NAME¶
Catalyst::Manual::ExtendingCatalyst - Extending The Framework
DESCRIPTION¶
This document will provide you with access points, techniques and best practices
to extend the Catalyst framework, or to find more elegant ways to abstract and
use your own code.
The design of Catalyst is such that the framework itself should not get in your
way. There are many entry points to alter or extend Catalyst's behaviour, and
this can be confusing. This document is written to help you understand the
possibilities, current practices and their consequences.
Please read the "BEST PRACTICES" section before deciding on a design,
especially if you plan to release your code to CPAN. The Catalyst developer
and user communities, which
you are part of, will benefit most if we
all work together and coordinate.
If you are unsure on an implementation or have an idea you would like to have
RFC'ed, it surely is a good idea to send your questions and suggestions to the
Catalyst mailing list (See "SUPPORT" in Catalyst) and/or come to the
"#catalyst" channel on the "irc.perl.org" network. You
might also want to refer to those places for research to see if a module doing
what you're trying to implement already exists. This might give you a solution
to your problem or a basis for starting.
BEST PRACTICES¶
During Catalyst's early days, it was common to write plugins to provide
functionality application wide. Since then, Catalyst has become a lot more
flexible and powerful. It soon became a best practice to use some other form
of abstraction or interface, to keep the scope of its influence as close as
possible to where it belongs.
For those in a hurry, here's a quick checklist of some fundamental points. If
you are going to read the whole thing anyway, you can jump forward to
"Namespaces".
Quick Checklist¶
- Use the "CatalystX::*" namespace if you can!
- If your extension isn't a Model, View, Controller, Plugin,
Engine, or Log, it's best to leave it out of the "Catalyst::"
namespace. Use <CatalystX::> instead.
- Don't make it a plugin unless you have to!
- A plugin should be careful since it's overriding Catalyst
internals. If your plugin doesn't really need to muck with the internals,
make it a base Controller or Model.
Also, if you think you really need a plugin, please instead consider using a
Moose::Role.
- There's a community. Use it!
- There are many experienced developers in the Catalyst
community, there's always the IRC channel and the mailing list to discuss
things.
- Add tests and documentation!
- This gives a stable basis for contribution, and even more
importantly, builds trust. The easiest way is a test application. See
Catalyst::Manual::Tutorial::Testing for more information.
Namespaces¶
While some core extensions (engines, plugins, etc.) have to be placed in the
"Catalyst::*" namespace, the Catalyst core would like to ask
developers to use the "CatalystX::*" namespace if possible.
Please
do not invent components which are outside the well known
"Model", "View", "Controller" or
"Plugin" namespaces!
When you try to put a base class for a "Model", "View" or
"Controller" directly under your "MyApp" directory as, for
example, "MyApp::Controller::Foo", you will have the problem that
Catalyst will try to load that base class as a component of your application.
The solution is simple: Use another namespace. Common ones are
"MyApp::Base::Controller::*" or "MyApp::ControllerBase::*"
as examples.
Can it be a simple module?¶
Sometimes you want to use functionality in your application that doesn't require
the framework at all. Remember that Catalyst is just Perl and you always can
just "use" a module. If you have application specific code that
doesn't need the framework, there is no problem in putting it in your
"MyApp::*" namespace. Just don't put it in "Model",
"Controller" or "View", because that would make Catalyst
try to load them as components.
Writing a generic component that only works with Catalyst is wasteful of your
time. Try writing a plain perl module, and then a small bit of glue that
integrates it with Catalyst. See Catalyst::Model::DBIC::Schema for a module
that takes the approach. The advantage here is that your "Catalyst"
DBIC schema works perfectly outside of Catalyst, making testing (and
command-line scripts) a breeze. The actual Catalyst Model is just a few lines
of glue that makes working with the schema convenient.
If you want the thinnest interface possible, take a look at
Catalyst::Model::Adaptor.
Using Moose roles to apply method modifiers¶
Rather than having a complex set of base classes which you have to mixin via
multiple inheritence, if your functionality is well structured, then it's
possible to use the composability of Moose roles, and method modifiers to hook
onto to provide functionality.
These can be applied to your models/views/controllers, and your application
class, and shipped to CPAN. Please see Catalyst::Manual::CatalystAndMoose for
specific information about using Roles in combination with Catalyst, and
Moose::Manual::Roles for more information about roles in general.
