except
that it appears in a
snit::type definition. Notice that every type
method gets an implicit argument called
type, which contains the
fully-qualified type name.
HOW DOES A CLIENT CALL A TYPE METHOD?¶
The type method name becomes a subcommand of the type's command. For example,
assuming that the constructor adds each pedigreed dog to the list of
pedigreedDogs,
snit::type dog {
option -pedigreed 0
# List of pedigreed dogs
typevariable pedigreed
typemethod pedigreedDogs {} {
return $pedigreed
}
# ...
}
dog spot -pedigreed 1
dog fido
foreach dog [dog pedigreedDogs] { ... }
ARE THERE ANY LIMITATIONS ON TYPE METHOD NAMES?¶
Not really, so long as you avoid the standard type method names:
create,
destroy, and
info.
HOW DO I MAKE A TYPE METHOD PRIVATE?¶
It's sometimes useful to define private type methods, that is, type methods
intended to be called only by other type or instance methods of the same
object.
Snit doesn't implement any access control on type methods; by convention, the
names of public methods begin with a lower-case letter, and the names of
private methods begin with an upper-case letter.
Alternatively, a Snit
proc can be used as a private type method; see
PROCS.
ARE THERE ANY LIMITATIONS ON TYPE METHOD ARGUMENTS?¶
Method argument lists are defined just like normal Tcl proc argument lists; in
particular, they can include arguments with default values and the
args
argument.
However, every type method is called with an implicit argument called
type that contains the name of the type command. In addition, type
methods should by convention avoid using the names of the arguments implicitly
defined for
INSTANCE METHODS.
HOW DOES AN INSTANCE OR TYPE METHOD CALL A TYPE METHOD?¶
If an instance or type method needs to call a type method, it should use
$type to do so:
snit::type dog {
typemethod pedigreedDogs {} { ... }
typemethod printPedigrees {} {
foreach obj [$type pedigreedDogs] { ... }
}
}
HOW DO I PASS A TYPE METHOD AS A CALLBACK?¶
It's common in Tcl to pass a snippet of code to another object, for it to call
later. Because types cannot be renamed, you can just use the type name, or, if
the callback is registered from within a type method,
type. For
example, suppose we want to print a list of pedigreed dogs when a Tk button is
pushed:
button .btn -text "Pedigrees" -command [list dog printPedigrees]
pack .btn
Alternatively, from a method or type method you can use the
mytypemethod
command, just as you would use
mymethod to define a callback command
for
INSTANCE METHODS.
CAN TYPE METHODS BE HIERARCHICAL?¶
Yes, you can define hierarchical type methods in just the same way as you can
define hierarchical instance methods. See
INSTANCE METHODS for more.
PROCS¶
WHAT IS A PROC?¶
A Snit
proc is really just a Tcl proc defined within the type's
namespace. You can use procs for private code that isn't related to any
particular instance.
HOW DO I DEFINE A PROC?¶
Procs are defined by including a
proc statement in the type definition:
snit::type mytype {
# Pops and returns the first item from the list stored in the
# listvar, updating the listvar
proc pop {listvar} { ... }
# ...
}
ARE THERE ANY LIMITATIONS ON PROC NAMES?¶
Any name can be used, so long as it does not begin with
Snit_; names
beginning with
Snit_ are reserved for Snit's own use. However, the wise
programmer will avoid
proc names (
set,
list,
if,
etc.) that would shadow standard Tcl command names.
proc names, being private, should begin with a capital letter according
to convention; however, as there are typically no public
procs in the
type's namespace it doesn't matter much either way.
HOW DOES A METHOD CALL A PROC?¶
Just like it calls any Tcl command. For example,
snit::type mytype {
# Pops and returns the first item from the list stored in the
# listvar, updating the listvar
proc pop {listvar} { ... }
variable requestQueue {}
# Get one request from the queue and process it.
method processRequest {} {
set req [pop requestQueue]
}
}
HOW CAN I PASS A PROC TO ANOTHER OBJECT AS A CALLBACK?¶
The
myproc command returns a callback command for the
proc, just
as
mymethod does for a method.
TYPE CONSTRUCTORS¶
WHAT IS A TYPE CONSTRUCTOR?¶
A type constructor is a body of code that initializes the type as a whole,
rather like a C++ static initializer. The body of a type constructor is
executed once when the type is defined, and never again.
A type can have at most one type constructor.
HOW DO I DEFINE A TYPE CONSTRUCTOR?¶
A type constructor is defined by using the
typeconstructor statement in
the type definition. For example, suppose the type uses an array-valued type
variable as a look-up table, and the values in the array have to be computed
at start-up.
% snit::type mytype {
typevariable lookupTable
typeconstructor {
array set lookupTable {key value...}
}
}
CONSTRUCTORS¶
WHAT IS A CONSTRUCTOR?¶
In object-oriented programming, an object's constructor is responsible for
initializing the object completely at creation time. The constructor receives
the list of options passed to the
snit::type command's
create
method and can then do whatever it likes. That might include computing
instance variable values, reading data from files, creating other objects,
updating type and instance variables, and so forth.
The constructor's return value is ignored (unless it's an error, of course).
HOW DO I DEFINE A CONSTRUCTOR?¶
A constructor is defined by using the
constructor statement in the type
definition. Suppose that it's desired to keep a list of all pedigreed dogs.
The list can be maintained in a type variable and retrieved by a type method.
Whenever a dog is created, it can add itself to the list--provided that it's
registered with the American Kennel Club.
% snit::type dog {
option -akc 0
typevariable akcList {}
constructor {args} {
$self configurelist $args
if {$options(-akc)} {
lappend akcList $self
}
}
typemethod akclist {} {
return $akcList
}
}
::dog
% dog spot -akc 1
::spot
% dog fido
::fido
% dog akclist
::spot
%
WHAT DOES THE DEFAULT CONSTRUCTOR DO?¶
If you don't provide a constructor explicitly, you get the default constructor,
which is identical to the explicitly-defined constructor shown here:
snit::type dog {
constructor {args} {
$self configurelist $args
}
}
When the constructor is called,
args will be set to the list of arguments
that follow the object's name. The constructor is allowed to interpret this
list any way it chooses; the normal convention is to assume that it's a list
of option names and values, as shown in the example above. If you simply want
to save the option values, you should use the
configurelist method, as
shown.
