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 If you need to hook (but not alter) the internals, then make it a L<Moose::Role>
61 =item There's a community. Use it!
63 There are many experienced developers in the Catalyst community,
64 there's always the IRC channel and the mailing list to discuss things.
66 =item Add tests and documentation!
68 This gives a stable basis for contribution, and even more importantly,
69 builds trust. The easiest way is a test application. See
70 L<Catalyst::Manual::Tutorial::Testing> for more information.
76 While some core extensions (engines, plugins, etc.) have to be placed
77 in the C<Catalyst::*> namespace, the Catalyst core would like to ask
78 developers to use the C<CatalystX::*> namespace if possible.
80 When you try to put a base class for a C<Model>, C<View> or
81 C<Controller> directly under your C<MyApp> directory as, for example,
82 C<MyApp::Controller::Foo>, you will have the problem that Catalyst
83 will try to load that base class as a component of your
84 application. The solution is simple: Use another namespace. Common
85 ones are C<MyApp::Base::Controller::*> or C<MyApp::ControllerBase::*>
88 =head2 Can it be a simple module?
90 Sometimes you want to use functionality in your application that
91 doesn't require the framework at all. Remember that Catalyst is just
92 Perl and you always can just C<use> a module. If you have application
93 specific code that doesn't need the framework, there is no problem in
94 putting it in your C<MyApp::*> namespace. Just don't put it in
95 C<Model>, C<Controller> or C<View>, because that would make Catalyst
96 try to load them as components.
98 Writing a generic component that only works with Catalyst is wasteful
99 of your time. Try writing a plain perl module, and then a small bit
100 of glue that integrates it with Catalyst. See
101 L<Catalyst::Model::DBIC::Schema|Catalyst::Model::DBIC::Schema> for a
102 module that takes the approach. The advantage here is that your
103 "Catalyst" DBIC schema works perfectly outside of Catalyst, making
104 testing (and command-line scripts) a breeze. The actual Catalyst
105 Model is just a few lines of glue that makes working with the schema
108 If you want the thinnest interface possible, take a look at
109 L<Catalyst::Model::Adaptor|Catalyst::Model::Adaptor>.
111 =head2 Using Moose roles to apply method modifiers
113 Rather than having a complex set of base classes which you have to mixin
114 via multiple inheritence, if your functionality is well structured, then
115 it's possible to use the composability of L<Moose> roles, and method modifiers
116 to hook onto to provide functionality.
118 For a simple example of this, see L<CatalystX::REPL>.
120 B<Note:> Currently, controllers with attributes will not function correctly
121 in conjunction with Moose roles.
123 =head2 Inheritance and overriding methods
125 While Catalyst itself is still based on L<NEXT> (for multiple
126 inheritance), extension developers are encouraged to use L<Class::C3>,
127 via L<MRO::Compat>, which is what Catalyst will be switching to in the
130 When overriding a method, keep in mind that some day additionally
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<@_>:
138 sub foo { my $self = shift;
139 my ($bar, $baz) = @_; # ... return
140 $self->next::method(@_); }
142 If you would do the common
144 my ($self, $foo, $bar) = @_;
146 you'd have to use a much uglier construct to ensure that all arguments
147 will be passed along and the method is future proof:
149 $self->next::method(@_[ 1 .. $#_ ]);
151 =head2 Tests and documentation
153 When you release your module to the CPAN, proper documentation and at
154 least a basic test suite (which means more than pod or even just
155 C<use_ok>, sorry) gives people a good base to contribute to the
156 module. It also shows that you care for your users. If you would like
157 your module to become a recommended addition, these things will prove
160 If you're just getting started, try using
161 L<CatalystX::Starter|CatalystX::Starter> to generate some example
162 tests for your module.
166 In planning to release a module to the community (Catalyst or CPAN and
167 Perl), you should consider if you have the resources to keep it up to
168 date, including fixing bugs and accepting contributions.