Inheritance and overriding methods¶
When overriding a method, keep in mind that some day additional arguments may be
provided to the method, if the last parameter is not a flat list. It is thus
better to override a method by shifting the invocant off of @_ and assign the
rest of the used arguments, so you can pass your complete arguments to the
original method via @_:
use MRO::Compat; ...
sub foo {
my $self = shift;
my ($bar, $baz) = @_; # ... return
$self->next::method(@_);
}
If you would do the common
my ($self, $foo, $bar) = @_;
you'd have to use a much uglier construct to ensure that all arguments will be
passed along and the method is future proof:
$self->next::method(@_[ 1 .. $#_ ]);
Tests and documentation¶
When you release your module to the CPAN, proper documentation and at least a
basic test suite (which means more than pod or even just "use_ok",
sorry) gives people a good base to contribute to the module. It also shows
that you care for your users. If you would like your module to become a
recommended addition, these things will prove invaluable.
If you're just getting started, try using CatalystX::Starter to generate some
example tests for your module.
Maintenance¶
In planning to release a module to the community (Catalyst or CPAN and Perl),
you should consider if you have the resources to keep it up to date, including
fixing bugs and accepting contributions.
If you're not sure about this, you can always ask in the proper Catalyst or Perl
channels if someone else might be interested in the project, and would jump in
as co-maintainer.
A public repository can further ease interaction with the community. Even read
only access enables people to provide you with patches to your current
development version. subversion, SVN and SVK, are broadly preferred in the
Catalyst community.
If you're developing a Catalyst extension, please consider asking the core team
for space in Catalyst's own subversion repository. You can get in touch about
this via IRC or the Catalyst developers mailing list.
The context object¶
Sometimes you want to get a hold of the context object in a component that was
created on startup time, where no context existed yet. Often this is about the
model reading something out of the stash or other context information (current
language, for example).
If you use the context object in your component you have tied it to an existing
request. This means that you might get into problems when you try to use the
component (e.g. the model - the most common case) outside of Catalyst, for
example in cronjobs.
A stable solution to this problem is to design the Catalyst model separately
from the underlying model logic. Let's take Catalyst::Model::DBIC::Schema as
an example. You can create a schema outside of Catalyst that knows nothing
about the web. This kind of design ensures encapsulation and makes development
and maintenance a whole lot easier. The you use the aforementioned model to
tie your schema to your application. This gives you a "MyApp::DBIC"
(the name is of course just an example) model as well as
"MyApp::DBIC::TableName" models to access your result sources
directly.
By creating such a thin layer between the actual model and the Catalyst
application, the schema itself is not at all tied to any application and the
layer in-between can access the model's API using information from the context
object.
A Catalyst component accesses the context object at request time with
"ACCEPT_CONTEXT($c, @args)" in Catalyst::Component.
CONFIGURATION¶
The application has to interact with the extension with some configuration.
There is of course again more than one way to do it.
Attributes¶
You can specify any valid Perl attribute on Catalyst actions you like. (See
"Syntax of Attribute Lists" in attributes for a description of what
is valid.) These will be available on the "Catalyst::Action"
instance via its "attributes" accessor. To give an example, this
action:
sub foo : Local Bar('Baz') {
my ($self, $c) = @_;
my $attributes = $self->action_for('foo')->attributes;
$c->res->body($attributes->{Bar}[0] );
}
will set the response body to "Baz". The values always come in an
array reference. As you can see, you can use attributes to configure your
actions. You can specify or alter these attributes via "Component
Configuration", or even react on them as soon as Catalyst encounters them
by providing your own component base class.
Component Configuration¶
At creation time, the class configuration of your component (the one available
via "$self->config") will be merged with possible configuration
settings from the applications configuration (either directly or via config
file). This is done by Catalyst, and the correctly merged configuration is
passed to your component's constructor (i.e. the new method).
Ergo, if you define an accessor for each configuration value that your component
takes, then the value will be automatically stored in the controller object's
hash reference, and available from the accessor.
The "config" accessor always only contains the original class
configuration and you
MUST NEVER call $self->config to get your
component configuration, as the data there is likely to be a subset of the
correct config.
For example:
package MyApp
use Moose;
extends 'Catalyst';
...
