NAME¶
CGI::XMLApplication -- Object Oriented Interface for CGI Script Applications
SYNOPSIS¶
use CGI::XMLApplication;
$script = new CGI::XMLApplication;
$script->setStylesheetPath( "the/path/to/the/stylesheets" );
# either this for simple scripts
$script->run();
# or if you need more control ...
$script->run(%context_hash); # or a context object
DESCRIPTION¶
CGI::XMLApplication is a CGI application class, that intends to enable perl
artists to implement CGIs that make use of XML/XSLT functionality, without
taking too much care about specialized errorchecking or even care too much
about XML itself. It provides the power of the XML::LibXML/ XML::LibXSLT
module package for content deliverment.
As well CGI::XMLApplication is designed to support project management on code
level. The class allows one to split web applications into several simple
parts. Through this most of the code stays simple and easy to maintain.
Throughout the whole runtime of a script CGI::XMLApplication tries to keep the
application stable. As well a programmer has not to bother about some of
XML::LibXML/ XML::LibXSLT transformation pitfalls.
The class module extends the CGI class. While all functionality of the original
CGI package is still available, it should be not such a big problem, to port
existing scripts to CGI::XMLApplication, although most functions used here are
the access function for client data such as
param().
CGI::XMLApplication, intended to be an application class should make writing of
XML enabled CGI scripts more easy. Especially because of the use of object
orientated concepts, this class enables much more transparent
implemententations with complex functionality compared to what is possible
with standard CGI-scripts.
The main difference with common perl CGI implementation is the fact, that the
client-output is not done from perl functions, but generated by an internally
build XML DOM that gets processed with an XSLT stylesheet. This fact helps to
remove a lot of the HTML related functions from the core code, so a script may
be much easier to read, since only application relevant code is visible, while
layout related information is left out (commonly in an XSLT file).
This helps to write and test a complete application faster and less layout
related. The design can be appended and customized later without effecting the
application code anymore.
Since the class uses the OO paradigma, it does not force anybody to implement a
real life application with the complete overhead of more or less redundant
code. Since most CGI-scripts are waiting for
events, which is usually
the code abstraction of a click of a submit button or an image,
CGI::XMLApplication implements a simple event system, that allows one to keep
event related code as separated as possible.
Therefore final application class is not meant to have a constructor anymore.
All functionality should be encapsulated into implicit or explicit event
handlers. Because of a lack in Perl's OO implementation the call of a
superclass constructor before the current constructor call is not default
behavior in Perl. For that reason I decided to have special
events to
enable the application to initialize correctly, excluding the danger of
leaving important variables undefined. Also this forces the programmer to
implement scripts more problem orientated, rather than class or content
focused.
Another design aspect for CGI::XMLApplication is the strict differentiation
between CODE and PRESENTATION. IMHO this, in fact being one of the major
problems in traditional CGI programming. To implement this, the XML::LibXML
and XML::LibXSLT modules are used by default but may be replaced easily by any
other XML/XSLT capable modules. Each CGI Script should generate an XML-DOM,
that can be processed with a given stylesheet.
Pay attention: In this Document XML-DOM means the DOM of XML::LibXML
and not XML::DOM!
Programflow of a CGI::XMLApplication¶
The following Flowchart illustratrates how CGI::XMLApplication behaves during
runtime. Also chart shows where specialized application code gets control
during script runtime.
------- CGI Script ------->|<--------- CGI::XMLApplication --------
.---------------------. .--------------------.
| app-class creation |--- | event registration |
`---------------------' | registerEvents() |
`--------------------'
.------------------------. |
| context initialization |------------'
| ( optional ) |
`------------------------'
|
.-----------------------. .------------------------.
| run() function called |--| application initialize |
`-----------------------' | event_init() |
`------------------------'
|
.--------'`------------.
/ event parameter found? \_
\ testEvent() / \
`--------.,------------' |
| |
yes | no |
| |
.------------. .------------------.
| call event | | call |
| event_*() | | event_default() |
`------------' `------------------'
| |
.---------------------. |
| application cleanup |-----'
| event_exit() |
`---------------------'
|
.---------'`------------.
