3 Catalyst::Manual::ExtendingCatalyst - Extending The Framework
7 This document will provide you with access points, techniques and best
8 practices to extend the L<Catalyst> framework, or to find more elegant
9 ways to abstract and use your own code.
11 The design of Catalyst is such that the framework itself should not
12 get in your way. There are many entry points to alter or extend
13 Catalyst's behaviour, and this can be confusing. This document is
14 written to help you understand the possibilities, current practices
15 and their consequences.
17 Please read the L<BEST PRACTICES> section before deciding on a design,
18 especially if you plan to release your code to CPAN. The Catalyst
19 developer and user communities, which B<you are part of>, will benefit
20 most if we all work together and coordinate.
22 If you are unsure on an implementation or have an idea you would like
23 to have RFC'ed, it surely is a good idea to send your questions and
24 suggestions to the Catalyst mailing list (See L<Catalyst/SUPPORT>)
25 and/or come to the C<#catalyst> channel on the C<irc.perl.org>
26 network. You might also want to refer to those places for research to
27 see if a module doing what you're trying to implement already
28 exists. This might give you a solution to your problem or a basis for
33 During Catalyst's early days, it was common to write plugins to
34 provide functionality application wide. Since then, Catalyst has
35 become a lot more flexible and powerful. It soon became a best
36 practice to use some other form of abstraction or interface, to keep
37 the scope of its influence as close as possible to where it belongs.
39 For those in a hurry, here's a quick checklist of some fundamental
40 points. If you are going to read the whole thing anyway, you can jump
41 forward to L</Namespaces>.
43 =head2 Quick Checklist
47 =item Use the C<CatalystX::*> namespace if you can!
49 If your extension isn't a Model, View, Controller, Plugin, Engine,
50 or Log, it's best to leave it out of the C<Catalyst::> namespace.
51 Use <CatalystX::> instead.
53 =item Don't make it a plugin unless you have to!
55 A plugin should be careful since it's overriding Catalyst internals.
56 If your plugin doesn't really need to muck with the internals, make it a
57 base Controller or Model.
59 Also, if you think you really need a plugin, please instead consider
60 using a L<Moose::Role>.
62 =item There's a community. Use it!
64 There are many experienced developers in the Catalyst community,
65 there's always the IRC channel and the mailing list to discuss things.
67 =item Add tests and documentation!
69 This gives a stable basis for contribution, and even more importantly,
70 builds trust. The easiest way is a test application. See
71 L<Catalyst::Manual::Tutorial::Testing> for more information.
77 While some core extensions (engines, plugins, etc.) have to be placed
78 in the C<Catalyst::*> namespace, the Catalyst core would like to ask
79 developers to use the C<CatalystX::*> namespace if possible.
81 Please B<do not> invent components which are outside the well
82 known C<Model>, C<View>, C<Controller> or C<Plugin> namespaces!
84 When you try to put a base class for a C<Model>, C<View> or
85 C<Controller> directly under your C<MyApp> directory as, for example,
86 C<MyApp::Controller::Foo>, you will have the problem that Catalyst
87 will try to load that base class as a component of your
88 application. The solution is simple: Use another namespace. Common
89 ones are C<MyApp::Base::Controller::*> or C<MyApp::ControllerBase::*>
92 =head2 Can it be a simple module?
94 Sometimes you want to use functionality in your application that
95 doesn't require the framework at all. Remember that Catalyst is just
96 Perl and you always can just C<use> a module. If you have application
97 specific code that doesn't need the framework, there is no problem in
98 putting it in your C<MyApp::*> namespace. Just don't put it in
99 C<Model>, C<Controller> or C<View>, because that would make Catalyst
100 try to load them as components.
102 Writing a generic component that only works with Catalyst is wasteful
103 of your time. Try writing a plain perl module, and then a small bit
104 of glue that integrates it with Catalyst. See
105 L<Catalyst::Model::DBIC::Schema|Catalyst::Model::DBIC::Schema> for a
106 module that takes the approach. The advantage here is that your
107 "Catalyst" DBIC schema works perfectly outside of Catalyst, making
108 testing (and command-line scripts) a breeze. The actual Catalyst
109 Model is just a few lines of glue that makes working with the schema
112 If you want the thinnest interface possible, take a look at
113 L<Catalyst::Model::Adaptor|Catalyst::Model::Adaptor>.
115 =head2 Using Moose roles to apply method modifiers
117 Rather than having a complex set of base classes which you have to mixin
118 via multiple inheritence, if your functionality is well structured, then
119 it's possible to use the composability of L<Moose> roles, and method modifiers
120 to hook onto to provide functionality.