170 If you're not sure about this, you can always ask in the proper
171 Catalyst or Perl channels if someone else might be interested in the
172 project, and would jump in as co-maintainer.
174 A public repository can further ease interaction with the
175 community. Even read only access enables people to provide you with
176 patches to your current development version. subversion, SVN and SVK,
177 are broadly preferred in the Catalyst community.
179 If you're developing a Catalyst extension, please consider asking the
180 core team for space in Catalyst's own subversion repository. You can
181 get in touch about this via IRC or the Catalyst developers mailing
184 =head2 The context object
186 Sometimes you want to get a hold of the context object in a component
187 that was created on startup time, where no context existed yet. Often
188 this is about the model reading something out of the stash or other
189 context information (current language, for example).
191 If you use the context object in your component you have tied it to an
192 existing request. This means that you might get into problems when
193 you try to use the component (e.g. the model - the most common case)
194 outside of Catalyst, for example in cronjobs.
196 A stable solution to this problem is to design the Catalyst model
197 separately from the underlying model logic. Let's take
198 L<Catalyst::Model::DBIC::Schema> as an example. You can create a
199 schema outside of Catalyst that knows nothing about the web. This kind
200 of design ensures encapsulation and makes development and maintenance
201 a whole lot easier. The you use the aforementioned model to tie your
202 schema to your application. This gives you a C<MyApp::DBIC> (the name
203 is of course just an example) model as well as
204 C<MyApp::DBIC::TableName> models to access your result sources
207 By creating such a thin layer between the actual model and the
208 Catalyst application, the schema itself is not at all tied to any
209 application and the layer in-between can access the model's API using
210 information from the context object.
212 A Catalyst component accesses the context object at request time with
213 L<Catalyst::Component/"ACCEPT_CONTEXT($c, @args)">.
217 The application has to interact with the extension with some
218 configuration. There is of course again more than one way to do it.
222 You can specify any valid Perl attribute on Catalyst actions you like.
223 (See L<attributes/"Syntax of Attribute Lists"> for a description of
224 what is valid.) These will be available on the C<Catalyst::Action>
225 instance via its C<attributes> accessor. To give an example, this
228 sub foo : Local Bar('Baz') {
231 $self->action_for('foo')->attributes;
232 $c->res->body($attributes->{Bar}[0] );
235 will set the response body to C<Baz>. The values always come in an
236 array reference. As you can see, you can use attributes to configure
237 your actions. You can specify or alter these attributes via
238 L</"Component Configuration">, or even react on them as soon as
239 Catalyst encounters them by providing your own L<component base
240 class|/"Component Base Classes">.
242 =head2 Creating custom accessors
244 L<Catalyst::Component> uses L<Class::Accessor::Fast> for accessor
245 creation. Please refer to the modules documentation for usage
248 =head2 Component configuration
250 At creation time, the class configuration of your component (the one
251 available via C<$self-E<gt>config>) will be merged with possible
252 configuration settings from the applications configuration (either
253 directly or via config file). This is then stored in the controller
254 object's hash reference. So, if you read possible configurations like:
256 my $model_name = $controller->{model_name};
258 you will get the right value. The C<config> accessor always only
259 contains the original class configuration and must not be used for
260 component configuration.
262 You are advised to create accessors on your component class for your
263 configuration values. This is good practice and makes it easier to
264 capture configuration key typos. You can do this with the
265 C<mk_ro_accessors> method provided to L<Catalyst::Component> via
266 L<Class::Accessor::Fast>:
268 use base 'Catalyst::Controller';
269 __PACKAGE__->mk_ro_accessors('model_name');
271 my $model_name = $controller->model_name;
273 =head1 IMPLEMENTATION
275 This part contains the technical details of various implementation
276 methods. Please read the L</"BEST PRACTICES"> before you start your
277 implementation, if you haven't already.