_/ avoid XML serialization \
/ \ skip_serialization() /
| `---------.,------------'
| |
yes | no |
| |
| .--------------------------.
| | XML generation, XSLT |
| | serialization and output |
| | serialization() |
| `--------------------------'
.---------------. | |
| END |-------+-------------'
`---------------'
What are Events and how to catch them¶
Most CGI Scripts handle the result of HTML-Forms or similar requests from
clients. Analouge to GUI Programming, CGI::XMLApplication calls this an
event. Spoken in CGI/HTML-Form words, a CGI-Script handles the various
situations a clients causes by pushing a submit button or follows a special
link. Because of this common events are thrown by arguments found in the CGI's
query string.
An event of CGI::XMLApplication has the same
name as the input field,
that should cause the event. The following example should illustrate this a
little better:
<!-- SOME HTML CODE -->
<input type="submit" name="dummy" value="whatever" />
<!-- SOME MORE HTML :) -->
If a user clicks the submitbutton and you have registered the event name
dummy for your script, CGI::XMLApplication will try to call the
function
event_dummy(). The script module to
handle the dummy event would look something like the following code:
# Application Module
package myApp;
use CGI::XMLApplication;
@ISA = qw(CGI::XMLApplication);
sub registerEvents { qw( dummy ); } # list of event names
# ...
sub event_dummy {
my ( $self, $context ) = @_;
# your event code goes here
return 0;
}
During the lifecircle of a CGI script, often the implementation starts with
ordinary submit buttons, which get often changed to so called input images, to
fit into the UI of the Website. One does not need to change the code to make
the scripts fit to these changes; CGI::XMLApplication already did it. The code
has not to be changed if the presentation of the form changes. Therefore there
is no need to declare separate events for input images. E.g. an event called
evname makes CGI::XMLApplication tests if evname
or evname.x exist in
the querystring.
So a perl artist can implement and test his code without caring if the design
crew have done their job, too ;-)
In many cases an web application is also confronted with events that can not be
represented in with querystring arguments. For these cases CGI::XMLApplication
offers the possibility to send
special events from the
event_init() function for example in case of
application errors. This is done with the
sendEvent() Function. This will set a new
parameter to the CGI's querystring after removing all other events.
One can
only send events that are already registred!.
Although a sendEvent function exists, CGI::XMLApplication doesn't implement an
event queqe. For GUI programmers this seems like a unnessecary restriction. In
terms of CGI it makes more sense to think of a script as a program, that is
only able to scan its event queqe only once during runtime and stopped before
the next event can be thrown. The only chance to stop the script from handling
a certain event is to send a new event or delete this (or even all) events
from inside the
event_init() function. This function is always called
at first from the run method. If another event uses the sendEvent function,
the call will have no effect.
- method registerEvents
- This method is called by the class constructor - namely
CGI::XMLApplication's new() function . Each
application should register the events it likes to handle with this
function. It should return an array of eventnames such as eg. 'remove' or
'store'. This list is used to find which event a user caused on the client
side.
- method run
- Being the main routine this should be the only method called by the script
apart from the constructor. All events are handled inside the method
run(). Since this method is extremly simple
and transparent to any kind of display type, there should be no need to
override this function. One can pass a context hash or context object, to
pass external or prefetched information to the application. This context
will be available and accessible in all events and most extra functions.
This function does all event and serialization related work. As well there
is some validation done as well, so catched events, that are not
implemented, will not cause any harm.
The Event System¶
A CGI::XMLApplication is split into two main parts: 1) The executable script
called by the webserver and 2) the application module which has to be loaded,
initialized and called by the script.
Commonly applications that make use of CGI::XMLApplication, will not bother
about the
run function too much. All functionality is kept inside
event- and (pseudo-)
callback functions. This forces one to
implement much more strict code than common perl would allow. What first looks
like a drawback, finally makes the code much easier to understand, maintain
and finally to extend.
CGI::XMLApplication knows two types of event handlers: implicit events, common
to all applications and explicit events, reflecting the application logic. The
class assumes that implicit events are implemented in any case. Those events
have reserved names and need not be specified through
registerEvents.
Since the class cannot know something about the application logic by itself,
names of events have to be explicitly passed to be handled by the application.
As well all event functions have to be implemented as member methods of the
application class right now. Because of perls OO interface a class has to be
written inside its own module.