122 These can be applied to your models/views/controllers, and your application
123 class, and shipped to CPAN.
124 Please see L<Catalyst::Manual::CatalystAndMoose> for specific information
125 about using Roles in combination with Catalyst, and L<Moose::Manual::Roles>
126 for more information about roles in general.
128 =head2 Inheritance and overriding methods
130 When overriding a method, keep in mind that some day additionall
131 arguments may be provided to the method, if the last parameter is not
132 a flat list. It is thus better to override a method by shifting the
133 invocant off of C<@_> and assign the rest of the used arguments, so
134 you can pass your complete arguments to the original method via C<@_>:
140 my ($bar, $baz) = @_; # ... return
141 $self->next::method(@_);
144 If you would do the common
146 my ($self, $foo, $bar) = @_;
148 you'd have to use a much uglier construct to ensure that all arguments
149 will be passed along and the method is future proof:
151 $self->next::method(@_[ 1 .. $#_ ]);
153 =head2 Tests and documentation
155 When you release your module to the CPAN, proper documentation and at
156 least a basic test suite (which means more than pod or even just
157 C<use_ok>, sorry) gives people a good base to contribute to the
158 module. It also shows that you care for your users. If you would like
159 your module to become a recommended addition, these things will prove
162 If you're just getting started, try using
163 L<CatalystX::Starter|CatalystX::Starter> to generate some example
164 tests for your module.
168 In planning to release a module to the community (Catalyst or CPAN and
169 Perl), you should consider if you have the resources to keep it up to
170 date, including fixing bugs and accepting contributions.
172 If you're not sure about this, you can always ask in the proper
173 Catalyst or Perl channels if someone else might be interested in the
174 project, and would jump in as co-maintainer.
176 A public repository can further ease interaction with the
177 community. Even read only access enables people to provide you with
178 patches to your current development version. subversion, SVN and SVK,
179 are broadly preferred in the Catalyst community.
181 If you're developing a Catalyst extension, please consider asking the
182 core team for space in Catalyst's own subversion repository. You can
183 get in touch about this via IRC or the Catalyst developers mailing
186 =head2 The context object
188 Sometimes you want to get a hold of the context object in a component
189 that was created on startup time, where no context existed yet. Often
190 this is about the model reading something out of the stash or other
191 context information (current language, for example).
193 If you use the context object in your component you have tied it to an
194 existing request. This means that you might get into problems when
195 you try to use the component (e.g. the model - the most common case)
196 outside of Catalyst, for example in cronjobs.
198 A stable solution to this problem is to design the Catalyst model
199 separately from the underlying model logic. Let's take
200 L<Catalyst::Model::DBIC::Schema> as an example. You can create a
201 schema outside of Catalyst that knows nothing about the web. This kind
202 of design ensures encapsulation and makes development and maintenance
203 a whole lot easier. The you use the aforementioned model to tie your
204 schema to your application. This gives you a C<MyApp::DBIC> (the name
205 is of course just an example) model as well as
206 C<MyApp::DBIC::TableName> models to access your result sources
209 By creating such a thin layer between the actual model and the
210 Catalyst application, the schema itself is not at all tied to any
211 application and the layer in-between can access the model's API using
212 information from the context object.
214 A Catalyst component accesses the context object at request time with
215 L<Catalyst::Component/"ACCEPT_CONTEXT($c, @args)">.
219 The application has to interact with the extension with some
220 configuration. There is of course again more than one way to do it.
224 You can specify any valid Perl attribute on Catalyst actions you like.
225 (See L<attributes/"Syntax of Attribute Lists"> for a description of
226 what is valid.) These will be available on the C<Catalyst::Action>
227 instance via its C<attributes> accessor. To give an example, this
230 sub foo : Local Bar('Baz') {
232 my $attributes = $self->action_for('foo')->attributes;
233 $c->res->body($attributes->{Bar}[0] );
236 will set the response body to C<Baz>. The values always come in an
237 array reference. As you can see, you can use attributes to configure
238 your actions. You can specify or alter these attributes via
239 L</"Component Configuration">, or even react on them as soon as
240 Catalyst encounters them by providing your own L<component base
241 class|/"Component Base Classes">.