CAN I CHOOSE A DIFFERENT SET OF ARGUMENTS FOR THE
CONSTRUCTOR?¶
Yes, you can. For example, suppose we wanted to be sure that the breed was
explicitly stated for every dog at creation time, and couldn't be changed
thereafter. One way to do that is as follows:
% snit::type dog {
variable breed
option -color brown
option -akc 0
constructor {theBreed args} {
set breed $theBreed
$self configurelist $args
}
method breed {} { return $breed }
}
::dog
% dog spot dalmatian -color spotted -akc 1
::spot
% spot breed
dalmatian
The drawback is that this syntax is non-standard, and may limit the
compatibility of your new type with other people's code. For example, Snit
assumes that it can create
COMPONENTS using the standard creation
syntax.
ARE THERE ANY LIMITATIONS ON CONSTRUCTOR ARGUMENTS?¶
Constructor argument lists are subject to the same limitations as those on
instance method argument lists. It has the same implicit arguments, and can
contain default values and the
args argument.
IS THERE ANYTHING SPECIAL ABOUT WRITING THE CONSTRUCTOR?¶
Yes. Writing the constructor can be tricky if you're delegating options to
components, and there are specific issues relating to
snit::widgets and
snit::widgetadaptors. See
DELEGATION,
WIDGETS,
WIDGET
ADAPTORS, and
THE TK OPTION DATABASE.
DESTRUCTORS¶
WHAT IS A DESTRUCTOR?¶
A destructor is a special kind of method that's called when an object is
destroyed. It's responsible for doing any necessary clean-up when the object
goes away: destroying
COMPONENTS, closing files, and so forth.
HOW DO I DEFINE A DESTRUCTOR?¶
Destructors are defined by using the
destructor statement in the type
definition.
Suppose we're maintaining a list of pedigreed dogs; then we'll want to remove
dogs from it when they are destroyed.
snit::type dog {
option -akc 0
typevariable akcList {}
constructor {args} {
$self configurelist $args
if {$options(-akc)} {
lappend akcList $self
}
}
destructor {
set ndx [lsearch $akcList $self]
if {$ndx != -1} {
set akcList [lreplace $akcList $ndx $ndx]
}
}
typemethod akclist {} {
return $akcList
}
}
ARE THERE ANY LIMITATIONS ON DESTRUCTOR ARGUMENTS?¶
Yes; a destructor has no explicit arguments.
WHAT IMPLICIT ARGUMENTS ARE PASSED TO THE DESTRUCTOR?¶
The destructor gets the same implicit arguments that are passed to
INSTANCE
METHODS:
type,
selfns,
win, and
self.
MUST COMPONENTS BE DESTROYED EXPLICITLY?¶
Yes and no.
Any Tk widgets created by a
snit::widget or
snit::widgetadaptor
will be destroyed automatically by Tk when the megawidget is destroyed, in
keeping with normal Tk behavior (destroying a parent widget destroys the whole
tree).
Components of normal
snit::types, on the other hand, are never destroyed
automatically, nor are non-widget components of Snit megawidgets. If your
object creates them in its constructor, then it should generally destroy them
in its destructor.
IS THERE ANY SPECIAL ABOUT WRITING A DESTRUCTOR?¶
Yes. If an object's constructor throws an error, the object's destructor will be
called to clean up; this means that the object might not be completely
constructed when the destructor is called. This can cause the destructor to
throw its own error; the result is usually misleading, confusing, and
unhelpful. Consequently, it's important to write your destructor so that it's
fail-safe.
For example, a
dog might create a
tail component; the component
will need to be destroyed. But suppose there's an error while processing the
creation options--the destructor will be called, and there will be no
tail to destroy. The simplest solution is generally to catch and ignore
any errors while destroying components.
snit::type dog {
component tail
constructor {args} {
$self configurelist $args
set tail [tail %AUTO%]
}
destructor {
catch {$tail destroy}
}
}
COMPONENTS¶
WHAT IS A COMPONENT?¶
Often an object will create and manage a number of other objects. A Snit
megawidget, for example, will often create a number of Tk widgets. These
objects are part of the main object; it is composed of them, so they are
called components of the object.
But Snit also has a more precise meaning for
COMPONENT. The components of
a Snit object are those objects to which methods or options can be delegated.
(See
DELEGATION for more information about delegation.)
HOW DO I DECLARE A COMPONENT?¶
First, you must decide what role a component plays within your object, and give
the role a name. Then, you declare the component using its role name and the
component statement. The
component statement declares an
instance variable which is used to store the component's command name
when the component is created.
For example, suppose your
dog object creates a
tail object (the
better to wag with, no doubt):
snit::type dog {
component mytail
constructor {args} {
# Create and save the component's command
set mytail [tail %AUTO% -partof $self]
$self configurelist $args
}
method wag {} {
$mytail wag
}
}
As shown here, it doesn't matter what the
tail object's real name is; the
dog object refers to it by its component name.
The above example shows one way to delegate the
wag method to the
mytail component; see
DELEGATION for an easier way.
HOW IS A COMPONENT NAMED?¶
A component has two names. The first name is that of the component variable;
this represents the role the component object plays within the Snit object.
This is the component name proper, and is the name used to refer to the
component within Snit code. The second name is the name of the actual
component object created by the Snit object's constructor. This second name is
always a Tcl command name, and is referred to as the component's object name.
In the example in the previous question, the component name is
mytail;
the
mytail component's object name is chosen automatically by Snit
since
%AUTO% was used when the component object was created.
ARE THERE ANY LIMITATIONS ON COMPONENT NAMES?¶
Yes.
snit::widget and
snit::widgetadaptor objects have a special
component called the
hull component; thus, the name
hull should
be used for no other purpose.
Otherwise, since component names are in fact instance variable names they must
follow the rules for
INSTANCE VARIABLES.
WHAT IS AN OWNED COMPONENT?¶
An
owned component is a component whose object command's lifetime is
controlled by the
snit::type or
snit::widget.
As stated above, a component is an object to which our object can delegate
methods or options. Under this definition, our object will usually create its
component objects, but not necessarily. Consider the following: a dog object
has a tail component; but tail knows that it's part of the dog:
snit::type dog {
component mytail
constructor {args} {
set mytail [tail %AUTO% -partof $self]
$self configurelist $args
}
destructor {
catch {$mytail destroy}
}
delegate method wagtail to mytail as wag
method bark {} {
return "$self barked."
}
}
snit::type tail {
component mydog
option -partof -readonly yes
constructor {args} {
$self configurelist $args
set mydog $options(-partof)
}
method wag {} {
return "Wag, wag."
}
method pull {} {
$mydog bark
}
}
Thus, if you ask a dog to wag its tail, it tells its tail to wag; and if you
pull the dog's tail, the tail tells the dog to bark. In this scenario, the
tail is a component of the dog, and the dog is a component of the tail, but
the dog owns the tail and not the other way around.