__PACKAGE__->config(
'Controller::Foo' => { some_value => 'bar' },
);
...
package MyApp::Controller::Foo;
use Moose;
use namespace::autoclean;
BEGIN { extends 'Catalyst::Controller' };
has some_value ( is => 'ro', required => 1 );
sub some_method {
my $self = shift;
return "the value of 'some_value' is " . $self->some_value;
}
...
my $controller = $c->controller('Foo');
warn $controller->some_value;
warn $controller->some_method;
IMPLEMENTATION¶
This part contains the technical details of various implementation methods.
Please read the "BEST PRACTICES" before you start your
implementation, if you haven't already.
Action classes¶
Usually, your action objects are of the class Catalyst::Action. You can override
this with the "ActionClass" attribute to influence execution and/or
dispatching of the action. A widely used example of this is
Catalyst::Action::RenderView, which is used in every newly created Catalyst
application in your root controller:
sub end : ActionClass('RenderView') { }
Usually, you want to override the "execute" and/or the
"match" method. The execute method of the action will naturally call
the methods code. You can surround this by overriding the method in a
subclass:
package Catalyst::Action::MyFoo;
use Moose;
use namespace::autoclean;
use MRO::Compat;
extends 'Catalyst::Action';
sub execute {
my $self = shift;
my ($controller, $c, @args) = @_;
# put your 'before' code here
my $r = $self->next::method(@_);
# put your 'after' code here
return $r;
}
1;
We are using MRO::Compat to ensure that you have the next::method call, from
Class::C3 (in older perls), or natively (if you are using perl 5.10) to
re-dispatch to the original "execute" method in the Catalyst::Action
class.
The Catalyst dispatcher handles an incoming request and, depending upon the
dispatch type, will call the appropriate target or chain. From time to time it
asks the actions themselves, or through the controller, if they would match
the current request. That's what the "match" method does. So by
overriding this, you can change on what the action will match and add new
matching criteria.
For example, the action class below will make the action only match on Mondays:
package Catalyst::Action::OnlyMondays;
use Moose;
use namespace::autoclean;
use MRO::Compat;
extends 'Catalyst::Action';
sub match {
my $self = shift;
return 0 if ( localtime(time) )[6] == 1;
return $self->next::method(@_);
}
1;
And this is how we'd use it:
sub foo: Local ActionClass('OnlyMondays') {
my ($self, $c) = @_;
$c->res->body('I feel motivated!');
}
If you are using action classes often or have some specific base classes that
you want to specify more conveniently, you can implement a component base
class providing an attribute handler.
It is not possible to use multiple action classes at once, however
Catalyst::Controller::ActionRole allows you to apply Moose Roles to actions.
For further information on action classes and roles, please refer to
Catalyst::Action and Catalyst::Manual::Actions.
Component base classes¶
Many Catalyst::Plugin that were written in Catalyst's early days should really
have been just controller base classes. With such a class, you could provide
functionality scoped to a single controller, not polluting the global
namespace in the context object.
You can provide regular Perl methods in a base class as well as actions which
will be inherited to the subclass. Please refer to "Controllers" for
an example of this.
You can introduce your own attributes by specifying a handler method in the
controller base. For example, to use a "FullClass" attribute to
specify a fully qualified action class name, you could use the following
implementation. Note, however, that this functionality is already provided via
the "+" prefix for action classes. A simple
sub foo : Local ActionClass('+MyApp::Action::Bar') { ... }
will use "MyApp::Action::Bar" as action class.
package MyApp::Base::Controller::FullClass;
use Moose;
use namespace::autoclean;
BEGIN { extends 'Catalyst::Controller'; }
sub _parse_FullClass_attr {
my ($self, $app_class, $action_name, $value, $attrs) = @_;
return( ActionClass => $value );
}
1;
Note that the full line of arguments is only provided for completeness sake. We
could use this attribute in a subclass like any other Catalyst attribute:
package MyApp::Controller::Foo;
use Moose;
use namespace::autoclean;
BEGIN { extends 'MyApp::Base::Controller::FullClass'; }
sub foo : Local FullClass('MyApp::Action::Bar') { ... }
1;
Controllers¶
Many things can happen in controllers, and it often improves maintainability to
abstract some of the code out into reusable base classes.