243 =head2 Creating custom accessors
245 L<Catalyst::Component> uses L<Class::Accessor::Fast> for accessor
246 creation. Please refer to the modules documentation for usage
249 =head2 Component configuration
251 At creation time, the class configuration of your component (the one
252 available via C<$self-E<gt>config>) will be merged with possible
253 configuration settings from the applications configuration (either
254 directly or via config file). This is then stored in the controller
255 object's hash reference. So, if you read possible configurations like:
257 my $model_name = $controller->{model_name};
259 you will get the right value. The C<config> accessor always only
260 contains the original class configuration and must not be used for
261 component configuration.
263 You are advised to create accessors on your component class for your
264 configuration values. This is good practice and makes it easier to
265 capture configuration key typos, or missing keys.
267 You can do this with L<Moose>:
269 package MyApp::Controller::Foo;
271 use namespace::autoclean;
272 BEGIN { extends 'Catalyst::Controller' };
274 has model_name ( is => 'ro', required => 1 );
277 my $model_name = $controller->model_name;
279 =head1 IMPLEMENTATION
281 This part contains the technical details of various implementation
282 methods. Please read the L</"BEST PRACTICES"> before you start your
283 implementation, if you haven't already.
285 =head2 Action classes
287 Usually, your action objects are of the class L<Catalyst::Action>.
288 You can override this with the C<ActionClass> attribute to influence
289 execution and/or dispatching of the action. A widely used example of
290 this is L<Catalyst::Action::RenderView>, which is used in every newly
291 created Catalyst application in your root controller:
293 sub end : ActionClass('RenderView') { }
295 Usually, you want to override the C<execute> and/or the C<match>
296 method. The execute method of the action will naturally call the
297 methods code. You can surround this by overriding the method in a
300 package Catalyst::Action::MyFoo;
302 use namespace::autoclean;
304 extends 'Catalyst::Action';
308 my ($controller, $c, @args) = @_;
309 # put your 'before' code here
310 my $r = $self->next::method(@_);
311 # put your 'after' code here
316 We are using L<MRO::Compat> to ensure that you have the next::method
317 call, from L<Class::C3> (in older perls), or natively (if you are using
318 perl 5.10) to re-dispatch to the original C<execute> method in the
319 L<Catalyst::Action> class.
321 The Catalyst dispatcher handles an incoming request and, depending
322 upon the dispatch type, will call the appropriate target or chain.
323 From time to time it asks the actions themselves, or through the
324 controller, if they would match the current request. That's what the
325 C<match> method does. So by overriding this, you can change on what
326 the action will match and add new matching criteria.
328 For example, the action class below will make the action only match on
331 package Catalyst::Action::OnlyMondays;
333 use namespace::autoclean;
335 extends 'Catalyst::Action';
339 return 0 if ( localtime(time) )[6] == 1;
340 return $self->next::method(@_);
344 And this is how we'd use it:
346 sub foo: Local ActionClass('OnlyMondays') {
348 $c->res->body('I feel motivated!');
351 If you are using action classes often or have some specific base
352 classes that you want to specify more conveniently, you can implement
353 a component base class providing an attribute handler.
355 It is not possible to use multiple action classes at once, however
356 L<Catalyst::Controller::ActionRole> allows you to apply L<Moose Roles|Moose::Role>
359 For further information on action classes and roles, please refer to
360 L<Catalyst::Action> and L<Catalyst::Manual::Actions>.
362 =head2 Component base classes
364 Many L<Catalyst::Plugin> that were written in Catalyst's early days
365 should really have been just controller base classes. With such a
366 class, you could provide functionality scoped to a single controller,
367 not polluting the global namespace in the context object.
369 You can provide regular Perl methods in a base class as well as
370 actions which will be inherited to the subclass. Please refer to
371 L</Controllers> for an example of this.
373 You can introduce your own attributes by specifying a handler method
374 in the controller base. For example, to use a C<FullClass> attribute
375 to specify a fully qualified action class name, you could use the
376 following implementation. Note, however, that this functionality is
377 already provided via the C<+> prefix for action classes. A simple
379 sub foo : Local ActionClass('+MyApp::Action::Bar') { ... }
381 will use C<MyApp::Action::Bar> as action class.