WHAT DOES THE INSTALL COMMAND DO?¶
The
install command creates an owned component using a specified command,
and assigns the result to the component's instance variable. For example:
snit::type dog {
component mytail
constructor {args} {
# set mytail [tail %AUTO% -partof $self]
install mytail using tail %AUTO% -partof $self
$self configurelist $args
}
}
In a
snit::type's code, the
install command shown above is
equivalent to the
set mytail command that's commented out. In a
snit::widget's or
snit::widgetadaptor's, code, however, the
install command also queries
THE TK OPTION DATABASE and
initializes the new component's options accordingly. For consistency, it's a
good idea to get in the habit of using
install for all owned
components.
MUST OWNED COMPONENTS BE CREATED IN THE CONSTRUCTOR?¶
No, not necessarily. In fact, there's no reason why an object can't destroy and
recreate a component multiple times over its own lifetime.
ARE THERE ANY LIMITATIONS ON COMPONENT OBJECT NAMES?¶
Yes.
Component objects which are Tk widgets or megawidgets must have valid Tk window
names.
Component objects which are not widgets or megawidgets must have fully-qualified
command names, i.e., names which include the full namespace of the command.
Note that Snit always creates objects with fully qualified names.
Next, the object names of components and owned by your object must be unique.
This is no problem for widget components, since widget names are always
unique; but consider the following code:
snit::type tail { ... }
snit::type dog {
delegate method wag to mytail
constructor {} {
install mytail using tail mytail
}
}
This code uses the component name,
mytail, as the component object name.
This is not good, and here's why: Snit instance code executes in the Snit
type's namespace. In this case, the
mytail component is created in the
::dog:: namespace, and will thus have the name
::dog::mytail.
Now, suppose you create two dogs. Both dogs will attempt to create a tail called
::dog::mytail. The first will succeed, and the second will fail, since
Snit won't let you create an object if its name is already a command. Here are
two ways to avoid this situation:
First, if the component type is a
snit::type you can specify
%AUTO% as its name, and be guaranteed to get a unique name. This is the
safest thing to do:
install mytail using tail %AUTO%
If the component type isn't a
snit::type you can create the component in
the object's instance namespace:
install mytail using tail ${selfns}::mytail
Make sure you pick a unique name within the instance namespace.
MUST I DESTROY THE COMPONENTS I OWN?¶
That depends. When a parent widget is destroyed, all child widgets are destroyed
automatically. Thus, if your object is a
snit::widget or
snit::widgetadaptor you don't need to destroy any components that are
widgets, because they will generally be children or descendants of your
megawidget.
If your object is an instance of
snit::type, though, none of its owned
components will be destroyed automatically, nor will be non-widget components
of a
snit::widget be destroyed automatically. All such owned components
must be destroyed explicitly, or they won't be destroyed at all.
CAN I EXPOSE A COMPONENT'S OBJECT COMMAND AS PART OF MY
INTERFACE?¶
Yes, and there are two ways to do it. The most appropriate way is usually to use
DELEGATION. Delegation allows you to pass the options and methods you
specify along to particular components. This effectively hides the components
from the users of your type, and ensures good encapsulation.
However, there are times when it's appropriate, not to mention simpler, just to
make the entire component part of your type's public interface.
HOW DO I EXPOSE A COMPONENT'S OBJECT COMMAND?¶
When you declare the component, specify the
component statement's
-public option. The value of this option is the name of a method which
will be delegated to your component's object command.
For example, supposed you've written a combobox megawidget which owns a listbox
widget, and you want to make the listbox's entire interface public. You can do
it like this:
snit::widget combobox {
component listbox -public listbox
constructor {args} {
install listbox using listbox $win.listbox ....
}
}
combobox .mycombo
.mycombo listbox configure -width 30
Your comobox widget,
.mycombo, now has a
listbox method which has
all of the same subcommands as the listbox widget itself. Thus, the above code
sets the listbox component's width to 30.
Usually you'll let the method name be the same as the component name; however,
you can name it anything you like.
TYPE COMPONENTS¶
WHAT IS A TYPE COMPONENT?¶
A type component is a component that belongs to the type itself instead of to a
particular instance of the type. The relationship between components and type
components is the same as the relationship between
INSTANCE VARIABLES
and
TYPE VARIABLES. Both
INSTANCE METHODS and
TYPE
METHODS can be delegated to type components.
Once you understand
COMPONENTS and
DELEGATION, type components are
just more of the same.
HOW DO I DECLARE A TYPE COMPONENT?¶
Declare a type component using the
typecomponent statement. It takes the
same options (
-inherit and
-public) as the
component
statement does, and defines a type variable to hold the type component's
object command.
Suppose in your model you've got many dogs, but only one veterinarian. You might
make the veterinarian a type component.
snit::type veterinarian { ... }
snit::type dog {
typecomponent vet
# ...
}
HOW DO I INSTALL A TYPE COMPONENT?¶
Just use the
set command to assign the component's object command to the
type component. Because types (even
snit::widget types) are not
widgets, and do not have options anyway, the extra features of the
install command are not needed.
You'll usually install type components in the type constructor, as shown here:
snit::type veterinarian { ... }
snit::type dog {
typecomponent vet
typeconstructor {
set vet [veterinarian %AUTO%]
}
}
ARE THERE ANY LIMITATIONS ON TYPE COMPONENT NAMES?¶
Yes, the same as on
INSTANCE VARIABLES,
TYPE VARIABLES, and normal
COMPONENTS.
DELEGATION¶
WHAT IS DELEGATION?¶
Delegation, simply put, is when you pass a task you've been given to one of your
assistants. (You do have assistants, don't you?) Snit objects can do the same
thing. The following example shows one way in which the
dog object can
delegate its
wag method and its
-taillength option to its
tail component.
snit::type dog {
variable mytail
option -taillength -configuremethod SetTailOption -cgetmethod GetTailOption
method SetTailOption {option value} {
$mytail configure $option $value
}
method GetTailOption {option} {
$mytail cget $option
}
method wag {} {
$mytail wag
}
constructor {args} {
install mytail using tail %AUTO% -partof $self
$self configurelist $args
}
}
This is the hard way to do it, by it demonstrates what delegation is all about.
See the following answers for the easy way to do it.
Note that the constructor calls the
configurelist method
after it
creates its
tail; otherwise, if
-taillength appeared in the list
of
args we'd get an error.
HOW CAN I DELEGATE A METHOD TO A COMPONENT OBJECT?¶
Delegation occurs frequently enough that Snit makes it easy. Any method can be
delegated to any component or type component by placing a single
delegate statement in the type definition. (See
COMPONENTS and
TYPE COMPONENTS for more information about component names.)