279 =head2 Action classes
281 Usually, your action objects are of the class L<Catalyst::Action>.
282 You can override this with the C<ActionClass> attribute to influence
283 execution and/or dispatching of the action. A widely used example of
284 this is L<Catalyst::Action::RenderView>, which is used in every newly
285 created Catalyst application in your root controller:
287 sub end : ActionClass('RenderView') { }
289 Usually, you want to override the C<execute> and/or the C<match>
290 method. The execute method of the action will naturally call the
291 methods code. You can surround this by overriding the method in a
294 package Catalyst::Action::MyFoo;
298 use base 'Catalyst::Action';
302 my ($controller, $c, @args) = @_;
303 # put your 'before' code here
304 my $r = $self->next::method(@_);
305 # put your 'after' code here
310 We are using L<MRO::Compat> to ensure that you have the next::method
311 call, from L<Class::C3> (in older perls), or natively (if you are using
312 perl 5.10) to re-dispatch to the original C<execute> method in the
313 L<Catalyst::Action> class.
315 The Catalyst dispatcher handles an incoming request and, depending
316 upon the dispatch type, will call the appropriate target or chain.
317 From time to time it asks the actions themselves, or through the
318 controller, if they would match the current request. That's what the
319 C<match> method does. So by overriding this, you can change on what
320 the action will match and add new matching criteria.
322 For example, the action class below will make the action only match on
325 package Catalyst::Action::OnlyMondays;
329 use base 'Catalyst::Action';
333 return 0 if ( localtime(time) )[6] == 1;
334 return $self->next::method(@_);
338 And this is how we'd use it:
340 sub foo: Local ActionClass('OnlyMondays') {
342 $c->res->body('I feel motivated!');
345 If you are using action classes often or have some specific base
346 classes that you want to specify more conveniently, you can implement
347 a component base class providing an attribute handler.
349 For further information on action classes, please refer to
350 L<Catalyst::Action> and L<Catalyst::Manual::Actions>.
352 =head2 Component base classes
354 Many L<Catalyst::Plugin> that were written in Catalyst's early days
355 should really have been just controller base classes. With such a
356 class, you could provide functionality scoped to a single controller,
357 not polluting the global namespace in the context object.
359 You can provide regular Perl methods in a base class as well as
360 actions which will be inherited to the subclass. Please refer to
361 L</Controllers> for an example of this.
363 You can introduce your own attributes by specifying a handler method
364 in the controller base. For example, to use a C<FullClass> attribute
365 to specify a fully qualified action class name, you could use the
366 following implementation. Note, however, that this functionality is
367 already provided via the C<+> prefix for action classes. A simple
369 sub foo : Local ActionClass('+MyApp::Action::Bar') { ... }
371 will use C<MyApp::Action::Bar> as action class.
373 package MyApp::Base::Controller::FullClass; use strict; use base
374 'Catalyst::Controller';
376 sub _parse_FullClass_attr {
377 my ($self, $app_class, $action_name, $value, $attrs) = @_;
378 return( ActionClass => $value );
382 Note that the full line of arguments is only provided for completeness
383 sake. We could use this attribute in a subclass like any other
386 package MyApp::Controller::Foo;
388 use base 'MyApp::Base::Controller::FullClass';
390 sub foo : Local FullClass('MyApp::Action::Bar') { ... }
396 Many things can happen in controllers, and it often improves
397 maintainability to abstract some of the code out into reusable base
400 You can provide usual Perl methods that will be available via your
401 controller object, or you can even define Catalyst actions which will
402 be inherited by the subclasses. Consider this controller base class:
404 package MyApp::Base::Controller::ModelBase;
406 use base 'Catalyst::Controller';
408 sub list : Chained('base') PathPart('') Args(0) {
410 my $model = $c->model( $self->{model_name} );
411 my $condition = $self->{model_search_condition} || {};
412 my $attrs = $self->{model_search_attrs} || {};
413 $c->stash(rs => $model->search($condition, $attrs);
416 sub load : Chained('base') PathPart('') CaptureArgs(1) {
417 my ($self, $c, $id) = @_;
418 my $model = $c->model( $self->{model_name} );
419 $c->stash(row => $model->find($id));
423 This example implements two simple actions. The C<list> action chains
424 to a (currently non-existent) C<base> action and puts a result-set
425 into the stash taking a configured C<model_name> as well as a search
426 condition and attributes. This action is a
427 L<chained|Catalyst::DispatchType::Chained> endpoint. The other action,
428 called C< load > is a chain midpoint that takes one argument. It takes
429 the value as an ID and loads the row from the configured model. Please
430 not that the above code is simplified for clarity. It misses error
431 handling, input validation, and probably other things.