You can provide usual Perl methods that will be available via your controller
object, or you can even define Catalyst actions which will be inherited by the
subclasses. Consider this controller base class:
package MyApp::Base::Controller::ModelBase;
use Moose;
use namespace::autoclean;
BEGIN { extends 'Catalyst::Controller'; }
sub list : Chained('base') PathPart('') Args(0) {
my ($self, $c) = @_;
my $model = $c->model( $self->{model_name} );
my $condition = $self->{model_search_condition} || {};
my $attrs = $self->{model_search_attrs} || {};
$c->stash(rs => $model->search($condition, $attrs);
}
sub load : Chained('base') PathPart('') CaptureArgs(1) {
my ($self, $c, $id) = @_;
my $model = $c->model( $self->{model_name} );
$c->stash(row => $model->find($id));
}
1;
This example implements two simple actions. The "list" action chains
to a (currently non-existent) "base" action and puts a result-set
into the stash taking a configured "model_name" as well as a search
condition and attributes. This action is a chained endpoint. The other action,
called " load " is a chain midpoint that takes one argument. It
takes the value as an ID and loads the row from the configured model. Please
not that the above code is simplified for clarity. It misses error handling,
input validation, and probably other things.
The class above is not very useful on its own, but we can combine it with some
custom actions by sub-classing it:
package MyApp::Controller::Foo;
use Moose;
use namespace::autoclean;
BEGIN { extends 'MyApp::Base::Controller::ModelBase'; }
__PACKAGE__->config( model_name => 'DB::Foo',
model_search_condition=> { is_active => 1 },
model_search_attrs => { order_by => 'name' },
);
sub base : Chained PathPart('foo') CaptureArgs(0) { }
sub view : Chained('load') Args(0) {
my ($self, $c) = @_;
my $row = $c->stash->{row};
$c->res->body(join ': ', $row->name,
$row->description); }
1;
This class uses the formerly created controller as a base class. First, we see
the configurations that were used in the parent class. Next comes the
"base" action, where everything chains off of.
Note that inherited actions act like they were declared in your controller
itself. You can therefor call them just by their name in "forward"s,
"detaches" and "Chained(..)" specifications. This is an
important part of what makes this technique so useful.
The new "view" action ties itself to the "load" action
specified in the base class and outputs the loaded row's "name" and
"description" columns. The controller
"MyApp::Controller::Foo" now has these publicly available paths:
- /foo
- Will call the controller's "base", then the base
classes "list" action.
- /foo/$id/view
- First, the controller's "base" will be called,
then it will "load" the row with the corresponding $id. After
that, "view" will display some fields out of the object.
Models and Views¶
If the functionality you'd like to add is really a data-set that you want to
manipulate, for example internal document types, images, files, it might be
better suited as a model.
The same applies for views. If your code handles representation or deals with
the applications interface and should be universally available, it could be a
perfect candidate for a view.
Please implement a "process" method in your views. This method will be
called by Catalyst if it is asked to forward to a component without a
specified action. Note that "process" is
not a Catalyst
action but a simple Perl method.
You are also encouraged to implement a "render" method corresponding
with the one in Catalyst::View::TT. This has proven invaluable, because people
can use your view for much more fine-grained content generation.
Here is some example code for a fictional view:
package Catalyst::View::MyView;
use Moose;
use namespace::autoclean;
extends 'Catalyst::View';
sub process {
my ($self, $c) = @_;
my $template = $c->stash->{template};
my $content = $self->render($c, $template, $c->stash);
$c->res->body( $content );
}
sub render {
my ($self, $c, $template, $args) = @_;
# prepare content here
return $content;
}
1;
Plugins¶
The first thing to say about plugins is that if you're not sure if your module
should be a plugin, it probably shouldn't. It once was common to add features
to Catalyst by writing plugins that provide accessors to said functionality.
As Catalyst grew more popular, it became obvious that this qualifies as bad
practice.
By designing your module as a Catalyst plugin, every method you implement,
import or inherit will be available via your applications context object. A
plugin pollutes the global namespace, and you should be only doing that when
you really need to.
Often, developers design extensions as plugins because they need to get hold of
the context object. Either to get at the stash or request/response objects are
the widely spread reasons. It is, however, perfectly possible to implement a
regular Catalyst component (read: model, view or controller) that receives the
current context object via "ACCEPT_CONTEXT($c, @args)" in
Catalyst::Component.