An event may return a integer value. If the event succeeds (no fatal errors,
e.g. database errors) the explicit or common event function should return a
value greater or eqal than 0. If the value is less than 0, CGI::XMLApplication
assumes an application panic, and will not try to generate a DOM or render it
with a stylesheet.
There are 4 defined panic levels:
- -1
- Stylesheet missing
- -2
- Stylesheet not available
- -3
- Event not implemented
- -4
- Application panic
Apart from
Application Panic the panic levels are set internally. An
Application Panic should be set if the application catches an error, that does
not allow any XML/XSLT processing. This can be for example, that a required
perl module is not installed on the system.
To make it clear: If CGI::XMLApplication throws a panic, the application is
broken, not completely implemented or stylesheets are missing or broken.
Application panics are meant for debugging purposes and to avoid
Internal
Server Errors. They are
not meant as a replacement of a propper
error handling!
But how does CGI::XMLApplication know about the correct event handler?
One needs to register the names of the events the application handles. This is
done by implmenting a
registerEvents() function that simply returns an
array of event names. Through this function one prepares the
CGI::XMLApplication to catch the listed names as events from the query string
the client browser sends back to the script. CGI::XMLApplication tries to call
a event handler if a name of a registred event is found. The coresponding
function-name of an event has to have the following format:
event_<eventname>
E.g. event_init handles the init event described below.
Each event has a single Parameter, the context. This can be an unblessed hash
reference or an object, where the user can store whatever needed. This context
is useful to pass scriptwide data between callbacks and event functions
around. The callback is even available and useable if the script does not
initialize the application context as earlier shown in the program flow chart.
If such a function is not implemented in the application module,
CGI::XMLApplication sets the
Event not implemented panic state.
All events have to return an integer that tells about their execution state as
already described.
By default CGI::XMLApplication does not test for other events if it already
found one. The most significant event is the first name of an event found in
the query string - all other names are simply ignored. One may change this
behaviour by overriding the
testEvent() function.
But still it is a good idea to choose the event names carefully and do not mix
them with ordinary datafield names.
- function testEvent
- If it is nesseccary to check which event is relevant for the current
script one can use this function to find out in event_init(). If
this function returns undef, the default event is active, otherwise
it returns the eventname as defined by registerEvents.
In case one needs a special algorithm for event selection one can override
this function. If done so, one can make use of the application context
inside this function since it is passed to
testEvent() by the
run() function.
- method sendEvent SCALAR
- Sometimes it could be necessary to send an event by your own (the
script's) initiative. A possible example could be if you don't have client
input but path_info data, which determinates how the script should behave
or session information is missing, so the client should not even get the
default output.
This can only be done during the event_init() method call. Some
coders would prefer the constructor, which is not a very good idea in this
case: While the constructor is running, the application is not completely
initialized. This can be only ashured in the event_init function.
Therefore all application specific errorhandling and initializing should
be done there.
sendEvent only can be called from event_init, because any
CGI::XMLApplication will handle just one event, plus the init and
the exit event. If sendEvent is called from another event
than event_init() it will take not effect.
It is possible through sendEvent() to keep the script logic clean.
Example:
package myApp;
use CGI::XMLApplication;
@ISA = qw(CGI::XMLApplication);
sub registerEvents { qw( missing ... ) ; }
# event_init is an implicit event
sub event_init {
my ( $self, $context ) = @_;
if ( not ( defined $self->param( $paraname ) && length $self->param( $paramname ) ) ){
# the parameter is not correctly filled
$self->sendEvent( 'missing' );
}
else {
... some more initialization ...
}
return 0;
}
... more code ...
# event_missing is an explicit event.
sub event_missing {
my ( $self , $context ) = @_;
... your error handling code goes ...
return -4 if $panic; # just for illustration
return 0;
}
Implicit Events¶
CGI::XMLApplication knows three implicit events which are more or less
independent to client responses: They are 'init', 'exit', and 'default'. These
events already exist for any CGI::XMLApplication. They need not to be
implemented separately if they make no sense for the application.
- event_init
- The init event is set before the CGI::XMLApplication tries to evaluate any
of script parameters. Therefore the event_init method should be used to
initialize the application.