383 package MyApp::Base::Controller::FullClass;
385 use namespace::autoclean;
386 BEGIN { extends 'Catalyst::Controller'; }
388 sub _parse_FullClass_attr {
389 my ($self, $app_class, $action_name, $value, $attrs) = @_;
390 return( ActionClass => $value );
394 Note that the full line of arguments is only provided for completeness
395 sake. We could use this attribute in a subclass like any other
398 package MyApp::Controller::Foo;
400 use namespace::autoclean;
401 BEGIN { extends 'MyApp::Base::Controller::FullClass'; }
403 sub foo : Local FullClass('MyApp::Action::Bar') { ... }
409 Many things can happen in controllers, and it often improves
410 maintainability to abstract some of the code out into reusable base
413 You can provide usual Perl methods that will be available via your
414 controller object, or you can even define Catalyst actions which will
415 be inherited by the subclasses. Consider this controller base class:
417 package MyApp::Base::Controller::ModelBase;
419 use namespace::autoclean;
421 BEGIN { extends 'Catalyst::Controller'; }
423 sub list : Chained('base') PathPart('') Args(0) {
425 my $model = $c->model( $self->{model_name} );
426 my $condition = $self->{model_search_condition} || {};
427 my $attrs = $self->{model_search_attrs} || {};
428 $c->stash(rs => $model->search($condition, $attrs);
431 sub load : Chained('base') PathPart('') CaptureArgs(1) {
432 my ($self, $c, $id) = @_;
433 my $model = $c->model( $self->{model_name} );
434 $c->stash(row => $model->find($id));
438 This example implements two simple actions. The C<list> action chains
439 to a (currently non-existent) C<base> action and puts a result-set
440 into the stash taking a configured C<model_name> as well as a search
441 condition and attributes. This action is a
442 L<chained|Catalyst::DispatchType::Chained> endpoint. The other action,
443 called C< load > is a chain midpoint that takes one argument. It takes
444 the value as an ID and loads the row from the configured model. Please
445 not that the above code is simplified for clarity. It misses error
446 handling, input validation, and probably other things.
448 The class above is not very useful on its own, but we can combine it
449 with some custom actions by sub-classing it:
451 package MyApp::Controller::Foo;
453 use namespace::autoclean;
455 BEGIN { extends 'MyApp::Base::Controller::ModelBase'; }
457 __PACKAGE__->config( model_name => 'DB::Foo',
458 model_search_condition=> { is_active => 1 },
459 model_search_attrs => { order_by => 'name' },
462 sub base : Chained PathPart('foo') CaptureArgs(0) { }
464 sub view : Chained('load') Args(0) {
466 my $row = $c->stash->{row};
467 $c->res->body(join ': ', $row->name,
468 $row->description); }
471 This class uses the formerly created controller as a base
472 class. First, we see the configurations that were used in the parent
473 class. Next comes the C<base> action, where everything chains off of.
475 Note that inherited actions act like they were declared in your
476 controller itself. You can therefor call them just by their name in
477 C<forward>s, C<detaches> and C<Chained(..)> specifications. This is an
478 important part of what makes this technique so useful.
480 The new C<view> action ties itself to the C<load> action specified in
481 the base class and outputs the loaded row's C<name> and C<description>
482 columns. The controller C<MyApp::Controller::Foo> now has these
483 publicly available paths:
489 Will call the controller's C<base>, then the base classes C<list>
494 First, the controller's C<base> will be called, then it will C<load>
495 the row with the corresponding C<$id>. After that, C<view> will
496 display some fields out of the object.
500 =head2 Models and Views
502 If the functionality you'd like to add is really a data-set that you
503 want to manipulate, for example internal document types, images,
504 files, it might be better suited as a model.
506 The same applies for views. If your code handles representation or
507 deals with the applications interface and should be universally
508 available, it could be a perfect candidate for a view.
510 Please implement a C<process> method in your views. This method will
511 be called by Catalyst if it is asked to forward to a component without
512 a specified action. Note that C<process> is B<not a Catalyst action>
513 but a simple Perl method.
515 You are also encouraged to implement a C<render> method corresponding
516 with the one in L<Catalyst::View::TT>. This has proven invaluable,
517 because people can use your view for much more fine-grained content
520 Here is some example code for a fictional view:
522 package Catalyst::View::MyView;
524 use namespace::autoclean;
526 extends 'Catalyst::View';
530 my $template = $c->stash->{template};
531 my $content = $self->render($c, $template, $c->stash);
532 $c->res->body( $content );
536 my ($self, $c, $template, $args) = @_;
537 # prepare content here
544 The first thing to say about plugins is that if you're not sure if
545 your module should be a plugin, it probably shouldn't. It once was
546 common to add features to Catalyst by writing plugins that provide
547 accessors to said functionality. As Catalyst grew more popular, it
548 became obvious that this qualifies as bad practice.
550 By designing your module as a Catalyst plugin, every method you
551 implement, import or inherit will be available via your applications
552 context object. A plugin pollutes the global namespace, and you
553 should be only doing that when you really need to.