For example, here's a much better way to delegate the
dog object's
wag method:
% snit::type dog {
delegate method wag to mytail
constructor {} {
install mytail using tail %AUTO%
}
}
::dog
% snit::type tail {
method wag {} { return "Wag, wag, wag."}
}
::tail
% dog spot
::spot
% spot wag
Wag, wag, wag.
This code has the same effect as the code shown under the previous question:
when a
dog's
wag method is called, the call and its arguments
are passed along automatically to the
tail object.
Note that when a component is mentioned in a
delegate statement, the
component's instance variable is defined implicitly. However, it's still good
practice to declare it explicitly using the
component statement.
Note also that you can define a method name using the
method statement,
or you can define it using
delegate; you can't do both.
CAN I DELEGATE TO A METHOD WITH A DIFFERENT NAME?¶
Suppose you wanted to delegate the
dog's
wagtail method to the
tail's
wag method. After all you wag the tail, not the dog. It's
easily done:
snit::type dog {
delegate method wagtail to mytail as wag
constructor {args} {
install mytail using tail %AUTO% -partof $self
$self configurelist $args
}
}
CAN I DELEGATE TO A METHOD WITH ADDITIONAL ARGUMENTS?¶
Suppose the
tail's
wag method takes as an argument the number of
times the tail should be wagged. You want to delegate the
dog's
wagtail method to the
tail's
wag method, specifying that
the tail should be wagged exactly three times. This is easily done, too:
snit::type dog {
delegate method wagtail to mytail as {wag 3}
# ...
}
snit::type tail {
method wag {count} {
return [string repeat "Wag " $count]
}
# ...
}
CAN I DELEGATE A METHOD TO SOMETHING OTHER THAN AN OBJECT?¶
Normal method delegation assumes that you're delegating a method (a subcommand
of an object command) to a method of another object (a subcommand of a
different object command). But not all Tcl objects follow Tk conventions, and
not everything you'd to which you'd like to delegate a method is necessary an
object. Consequently, Snit makes it easy to delegate a method to pretty much
anything you like using the
delegate statement's
using clause.
Suppose your dog simulation stores dogs in a database, each dog as a single
record. The database API you're using provides a number of commands to manage
records; each takes the record ID (a string you choose) as its first argument.
For example,
saverec saves a record. If you let the record ID be the
name of the dog object, you can delegate the dog's
save method to the
saverec command as follows:
snit::type dog {
delegate method save using {saverec %s}
}
The
%s is replaced with the instance name when the
save method is
called; any additional arguments are the appended to the resulting command.
The
using clause understands a number of other %-conversions; in addition
to the instance name, you can substitute in the method name (
%m), the
type name (
%t), the instance namespace (
%n), the Tk window
name (
%w), and, if a component or typecomponent name was given in the
delegate statement, the component's object command (
%c).
HOW CAN I DELEGATE A METHOD TO A TYPE COMPONENT OBJECT?¶
Just exactly as you would to a component object. The
delegate method
statement accepts both component and type component names in its
to
clause.
HOW CAN I DELEGATE A TYPE METHOD TO A TYPE COMPONENT OBJECT?¶
Use the
delegate typemethod statement. It works like
delegate
method, with these differences: first, it defines a type method instead of
an instance method; second, the
using clause ignores the
%s,
%n, and
%w %-conversions.
Naturally, you can't delegate a type method to an instance component...Snit
wouldn't know which instance should receive it.
HOW CAN I DELEGATE AN OPTION TO A COMPONENT OBJECT?¶
The first question in this section (see
DELEGATION) shows one way to
delegate an option to a component; but this pattern occurs often enough that
Snit makes it easy. For example, every
tail object has a
-length
option; we want to allow the creator of a
dog object to set the tail's
length. We can do this:
% snit::type dog {
delegate option -length to mytail
constructor {args} {
install mytail using tail %AUTO% -partof $self
$self configurelist $args
}
}
::dog
% snit::type tail {
option -partof
option -length 5
}
::tail
% dog spot -length 7
::spot
% spot cget -length
7
This produces nearly the same result as the
-configuremethod and
-cgetmethod shown under the first question in this section: whenever a
dog object's
-length option is set or retrieved, the underlying
tail object's option is set or retrieved in turn.
Note that you can define an option name using the
option statement, or
you can define it using
delegate; you can't do both.
CAN I DELEGATE TO AN OPTION WITH A DIFFERENT NAME?¶
In the previous answer we delegated the
dog's
-length option down
to its
tail. This is, of course, wrong. The dog has a length, and the
tail has a length, and they are different. What we'd really like to do is give
the
dog a
-taillength option, but delegate it to the
tail's
-length option:
snit::type dog {
delegate option -taillength to mytail as -length
constructor {args} {
set mytail [tail %AUTO% -partof $self]
$self configurelist $args
}
}
HOW CAN I DELEGATE ANY UNRECOGNIZED METHOD OR OPTION TO A
COMPONENT OBJECT?¶
It may happen that a Snit object gets most of its behavior from one of its
components. This often happens with
snit::widgetadaptors, for example,
where we wish to slightly the modify the behavior of an existing widget. To
carry on with our
dog example, however, suppose that we have a
snit::type called
animal that implements a variety of animal
behaviors--moving, eating, sleeping, and so forth. We want our
dog
objects to inherit these same behaviors, while adding dog-like behaviors of
its own. Here's how we can give a
dog methods and options of its own
while delegating all other methods and options to its
animal component:
snit::type dog {
delegate option * to animal
delegate method * to animal
option -akc 0
constructor {args} {
install animal using animal %AUTO% -name $self
$self configurelist $args
}
method wag {} {
return "$self wags its tail"
}
}
That's it. A
dog is now an
animal that has a
-akc option
and can
wag its tail.
Note that we don't need to specify the full list of method names or option names
that
animal will receive. It gets anything
dog doesn't
recognize--and if it doesn't recognize it either, it will simply throw an
error, just as it should.
You can also delegate all unknown type methods to a type component using
delegate typemethod *.
HOW CAN I DELEGATE ALL BUT CERTAIN METHODS OR OPTIONS TO A
COMPONENT?¶
In the previous answer, we said that every
dog is an
animal by
delegating all unknown methods and options to the
animal component. But
what if the
animal type has some methods or options that we'd like to
suppress?
One solution is to explicitly delegate all the options and methods, and forgo
the convenience of
delegate method * and
delegate option *. But
if we wish to suppress only a few options or methods, there's an easier way:
snit::type dog {
delegate option * to animal except -numlegs
delegate method * to animal except {fly climb}
# ...
constructor {args} {
install animal using animal %AUTO% -name $self -numlegs 4
$self configurelist $args
}
# ...
}
Dogs have four legs, so we specify that explicitly when we create the
animal component, and explicitly exclude
-numlegs from the set
of delegated options. Similarly, dogs can neither
fly nor
climb,
so we exclude those
animal methods as shown.