- event_exit
- The event_exit method is called after all other events have been
processed, but just before the rendering is done. This should be used, if
you need to do something independend from all events before the data is
send to the user.
- event_default
- This event is called as a fallback mechanism if CGI::XMLApplication did
not receive a stylesheet id by another event handler; for example if no
event matched.
the XML Serialization¶
The presentation is probably the main part of a CGI script. By using XML and
XSLT this can be done in a standartised manner. From the application view all
this can be isolated in a separate subsystem as well. In CGI::XMLApplication
this subsystem is implemented inside the
serialize() function.
For XML phobic perl programmers it should be cleared, that CGI::XMLApplication
makes real use of XML/XSLT functionalty only inside this function. For all
code explained above it is not required to make use of XML at all.
The XML serialization subsystem of CGI::XMLApplication tries to hide most of non
application specific code from the application programmer.
This method renders the data stored in the DOM with the stylesheet returned by
the event handler. You should override this function if you like to use a
different way of displaying your data.
If the serialization should be skipped, CGI::XMLApplication will not print any
headers. In such case the application is on its own to pass all the output.
The algorithm used by serialization is simple:
- •
- request the appplication DOM through
getDOM()
- •
- test for XML passthru
- •
- get the stylesheet the application preferes through
selectStylesheet()
- •
- parse the stylesheet
- •
- transform the DOM with the stylesheet
- •
- set Content-Type and headers
- •
- return the content to the client
If errors occour on a certain stage of serialization, the application is stopped
and the generated error messages are returned.
CGI::XMLApplication provides four pseudo-callbacks, that are used to get the
application specific information during serialization. In order of being
called by
CGI::XMLApplication::serialization() they are:
- •
- getDOM
- •
- setHttpHeader
- •
- getStylesheet
- •
- getXSLTParameter
In fact only getStylesheet has to be implemented. In most cases it will be a
good idea to provide the getDOM function as well. The other functions provider
a interface to make the CGI output more generic. For example one can set
cookies or pass XSL parameters to XML::LibXSLT's xsl processor.
These methods are used by the serialization function, to create the content
related datastructure. Like event functions these functions have to be
implemented as class member, and like event funcitons the functions will have
the context passed as the single parameter.
- getDOM()
- getDOM() should return the application data as XML-DOM.
CGI::XMLApplication is quite lax if this function does not return anything
- its simply assumed that an empty DOM should be rendered. In this case a
dummy root element is created to avoid error messages from
XML::LibXSLT.
- setHttpHeader()
- setHttpHeader should return a hash of headers (but not the
Content-Type). This can be used to set the nocache pragma, to set
or remove cookies. The keys of the hash must be the same as the named
parameters of CGI.pm's header method. One does not need to care about the
output of these headers, this is done by CGI::XMLApplication
automatically.
The content type of the returned data is usually not required to be set this
way, since the XSLT processor knows about the content type, too.
- getStylesheet()
- If the getStylesheet is implemented the CGI::XMLApplication will
assume the returned value either as a filename of a stylesheet or as a XML
DOM representation of the same. If Stylesheets are stored in a file
accessible from the , one should set the common path for the stylesheets
and let CGI::XMLApplication do the parsing job.
In cases the stylesheet is already present as a string (e.g. as a result of
a database query) one may pass this string directly to
CGI::XMLApplication.
selectStylesheet is an alias for getStylesheet left for
compatibility reasons.
If none of these stylesheet selectors succeeds the Stylesheet
missing panic code is thrown. If the parsing of the stylesheet XML
fails Stylesheet not available is thrown. The latter case will also
give some information where the stylesheet selection failed.
selectStylesheet() has to return a valid
path/filename for the stylesheet requested.
- getXSLTParameter()
- This function allows one to pass a set of parameters to XML::LibXSLT's xsl
processor. The function needs only to return a hash and does not need to
encode the parameters.
The function is the last callback called before the XSLT processing is
done.
Flow Control¶
Besides the
sendEvent() function does CGI::XMLApplication provide to
other functions that allow to controll the flow of the application.
These two functions are related to the XML serialization and have not affect to
the event handling.