433 The class above is not very useful on its own, but we can combine it
434 with some custom actions by sub-classing it:
436 package MyApp::Controller::Foo;
438 use base 'MyApp::Base::Controller::ModelBase';
440 __PACKAGE__->config( model_name => 'DB::Foo',
441 model_search_condition=> { is_active => 1 },
442 model_search_attrs => { order_by => 'name' },
445 sub base : Chained PathPart('foo') CaptureArgs(0) { }
447 sub view : Chained('load') Args(0) {
449 my $row = $c->stash->{row};
450 $c->res->body(join ': ', $row->name,
451 $row->description); }
454 This class uses the formerly created controller as a base
455 class. First, we see the configurations that were used in the parent
456 class. Next comes the C<base> action, where everything chains off of.
458 Note that inherited actions act like they were declared in your
459 controller itself. You can therefor call them just by their name in
460 C<forward>s, C<detaches> and C<Chained(..)> specifications. This is an
461 important part of what makes this technique so useful.
463 The new C<view> action ties itself to the C<load> action specified in
464 the base class and outputs the loaded row's C<name> and C<description>
465 columns. The controller C<MyApp::Controller::Foo> now has these
466 publicly available paths:
472 Will call the controller's C<base>, then the base classes C<list>
477 First, the controller's C<base> will be called, then it will C<load>
478 the row with the corresponding C<$id>. After that, C<view> will
479 display some fields out of the object.
483 =head2 Models and Views
485 If the functionality you'd like to add is really a data-set that you
486 want to manipulate, for example internal document types, images,
487 files, it might be better suited as a model.
489 The same applies for views. If your code handles representation or
490 deals with the applications interface and should be universally
491 available, it could be a perfect candidate for a view.
493 Please implement a C<process> method in your views. This method will
494 be called by Catalyst if it is asked to forward to a component without
495 a specified action. Note that C<process> is B<not a Catalyst action>
496 but a simple Perl method.
498 You are also encouraged to implement a C<render> method corresponding
499 with the one in L<Catalyst::View::TT>. This has proven invaluable,
500 because people can use your view for much more fine-grained content
503 Here is some example code for a fictional view:
505 package CatalystX::View::MyView;
507 use base 'Catalyst::View';
511 my $template = $c->stash->{template};
512 my $content = $self->render($c, $template, $c->stash);
513 $c->res->body( $content );
517 my ($self, $c, $template, $args) = @_;
518 # prepare content here
525 The first thing to say about plugins is that if you're not sure if
526 your module should be a plugin, it probably shouldn't. It once was
527 common to add features to Catalyst by writing plugins that provide
528 accessors to said functionality. As Catalyst grew more popular, it
529 became obvious that this qualifies as bad practice.
531 By designing your module as a Catalyst plugin, every method you
532 implement, import or inherit will be available via your applications
533 context object. A plugin pollutes the global namespace, and you
534 should be only doing that when you really need to.
536 Often, developers design extensions as plugins because they need to
537 get hold of the context object. Either to get at the stash or
538 request/response objects are the widely spread reasons. It is,
539 however, perfectly possible to implement a regular Catalyst component
540 (read: model, view or controller) that receives the current context
541 object via L<Catalyst::Component/"ACCEPT_CONTEXT($c, @args)">.