555 Often, developers design extensions as plugins because they need to
556 get hold of the context object. Either to get at the stash or
557 request/response objects are the widely spread reasons. It is,
558 however, perfectly possible to implement a regular Catalyst component
559 (read: model, view or controller) that receives the current context
560 object via L<Catalyst::Component/"ACCEPT_CONTEXT($c, @args)">.
562 When is a plugin suited to your task? Your code needs to be a
563 plugin to act upon or alter specific parts of Catalyst's request
564 lifecycle. If your functionality needs to change some C<prepare_*> or
565 C<finalize_*> stages, you won't get around a plugin.
567 Note, if you just want to hook into such a stage, and run code before,
568 or after it, then it is recommended that you use L<Moose>s method modifiers
571 Another valid target for a plugin architecture are things that
572 B<really> have to be globally available, like sessions or
575 B<Please do not> release Catalyst extensions as plugins only to
576 provide some functionality application wide. Design it as a controller
577 base class or another better suited technique with a smaller scope, so that
578 your code only influences those parts of the application where it is
579 needed, and namespace clashes and conflicts are ruled out.
581 The implementation is pretty easy. Your plugin will be inserted in the
582 application's inheritance list, above Catalyst itself. You can by this
583 alter Catalyst's request lifecycle behaviour. Every method you
584 declare, every import in your package will be available as method on
585 the application and the context object. As an example, let's say you
586 want Catalyst to warn you every time uri_for was called without an action
587 object as the first parameter, for example to test that all your chained
588 uris are generated from actions (a recommended best practice).
589 You could do this with this simple
590 implementation (excuse the lame class name, it's just an example):
592 package Catalyst::Plugin::UriforUndefWarning;
594 use Scalar::Util qw/blessed/;
599 my $uri = $c->next::method(@_);
600 $c->log->warn( 'uri_for with non action: ', join(', ', @_), )
601 if (!blessed($_[0]) || !$_[0]->isa('Catalyst::Action'));
607 This would override Catalyst's C<uri_for> method and emit a C<warn>
608 log entry containing the arguments to uri_for.
610 Please note this is not a practical example, as string URLs are fine for
613 A simple example like this is actually better as a L<Moose> role, for example:
615 package CatalystX::UriforUndefWarning;
617 use namespace::autoclean;
619 after 'uri_for' => sub {
621 $c->log->warn( 'uri_for with non action: ', join(', ', @_), )
622 if (!blessed($_[0]) || !$_[0]->isa('Catalyst::Action'));
626 Note that Catalyst will load any Moose Roles in the plugin list,
627 and apply them to your application class.
629 =head2 Factory components with COMPONENT()
631 Every component inheriting from L<Catalyst::Component> contains a
632 C<COMPONENT> method. It is used on application startup by
633 C<setup_components> to instantiate the component object for the
634 Catalyst application. By default, this will merge the components own
635 C<config>uration with the application wide overrides and call the
636 class' C<new> method to return the component object.
638 You can override this method and do and return whatever you want.
639 However, you should use L<Class::C3> (via L<MRO::Compat>) to forward
640 to the original C<COMPONENT> method to merge the configuration of
643 Here is a stub C<COMPONENT> method:
645 package CatalystX::Component::Foo;
647 use namespace::autoclean;
649 extends 'Catalyst::Component';
653 # Note: $app is like $c, but since the application isn't fully
654 # initialized, we don't want to call it $c yet. $config
655 # is a hashref of config options possibly set on this component.
656 my ($app, $config) = @_;
658 # Do things here before instantiation
659 $new = $class->next::method(@_);
660 # Do things to object after instantiation
664 The arguments are the class name of the component, the class name of
665 the application instantiating the component, and a hash reference with
666 the controller's configuration.
668 You are free to re-bless the object, instantiate a whole other
669 component or really do anything compatible with Catalyst's
670 expectations on a component.
672 For more information, please see
673 L<Catalyst::Component/"COMPONENT($c,$arguments)">.
675 =head2 Applying roles to parts of the framework
677 L<CatalystX::RoleApplicator> will allow you to apply Roles to
678 the following classes:
694 These roles can add new methods to these classes, or wrap preexisting methods.
696 The namespace for roles like this is C<Catalyst::TraitFor::XXX::YYYY>.
698 For an example of a CPAN component implemented in this manor, see
699 L<Catalyst::TraitFor::Request::BrowserDetect>.
703 L<Catalyst>, L<Catalyst::Manual::Actions>, L<Catalyst::Component>
707 Catalyst Contributors, see Catalyst.pm
711 This library is free software. You can redistribute it and/or modify it under
712 the same terms as Perl itself.