- passthru()
- Originally for debugging purposes CGI::XMLApplication supports the
passthru argument in the CGI query string. It can be used to
directly pass the stringified XML-DOM to the client.
Since there are cases one needs to decide from within the application if an
untransformed XML Document has to be returned, this function was
introduced.
If is called without parameters passthru()
returns the current passthru state of the application. E.g. this is done
inside serialization(). Where TRUE \fIs0(1)
means the XML DOM should be passed directly to the client and FALSE
\fIs0(0) marks that the DOM must get XSL transformed first.
Optional the function takes a single parameter, which shows if the function
should be used in set rather than get mode. The parameter is interpreted
as just described.
If an application sets passthru by itself any external 'passthru' parameter
will be lost. This is useful if one likes to avoid, someone can fetch the
plain (untransformed) XML Data.
- skipSerialization()
- To avoid the call of serialization() one
should set skipSerialization.
event_default {
my $self = shift;
# avoid serialization call
$self->skipSerialization( 1 ); # use 0 to unset
# now you can directly print to the client, but don't forget the
# headers.
return 0;
}
Helperfunctions for internal use¶
- function checkPush LIST
- This function searches the query string for a parameter with the passed
name. The implementation is "imagesave" meaning there is no
change in the code needed, if you switch from input.type=submit to
input.type=image or vv. The algorithm tests whether a full name is found
in the querystring, if not it tries tests for the name expanded by a '.x'.
In context of events this function interprets each item part in the query
string list as an event. Because of that, the algorithm returns only the
first item matched.
If you use the event interface on this function, make sure, the HTML-forms
pass unique events to the script. This is necessary to avoid confusing
behaviour.
This function is used by testEvent() so if it is required to change
the way CGI::XMLApplication selects events, override that function.
- method panic SCALAR
- This a simple error message handler. By default this function will print
some information to the client where the application failed. While
development this is a useful feature on production system this may pass
vunerable information about the system to the outside. To change the
default behaviour, one may write his own panic method or simply set
$CGI::XMLApplication::Quiet to 1. The latter still
causes the error page but does not send any error message.
The current implementation send the 404 status to the client if any low
level errors occour ( e.g. panic levels > -4 aka Application Panic).
Commonly this really shows a "Not Found" on the application
Level. Application Panics will set the 500 error state. This makes this
implementation work perfect with a mod_perl installation.
In case mod_perl is used to handle the script one likes to set
CGI::XMLApplication::Quiet to 2 which will cause
CGI::XMLApplication just to return the error state while mod_perl does the
rest.
- method setPanicMsg $SCALAR
- This useful method, helps to pass more specific error messages to the
user. Currently this method is not very sophisticated: if the method is
called twice, only the last string will be displayed.
- function getPanicMsg
- This method returns the panic message set by setPanicMsg().
The following functions are some neat features missing in CGI.pm
- function checkFields LIST
- This is an easy way to test wether all required fields are filled out
correctly. Called in array context the function returns the list of
missing parameter. (Different to param() which returns all
parameter names). In scalar context the function returns a boolean
value.
- function getParamHash LIST
- This function is a bit better for general data processing as the standard
CGI::Vars function. While Vars sets a keys for each parameter found in the
query string, getFieldsAsHash returns only the requested fields (as long
they aren't NULL). This is useful in scripts where the script itself
handles different kind of data within the same event.
Since the function relies on Vars the returned data has the same structure
Vars returns.
some extra functions for stylesheet handling¶
The
getStylesheet() function should return either a filename or a
stringnyfied XSL-DOM. For the firstcase it can be a restriction to return the
fully qualified path. The following functions allow to set the stylesheetpath
systemwide.
- method setStylesheetDir DIRNAME
- alias for setStylesheetPath
- method setStylesheetPath DIRNAME
- This method is for telling the application where the stylesheets can be
found. If you keep your stylesheets in the same directory as your script
-- IMHO a bad idea -- you might leave this untouched.
- function getStylesheetPath
- This function is only relevant if you write your own
serialization() method. It returns the
current path to the application stylesheets.
SEE ALSO¶
CGI, perlobj, perlmod, XML::LibXML, XML::LibXSLT
AUTHOR¶
Christian Glahn, christian.glahn@uibk.ac.at
VERSION¶
1.1.1