543 When is a plugin suited to your task? Your code needs to be a
544 plugin to act upon or alter specific parts of Catalyst's request
545 lifecycle. If your functionality needs to change some C<prepare_*> or
546 C<finalize_*> stages, you won't get around a plugin.
548 Note, if you just want to hook into such a stage, and run code before,
549 or after it, then it is recommended that you use L<Moose>s method modifiers
552 Another valid target for a plugin architecture are things that
553 B<really> have to be globally available, like sessions or
556 B<Please do not> release Catalyst extensions as plugins only to
557 provide some functionality application wide. Design it as a controller
558 base class or another suiting technique with a smaller scope, so that
559 your code only influences those parts of the application where it is
560 needed, and namespace clashes and conflicts are ruled out.
562 The implementation is pretty easy. Your plugin will be inserted in the
563 application's inheritance list, above Catalyst itself. You can by this
564 alter Catalyst's request lifecycle behaviour. Every method you
565 declare, every import in your package will be available as method on
566 the application and the context object. As an example, let's say you
567 want Catalyst to warn you every time uri_for was called without an action
568 object as the first parameter, for example to test that all your chained
569 uris are generated from actions (a recommended best practice).
570 You could do this with this simple
571 implementation (excuse the lame class name, it's just an example):
573 package Catalyst::Plugin::UriforUndefWarning;
575 use Scalar::Util qw/blessed/;
580 my $uri = $c->next::method(@_);
581 $c->log->warn( 'uri_for with non action: ', join(', ', @_), )
582 if (!blessed($_[0]) || !$_[0]->isa('Catalyst::Action'));
588 This would override Catalyst's C<uri_for> method and emit a C<warn>
589 log entry containing the arguments to uri_for.
591 Please note this is not a practical example, as string URLs are fine for
594 A simple example like this is actually better as a L<Moose> role, for example:
596 package CatalystX::UriforUndefWarning;
598 use namespace::clean -except => 'meta';
600 after 'uri_for' => sub {
602 $c->log->warn( 'uri_for with non action: ', join(', ', @_), )
603 if (!blessed($_[0]) || !$_[0]->isa('Catalyst::Action'));
607 =head2 Factory components with COMPONENT()
609 Every component inheriting from L<Catalyst::Component> contains a
610 C<COMPONENT> method. It is used on application startup by
611 C<setup_components> to instantiate the component object for the
612 Catalyst application. By default, this will merge the components own
613 C<config>uration with the application wide overrides and call the
614 class' C<new> method to return the component object.
616 You can override this method and do and return whatever you want.
617 However, you should use L<Class::C3> (via L<MRO::Compat>) to forward
618 to the original C<COMPONENT> method to merge the configuration of
621 Here is a stub C<COMPONENT> method:
623 package CatalystX::Component::Foo;
625 use base 'Catalyst::Component';
631 my ($app_class, $config) = @_;
633 # do things here before instantiation my
634 $obj = $self->next::method(@_);
635 # do things to object after instantiation
639 The arguments are the class name of the component, the class name of
640 the application instantiating the component, and a hash reference with
641 the controller's configuration.
643 You are free to re-bless the object, instantiate a whole other
644 component or really do anything compatible with Catalyst's
645 expectations on a component.
647 For more information, please see L<Catalyst::Component/"COMPONENT($c,$arguments)">.
651 L<Catalyst>, L<Catalyst::Manual::Actions>, L<Catalyst::Component>
655 Robert Sedlacek C<< <rs@474.at> >>
657 Jonathan Rockway C<< <jrockway@cpan.org> >>
659 =head1 LICENSE AND COPYRIGHT
661 This document is free, you can redistribute it and/or modify it under
662 the same terms as Perl itself.