CAN A HIERARCHICAL METHOD BE DELEGATED?¶
Yes; just specify multiple words in the delegated method's name:
snit::type tail {
method wag {} {return "Wag, wag"}
method droop {} {return "Droop, droop"}
}
snit::type dog {
delegate method {tail wag} to mytail
delegate method {tail droop} to mytail
# ...
constructor {args} {
install mytail using tail %AUTO%
$self configurelist $args
}
# ...
}
Unrecognized hierarchical methods can also be delegated; the following code
delegates all subcommands of the "tail" method to the
"mytail" component:
snit::type dog {
delegate method {tail *} to mytail
# ...
}
A
snit::widget is the Snit version of what Tcl programmers usually call a
megawidget: a widget-like object usually consisting of one or more Tk
widgets all contained within a Tk frame.
A
snit::widget is also a special kind of
snit::type. Just about
everything in this FAQ list that relates to
snit::types also applies to
snit::widgets.
snit::widgets are defined using the
snit::widget command, just as
snit::types are defined by the
snit::type command.
The body of the definition can contain all of the same kinds of statements, plus
a couple of others which will be mentioned below.
- •
- The name of an instance of a snit::type can be any
valid Tcl command name, in any namespace. The name of an instance of a
snit::widget must be a valid Tk widget name, and its parent widget
must already exist.
- •
- An instance of a snit::type can be destroyed by
calling its destroy method. Instances of a snit::widget have
no destroy method; use the Tk destroy command instead.
- •
- Every instance of a snit::widget has one predefined
component called its hull component. The hull is usually a Tk
frame or toplevel widget; any other widgets created as part
of the snit::widget will usually be contained within the hull.
- •
- snit::widgets can have their options receive default
values from THE TK OPTION DATABASE.
WHAT IS A HULL COMPONENT?¶
Snit can't create a Tk widget object; only Tk can do that. Thus, every instance
of a
snit::widget must be wrapped around a genuine Tk widget; this Tk
widget is called the
hull component. Snit effectively piggybacks the
behavior you define (methods, options, and so forth) on top of the hull
component so that the whole thing behaves like a standard Tk widget.
For
snit::widgets the hull component must be a Tk widget that defines the
-class option.
snit::widgetadaptors differ from
snit::widgets chiefly in that any
kind of widget can be used as the hull component; see
WIDGET ADAPTORS.
A
snit::widget's hull component will usually be a Tk
frame widget;
however, it may be any Tk widget that defines the
-class option. You
can explicitly choose the hull type you prefer by including the
hulltype command in the widget definition:
snit::widget mytoplevel {
hulltype toplevel
# ...
}
If no
hulltype command appears, the hull will be a
frame.
By default, Snit recognizes the following hull types: the Tk widgets
frame,
labelframe,
toplevel, and the Tile widgets
ttk::frame,
ttk::labelframe, and
ttk::toplevel. To enable
the use of some other kind of widget as the hull type, you can
lappend
the widget command to the variable
snit::hulltypes (always provided the
widget defines the
-class option. For example, suppose Tk gets a new
widget type called a
prettyframe:
lappend snit::hulltypes prettyframe
snit::widget mywidget {
hulltype prettyframe
# ...
}
Every widget, whether a genuine Tk widget or a Snit megawidget, has to have a
valid Tk window name. When a
snit::widget is first created, its
instance name,
self, is a Tk window name; however, if the
snit::widget is used as the hull component by a
snit::widgetadaptor its instance name will be changed to something
else. For this reason, every
snit::widget method, constructor,
destructor, and so forth is passed another implicit argument,
win,
which is the window name of the megawidget. Any children should be named using
win as the root.
Thus, suppose you're writing a toolbar widget, a frame consisting of a number of
buttons placed side-by-side. It might look something like this:
snit::widget toolbar {
delegate option * to hull
constructor {args} {
button $win.open -text Open -command [mymethod open]
button $win.save -text Save -command [mymethod save]
# ....
$self configurelist $args
}
}
See also the question on renaming objects, toward the top of this file.
A
snit::widgetadaptor is a kind of
snit::widget. Whereas a
snit::widget's hull is automatically created and is always a Tk frame,
a
snit::widgetadaptor can be based on any Tk widget--or on any Snit
megawidget, or even (with luck) on megawidgets defined using some other
package.
It's called a
widget adaptor because it allows you to take an existing
widget and customize its behavior.
Use the
snit::widgetadaptor command. The definition for a
snit::widgetadaptor looks just like that for a
snit::type or
snit::widget, except that the constructor must create and install the
hull component.
For example, the following code creates a read-only text widget by the simple
device of turning its
insert and
delete methods into no-ops.
Then, we define new methods,
ins and
del, which get delegated to
the hull component as
insert and
delete. Thus, we've adapted the
text widget and given it new behavior while still leaving it fundamentally a
text widget.
::snit::widgetadaptor rotext {
constructor {args} {
# Create the text widget; turn off its insert cursor
installhull using text -insertwidth 0
# Apply any options passed at creation time.
$self configurelist $args
}
# Disable the text widget's insert and delete methods, to
# make this readonly.
method insert {args} {}
method delete {args} {}
# Enable ins and del as synonyms, so the program can insert and
# delete.
delegate method ins to hull as insert
delegate method del to hull as delete
# Pass all other methods and options to the real text widget, so
# that the remaining behavior is as expected.
delegate method * to hull
delegate option * to hull
}
The most important part is in the constructor. Whereas
snit::widget
creates the hull for you,
snit::widgetadaptor cannot -- it doesn't know
what kind of widget you want. So the first thing the constructor does is
create the hull component (a Tk text widget in this case), and then installs
it using the
installhull command.
Note: There is no instance command until you create one by installing a
hull component. Any attempt to pass methods to
$self prior to calling
installhull will fail.
Yes.
At times, it can be convenient to adapt a pre-existing widget instead of
creating your own. For example, the Bwidget
PagesManager widget manages
a set of
frame widgets, only one of which is visible at a time. The
application chooses which
frame is visible. All of the These
frames are created by the
PagesManager itself, using its
add method. It's convenient to adapt these frames to do what we'd like
them to do.
In a case like this, the Tk widget will already exist when the
snit::widgetadaptor is created. Snit provides an alternate form of the
installhull command for this purpose:
snit::widgetadaptor pageadaptor {
constructor {args} {
# The widget already exists; just install it.
installhull $win
# ...
}
}
Maybe. If the other megawidget is a
snit::widget or
snit::widgetadaptor, then yes. If it isn't then, again, maybe. You'll
have to try it and see. You're most likely to have trouble with widget
destruction--you have to make sure that your megawidget code receives the
<Destroy> event before the megawidget you're adapting does.