When is a plugin suited to your task? Your code needs to be a plugin to act upon
or alter specific parts of Catalyst's request lifecycle. If your functionality
needs to change some "prepare_*" or "finalize_*" stages,
you won't get around a plugin.
Note, if you just want to hook into such a stage, and run code before, or after
it, then it is recommended that you use Mooses method modifiers to do this.
Another valid target for a plugin architecture are things that
really
have to be globally available, like sessions or authentication.
Please do not release Catalyst extensions as plugins only to provide some
functionality application wide. Design it as a controller base class or
another better suited technique with a smaller scope, so that your code only
influences those parts of the application where it is needed, and namespace
clashes and conflicts are ruled out.
The implementation is pretty easy. Your plugin will be inserted in the
application's inheritance list, above Catalyst itself. You can by this alter
Catalyst's request lifecycle behaviour. Every method you declare, every import
in your package will be available as method on the application and the context
object. As an example, let's say you want Catalyst to warn you every time
uri_for was called without an action object as the first parameter, for
example to test that all your chained uris are generated from actions (a
recommended best practice). You could do this with this simple implementation
(excuse the lame class name, it's just an example):
package Catalyst::Plugin::UriforUndefWarning;
use strict;
use Scalar::Util qw/blessed/;
use MRO::Compat;
sub uri_for {
my $c = shift;
my $uri = $c->next::method(@_);
$c->log->warn( 'uri_for with non action: ', join(', ', @_), )
if (!blessed($_[0]) || !$_[0]->isa('Catalyst::Action'));
return $uri;
}
1;
This would override Catalyst's "uri_for" method and emit a
"warn" log entry containing the arguments to uri_for.
Please note this is not a practical example, as string URLs are fine for static
content etc.
A simple example like this is actually better as a Moose role, for example:
package CatalystX::UriforUndefWarning;
use Moose::Role;
use namespace::autoclean;
after 'uri_for' => sub {
my ($c, $arg) = @_;
$c->log->warn( 'uri_for with non action: ', join(', ', @_), )
if (!blessed($_[0]) || !$_[0]->isa('Catalyst::Action'));
return $uri;
};
Note that Catalyst will load any Moose Roles in the plugin list, and apply them
to your application class.
Factory components with COMPONENT()¶
Every component inheriting from Catalyst::Component contains a
"COMPONENT" method. It is used on application startup by
"setup_components" to instantiate the component object for the
Catalyst application. By default, this will merge the components own
"config"uration with the application wide overrides and call the
class' "new" method to return the component object.
You can override this method and do and return whatever you want. However, you
should use Class::C3 (via MRO::Compat) to forward to the original
"COMPONENT" method to merge the configuration of your component.
Here is a stub "COMPONENT" method:
package CatalystX::Component::Foo;
use Moose;
use namespace::autoclean;
extends 'Catalyst::Component';
sub COMPONENT {
my $class = shift;
# Note: $app is like $c, but since the application isn't fully
# initialized, we don't want to call it $c yet. $config
# is a hashref of config options possibly set on this component.
my ($app, $config) = @_;
# Do things here before instantiation
$new = $class->next::method(@_);
# Do things to object after instantiation
return $new;
}
The arguments are the class name of the component, the class name of the
application instantiating the component, and a hash reference with the
controller's configuration.
You are free to re-bless the object, instantiate a whole other component or
really do anything compatible with Catalyst's expectations on a component.
For more information, please see "COMPONENT($c,$arguments)" in
Catalyst::Component.
Applying roles to parts of the framework¶
CatalystX::RoleApplicator will allow you to apply Roles to the following
classes:
- Request
- Response
- Engine
- Dispatcher
- Stats
These roles can add new methods to these classes, or wrap preexisting methods.
The namespace for roles like this is "Catalyst::TraitFor::XXX::YYYY".
For an example of a CPAN component implemented in this manor, see
Catalyst::TraitFor::Request::BrowserDetect.
SEE ALSO¶
Catalyst, Catalyst::Manual::Actions, Catalyst::Component
AUTHORS¶
Catalyst Contributors, see Catalyst.pm
COPYRIGHT¶
This library is free software. You can redistribute it and/or modify it under
the same terms as Perl itself.