THE TK OPTION DATABASE¶
WHAT IS THE TK OPTION DATABASE?¶
The Tk option database is a database of default option values maintained by Tk
itself; every Tk application has one. The concept of the option database
derives from something called the X Windows resource database; however, the
option database is available in every Tk implementation, including those which
do not use the X Windows system (e.g., Microsoft Windows).
Full details about the Tk option database are beyond the scope of this document;
both
Practical Programming in Tcl and Tk by Welch, Jones, and Hobbs,
and
Effective Tcl/Tk Programming by Harrison and McClennan., have good
introductions to it.
Snit is implemented so that most of the time it will simply do the right thing
with respect to the option database, provided that the widget developer does
the right thing by Snit. The body of this section goes into great deal about
what Snit requires. The following is a brief statement of the requirements,
for reference.
- •
- If the widget's default widget class is not what is
desired, set it explicitly using the widgetclass statement in the
widget definition.
- •
- When defining or delegating options, specify the resource
and class names explicitly when necessary.
- •
- Use the installhull using command to create and
install the hull for snit::widgetadaptors.
- •
- Use the install command to create and install all
components which are widgets.
- •
- Use the install command to create and install
components which aren't widgets if you'd like them to receive option
values from the option database.
The interaction of Tk widgets with the option database is a complex thing; the
interaction of Snit with the option database is even more so, and repays
attention to detail.
DO SNIT::TYPES USE THE TK OPTION DATABASE?¶
No, they don't; querying the option database requires a Tk window name, and
snit::types don't have one.
If you create an instance of a
snit::type as a component of a
snit::widget or
snit::widgetadaptor, on the other hand, and if
any options are delegated to the component, and if you use
install to
create and install it, then the megawidget will query the option database on
the
snit::type's behalf. This might or might not be what you want, so
take care.
Every Tk widget has a "widget class": a name that is used when adding
option settings to the database. For Tk widgets, the widget class is the same
as the widget command name with an initial capital. For example, the widget
class of the Tk
button widget is
Button.
Similarly, the widget class of a
snit::widget defaults to the unqualified
type name with the first letter capitalized. For example, the widget class of
snit::widget ::mylibrary::scrolledText { ... }
is
ScrolledText.
The widget class can also be set explicitly using the
widgetclass
statement within the
snit::widget definition:
snit::widget ::mylibrary::scrolledText {
widgetclass Text
# ...
}
The above definition says that a
scrolledText megawidget has the same
widget class as an ordinary
text widget. This might or might not be a
good idea, depending on how the rest of the megawidget is defined, and how its
options are delegated.
The widget class of a
snit::widgetadaptor is just the widget class of its
hull widget; Snit has no control over this.
Note that the widget class can be changed only for
frame and
toplevel widgets, which is why these are the valid hull types for
snit::widgets.
Try to use
snit::widgetadaptors only to make small modifications to
another widget's behavior. Then, it will usually not make sense to change the
widget's widget class anyway.
WHAT ARE OPTION RESOURCE AND CLASS NAMES?¶
Every Tk widget option has three names: the option name, the resource name, and
the class name. The option name begins with a hyphen and is all lowercase;
it's used when creating widgets, and with the
configure and
cget
commands.
The resource and class names are used to initialize option default values by
querying the option database. The resource name is usually just the option
name minus the hyphen, but may contain uppercase letters at word boundaries;
the class name is usually just the resource name with an initial capital, but
not always. For example, here are the option, resource, and class names for
several Tk
text widget options:
-background background Background
-borderwidth borderWidth BorderWidth
-insertborderwidth insertBorderWidth BorderWidth
-padx padX Pad
As is easily seen, sometimes the resource and class names can be inferred from
the option name, but not always.
For options implicitly delegated to a component using
delegate option *,
the resource and class names will be exactly those defined by the component.
The
configure method returns these names, along with the option's
default and current values:
% snit::widget mytext {
delegate option * to text
constructor {args} {
install text using text .text
# ...
}
# ...
}
::mytext
% mytext .text
.text
% .text configure -padx
-padx padX Pad 1 1
%
For all other options (whether locally defined or explicitly delegated), the
resource and class names can be defined explicitly, or they can be allowed to
have default values.
By default, the resource name is just the option name minus the hyphen; the the
class name is just the option name with an initial capital letter. For
example, suppose we explicitly delegate "-padx":
% snit::widget mytext {
option -myvalue 5
delegate option -padx to text
delegate option * to text
constructor {args} {
install text using text .text
# ...
}
# ...
}
::mytext
% mytext .text
.text
% .text configure -myvalue
-myvalue myvalue Myvalue 5 5
% .text configure -padx
-padx padx Padx 1 1
%
Here the resource and class names are chosen using the default rules. Often
these rules are sufficient, but in the case of "-padx" we'd most
likely prefer that the option's resource and class names are the same as for
the built-in Tk widgets. This is easily done:
% snit::widget mytext {
delegate option {-padx padX Pad} to text
# ...
}
::mytext
% mytext .text
.text
% .text configure -padx
-padx padX Pad 1 1
%
The option database is queried for each of the megawidget's locally-defined
options, using the option's resource and class name. If the result isn't
"", then it replaces the default value given in widget definition.
In either case, the default can be overridden by the caller. For example,
option add *Mywidget.texture pebbled
snit::widget mywidget {
option -texture smooth
# ...
}
mywidget .mywidget -texture greasy
Here,
-texture would normally default to "smooth", but because
of the entry added to the option database it defaults to "pebbled".
However, the caller has explicitly overridden the default, and so the new
widget will be "greasy".
HOW DOES SNIT INITIALIZE DELEGATED OPTIONS?¶
That depends on whether the options are delegated to the hull, or to some other
component.
HOW DOES SNIT INITIALIZE OPTIONS DELEGATED TO THE HULL?¶
A
snit::widget's hull is a widget, and given that its class has been set
it is expected to query the option database for itself. The only exception
concerns options that are delegated to it with a different name. Consider the
following code:
option add *Mywidget.borderWidth 5
option add *Mywidget.relief sunken
option add *Mywidget.hullbackground red
option add *Mywidget.background green
snit::widget mywidget {
delegate option -borderwidth to hull
delegate option -hullbackground to hull as -background
delegate option * to hull
# ...
}
mywidget .mywidget
set A [.mywidget cget -relief]
set B [.mywidget cget -hullbackground]
set C [.mywidget cget -background]
set D [.mywidget cget -borderwidth]
The question is, what are the values of variables A, B, C and D?
The value of A is "sunken". The hull is a Tk frame which has been
given the widget class
Mywidget; it will automatically query the option
database and pick up this value. Since the
-relief option is implicitly
delegated to the hull, Snit takes no action.
The value of B is "red". The hull will automatically pick up the value
"green" for its
-background option, just as it picked up the
-relief value. However, Snit knows that
-hullbackground is
mapped to the hull's
-background option; hence, it queries the option
database for
-hullbackground and gets "red" and updates the
hull accordingly.
The value of C is also "red", because
-background is implicitly
delegated to the hull; thus, retrieving it is the same as retrieving
-hullbackground. Note that this case is unusual; the
-background
option should probably have been excluded using the delegate statement's
except clause, or (more likely) delegated to some other component.
The value of D is "5", but not for the reason you think. Note that as
it is defined above, the resource name for
-borderwidth defaults to
borderwidth, whereas the option database entry is
borderWidth,
in accordance with the standard Tk naming for this option. As with
-relief, the hull picks up its own
-borderwidth option before
Snit does anything. Because the option is delegated under its own name, Snit
assumes that the correct thing has happened, and doesn't worry about it any
further. To avoid confusion, the
-borderwidth option should have been
delegated like this:
delegate option {-borderwidth borderWidth BorderWidth} to hull
For
snit::widgetadaptors, the case is somewhat altered. Widget adaptors
retain the widget class of their hull, and the hull is not created
automatically by Snit. Instead, the
snit::widgetadaptor must call
installhull in its constructor. The normal way to do this is as
follows:
snit::widgetadaptor mywidget {
# ...
constructor {args} {
# ...
installhull using text -foreground white
# ...
}
# ...
}
In this case, the
installhull command will create the hull using a
command like this:
set hull [text $win -foreground white]
The hull is a
text widget, so its widget class is
Text. Just as
with
snit::widget hulls, Snit assumes that it will pick up all of its
normal option values automatically, without help from Snit. Options delegated
from a different name are initialized from the option database in the same way
as described above.
In earlier versions of Snit,
snit::widgetadaptors were expected to call
installhull like this:
installhull [text $win -foreground white]
This form still works--but Snit will not query the option database as described
above.
HOW DOES SNIT INITIALIZE OPTIONS DELEGATED TO OTHER
COMPONENTS?¶
For hull components, Snit assumes that Tk will do most of the work
automatically. Non-hull components are somewhat more complicated, because they
are matched against the option database twice.
A component widget remains a widget still, and is therefore initialized from the
option database in the usual way. A
text widget remains a
text
widget whether it is a component of a megawidget or not, and will be created
as such.
But then, the option database is queried for all options delegated to the
component, and the component is initialized accordingly--provided that the
install command is used to create it.
Before option database support was added to Snit, the usual way to create a
component was to simply create it in the constructor and assign its command
name to the component variable:
snit::widget mywidget {
delegate option -background to myComp
constructor {args} {
set myComp [text $win.text -foreground black]
}
}
The drawback of this method is that Snit has no opportunity to initialize the
component properly. Hence, the following approach is now used:
snit::widget mywidget {
delegate option -background to myComp
constructor {args} {
install myComp using text $win.text -foreground black
}
}
The
install command does the following:
- •
- Builds a list of the options explicitly included in the
install command--in this case, -foreground.
- •
- Queries the option database for all options delegated
explicitly to the named component.
- •
- Creates the component using the specified command, after
inserting into it a list of options and values read from the option
database. Thus, the explicitly included options (like -foreground)
will override anything read from the option database.
- •
- If the widget definition implicitly delegated options to
the component using delegate option *, then Snit calls the newly
created component's configure method to receive a list of all of
the component's options. From this Snit builds a list of options
implicitly delegated to the component which were not explicitly included
in the install command. For all such options, Snit queries the
option database and configures the component accordingly.
You don't really need to know all of this; just use
install to install
your components, and Snit will try to do the right thing.
A
snit::type never queries the option database. However, a
snit::widget can have non-widget components. And if options are
delegated to those components, and if the
install command is used to
install those components, then they will be initialized from the option
database just as widget components are.
However, when used within a megawidget,
install assumes that the created
component uses a reasonably standard widget-like creation syntax. If it
doesn't, don't use
install.
ENSEMBLE COMMANDS¶
WHAT IS AN ENSEMBLE COMMAND?¶
An ensemble command is a command with subcommands. Snit objects are all ensemble
commands; however, the term more usually refers to commands like the standard
Tcl commands
string,
file, and
clock. In a sense, these
are singleton objects--there's only one instance of them.
HOW CAN I CREATE AN ENSEMBLE COMMAND USING SNIT?¶
There are two ways--as a
snit::type, or as an instance of a
snit::type.
HOW CAN I CREATE AN ENSEMBLE COMMAND USING AN INSTANCE OF A
SNIT::TYPE?¶
Define a type whose
INSTANCE METHODS are the subcommands of your ensemble
command. Then, create an instance of the type with the desired name.
For example, the following code uses
DELEGATION to create a work-alike
for the standard
string command:
snit::type ::mynamespace::mystringtype {
delegate method * to stringhandler
constructor {} {
set stringhandler string
}
}
::mynamespace::mystringtype mystring
We create the type in a namespace, so that the type command is hidden; then we
create a single instance with the desired name--
mystring, in this
case.
This method has two drawbacks. First, it leaves the type command floating about.
More seriously, your shiny new ensemble command will have
info and
destroy subcommands that you probably have no use for. But read on.
HOW CAN I CREATE AN ENSEMBLE COMMAND USING A SNIT::TYPE?¶
Define a type whose
TYPE METHODS are the subcommands of your ensemble
command.
For example, the following code uses
DELEGATION to create a work-alike
for the standard
string command:
snit::type mystring {
delegate typemethod * to stringhandler
typeconstructor {
set stringhandler string
}
}
Now the type command itself is your ensemble command.
This method has only one drawback, and though it's major, it's also
surmountable. Your new ensemble command will have
create,
info
and
destroy subcommands you don't want. And worse yet, since the
create method can be implicit, users of your command will accidentally
be creating instances of your
mystring type if they should mispell one
of the subcommands. The command will succeed--the first time--but won't do
what's wanted. This is very bad.
The work around is to set some
PRAGMAS, as shown here:
snit::type mystring {
pragma -hastypeinfo no
pragma -hastypedestroy no
pragma -hasinstances no
delegate typemethod * to stringhandler
typeconstructor {
set stringhandler string
}
}
Here we've used the
pragma statement to tell Snit that we don't want the
info typemethod or the
destroy typemethod, and that our type has
no instances; this eliminates the
create typemethod and all related
code. As a result, our ensemble command will be well-behaved, with no
unexpected subcommands.
PRAGMAS¶
WHAT IS A PRAGMA?¶
A pragma is an option you can set in your type definitions that affects how the
type is defined and how it works once it is defined.
HOW DO I SET A PRAGMA?¶
Use the
pragma statement. Each pragma is an option with a value; each
time you use the
pragma statement you can set one or more of them.
HOW CAN I GET RID OF THE INFO" TYPE METHOD?"¶
Set the
-hastypeinfo pragma to
no:
snit::type dog {
pragma -hastypeinfo no
# ...
}
Snit will refrain from defining the
info type method.
HOW CAN I GET RID OF THE DESTROY" TYPE METHOD?"¶
Set the
-hastypedestroy pragma to
no:
snit::type dog {
pragma -hastypedestroy no
# ...
}
Snit will refrain from defining the
destroy type method.
HOW CAN I GET RID OF THE CREATE" TYPE METHOD?"¶
Set the
-hasinstances pragma to
no:
snit::type dog {
pragma -hasinstances no
# ...
}
Snit will refrain from defining the
create type method; if you call the
type command with an unknown method name, you'll get an error instead of a new
instance of the type.
This is useful if you wish to use a
snit::type to define an ensemble
command rather than a type with instances.
Pragmas
-hastypemethods and
-hasinstances cannot both be false (or
there'd be nothing left).
HOW CAN I GET RID OF TYPE METHODS ALTOGETHER?¶
Normal Tk widget type commands don't have subcommands; all they do is create
widgets--in Snit terms, the type command calls the
create type method
directly. To get the same behavior from Snit, set the
-hastypemethods
pragma to
no:
snit::type dog {
pragma -hastypemethods no
#...
}
# Creates ::spot
dog spot
# Tries to create an instance called ::create
dog create spot
Pragmas
-hastypemethods and
-hasinstances cannot both be false (or
there'd be nothing left).
WHY CAN'T I CREATE AN OBJECT THAT REPLACES AN OLD OBJECT WITH THE
SAME NAME?¶
Up until Snit 0.95, you could use any name for an instance of a
snit::type, even if the name was already in use by some other object or
command. You could do the following, for example:
snit::type dog { ... }
dog proc
You now have a new dog named "proc", which is probably not something
that you really wanted to do. As a result, Snit now throws an error if your
chosen instance name names an existing command. To restore the old behavior,
set the
-canreplace pragma to
yes:
snit::type dog {
pragma -canreplace yes
# ...
}
HOW CAN I MAKE MY SIMPLE TYPE RUN FASTER?¶
In Snit 1.x, you can set the
-simpledispatch pragma to
yes.
Snit 1.x method dispatch is both flexible and fast, but the flexibility comes
with a price. If your type doesn't require the flexibility, the
-simpledispatch pragma allows you to substitute a simpler dispatch
mechanism that runs quite a bit faster. The limitations are these:
- •
- Methods cannot be delegated.
- •
- uplevel and upvar do not work as expected:
the caller's scope is two levels up rather than one.
- •
- The option-handling methods ( cget,
configure, and configurelist) are very slightly slower.
In Snit 2.2, the
-simpledispatch macro is obsolete, and ignored; all Snit
2.2 method dispatch is faster than Snit 1.x's
-simpledispatch.
MACROS¶
WHAT IS A MACRO?¶
A Snit macro is nothing more than a Tcl proc that's defined in the Tcl
interpreter used to compile Snit type definitions.
WHAT ARE MACROS GOOD FOR?¶
You can use Snit macros to define new type definition syntax, and to support
conditional compilation.
HOW DO I DO CONDITIONAL COMPILATION?¶
Suppose you want your type to use a fast C extension if it's available;
otherwise, you'll fallback to a slower Tcl implementation. You want to define
one set of methods in the first case, and another set in the second case. But
how can your type definition know whether the fast C extension is available or
not?
It's easily done. Outside of any type definition, define a macro that returns 1
if the extension is available, and 0 otherwise:
if {$gotFastExtension} {
snit::macro fastcode {} {return 1}
} else {
snit::macro fastcode {} {return 0}
}
Then, use your macro in your type definition:
snit::type dog {
if {[fastcode]} {
# Fast methods
method bark {} {...}
method wagtail {} {...}
} else {
# Slow methods
method bark {} {...}
method wagtail {} {...}
}
}
HOW DO I DEFINE NEW TYPE DEFINITION SYNTAX?¶
Use a macro. For example, your
snit::widget's
-background option
should be propagated to a number of component widgets. You could implement
that like this:
snit::widget mywidget {
option -background -default white -configuremethod PropagateBackground
method PropagateBackground {option value} {
$comp1 configure $option $value
$comp2 configure $option $value
$comp3 configure $option $value
}
}
For one option, this is fine; if you've got a number of options, it becomes
tedious and error prone. So package it as a macro:
snit::macro propagate {option "to" components} {
option $option -configuremethod Propagate$option
set body "\n"
foreach comp $components {
append body "\$$comp configure $option \$value\n"
}
method Propagate$option {option value} $body
}
Then you can use it like this:
snit::widget mywidget {
option -background default -white
option -foreground default -black
propagate -background to {comp1 comp2 comp3}
propagate -foreground to {comp1 comp2 comp3}
}
ARE THERE ARE RESTRICTIONS ON MACRO NAMES?¶
Yes, there are. You can't redefine any standard Tcl commands or Snit type
definition statements. You can use any other command name, including the name
of a previously defined macro.
If you're using Snit macros in your application, go ahead and name them in the
global namespace, as shown above. But if you're using them to define types or
widgets for use by others, you should define your macros in the same namespace
as your types or widgets. That way, they won't conflict with other people's
macros.
If my fancy
snit::widget is called
::mylib::mywidget, for example,
then I should define my
propagate macro as
::mylib::propagate:
snit::macro mylib::propagate {option "to" components} { ... }
snit::widget ::mylib::mywidget {
option -background default -white
option -foreground default -black
mylib::propagate -background to {comp1 comp2 comp3}
mylib::propagate -foreground to {comp1 comp2 comp3}
}
BUGS, IDEAS, FEEDBACK¶
This document, and the package it describes, will undoubtedly contain bugs and
other problems. Please report such in the category
snit of the
Tcllib SF Trackers [
http://sourceforge.net/tracker/?group_id=12883].
Please also report any ideas for enhancements you may have for either package
and/or documentation.
KEYWORDS¶
BWidget, C++, Incr Tcl, adaptors, class, mega widget, object, object oriented,
widget, widget adaptors
CATEGORY¶
Programming tools
COPYRIGHT¶
Copyright (c) 2003-2006, by William H. Duquette