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 For a simple example of this, see L<CatalystX::REPL>.
124 =head2 Inheritance and overriding methods
126 When overriding a method, keep in mind that some day additionally
127 arguments may be provided to the method, if the last parameter is not
128 a flat list. It is thus better to override a method by shifting the
129 invocant off of C<@_> and assign the rest of the used arguments, so
130 you can pass your complete arguments to the original method via C<@_>:
136 my ($bar, $baz) = @_; # ... return
137 $self->next::method(@_);
140 If you would do the common
142 my ($self, $foo, $bar) = @_;
144 you'd have to use a much uglier construct to ensure that all arguments
145 will be passed along and the method is future proof:
147 $self->next::method(@_[ 1 .. $#_ ]);
149 =head2 Tests and documentation
151 When you release your module to the CPAN, proper documentation and at
152 least a basic test suite (which means more than pod or even just
153 C<use_ok>, sorry) gives people a good base to contribute to the
154 module. It also shows that you care for your users. If you would like
155 your module to become a recommended addition, these things will prove
158 If you're just getting started, try using
159 L<CatalystX::Starter|CatalystX::Starter> to generate some example
160 tests for your module.
164 In planning to release a module to the community (Catalyst or CPAN and
165 Perl), you should consider if you have the resources to keep it up to
166 date, including fixing bugs and accepting contributions.
168 If you're not sure about this, you can always ask in the proper
169 Catalyst or Perl channels if someone else might be interested in the
170 project, and would jump in as co-maintainer.
172 A public repository can further ease interaction with the
173 community. Even read only access enables people to provide you with
174 patches to your current development version. subversion, SVN and SVK,
175 are broadly preferred in the Catalyst community.
177 If you're developing a Catalyst extension, please consider asking the
178 core team for space in Catalyst's own subversion repository. You can
179 get in touch about this via IRC or the Catalyst developers mailing
182 =head2 The context object
184 Sometimes you want to get a hold of the context object in a component
185 that was created on startup time, where no context existed yet. Often
186 this is about the model reading something out of the stash or other
187 context information (current language, for example).
189 If you use the context object in your component you have tied it to an
190 existing request. This means that you might get into problems when
191 you try to use the component (e.g. the model - the most common case)
192 outside of Catalyst, for example in cronjobs.
194 A stable solution to this problem is to design the Catalyst model
195 separately from the underlying model logic. Let's take
196 L<Catalyst::Model::DBIC::Schema> as an example. You can create a
197 schema outside of Catalyst that knows nothing about the web. This kind
198 of design ensures encapsulation and makes development and maintenance
199 a whole lot easier. The you use the aforementioned model to tie your
200 schema to your application. This gives you a C<MyApp::DBIC> (the name
201 is of course just an example) model as well as
202 C<MyApp::DBIC::TableName> models to access your result sources
205 By creating such a thin layer between the actual model and the
206 Catalyst application, the schema itself is not at all tied to any
207 application and the layer in-between can access the model's API using
208 information from the context object.
210 A Catalyst component accesses the context object at request time with
211 L<Catalyst::Component/"ACCEPT_CONTEXT($c, @args)">.
215 The application has to interact with the extension with some
216 configuration. There is of course again more than one way to do it.
220 You can specify any valid Perl attribute on Catalyst actions you like.
221 (See L<attributes/"Syntax of Attribute Lists"> for a description of
222 what is valid.) These will be available on the C<Catalyst::Action>
223 instance via its C<attributes> accessor. To give an example, this
226 sub foo : Local Bar('Baz') {
229 $self->action_for('foo')->attributes;
230 $c->res->body($attributes->{Bar}[0] );
233 will set the response body to C<Baz>. The values always come in an
234 array reference. As you can see, you can use attributes to configure
235 your actions. You can specify or alter these attributes via
236 L</"Component Configuration">, or even react on them as soon as
237 Catalyst encounters them by providing your own L<component base
238 class|/"Component Base Classes">.
240 =head2 Creating custom accessors
242 L<Catalyst::Component> uses L<Class::Accessor::Fast> for accessor
243 creation. Please refer to the modules documentation for usage
246 =head2 Component configuration
248 At creation time, the class configuration of your component (the one
249 available via C<$self-E<gt>config>) will be merged with possible
250 configuration settings from the applications configuration (either
251 directly or via config file). This is then stored in the controller
252 object's hash reference. So, if you read possible configurations like:
254 my $model_name = $controller->{model_name};
256 you will get the right value. The C<config> accessor always only
257 contains the original class configuration and must not be used for
258 component configuration.
260 You are advised to create accessors on your component class for your
261 configuration values. This is good practice and makes it easier to
262 capture configuration key typos. You can do this with the
263 C<mk_ro_accessors> method provided to L<Catalyst::Component> via
264 L<Class::Accessor::Fast>:
266 use base 'Catalyst::Controller';
267 __PACKAGE__->mk_ro_accessors('model_name');
269 my $model_name = $controller->model_name;
271 =head1 IMPLEMENTATION
273 This part contains the technical details of various implementation
274 methods. Please read the L</"BEST PRACTICES"> before you start your
275 implementation, if you haven't already.
277 =head2 Action classes
279 Usually, your action objects are of the class L<Catalyst::Action>.
280 You can override this with the C<ActionClass> attribute to influence
281 execution and/or dispatching of the action. A widely used example of
282 this is L<Catalyst::Action::RenderView>, which is used in every newly
283 created Catalyst application in your root controller:
285 sub end : ActionClass('RenderView') { }
287 Usually, you want to override the C<execute> and/or the C<match>
288 method. The execute method of the action will naturally call the
289 methods code. You can surround this by overriding the method in a
292 package Catalyst::Action::MyFoo;
296 use base 'Catalyst::Action';
300 my ($controller, $c, @args) = @_;
301 # put your 'before' code here
302 my $r = $self->next::method(@_);
303 # put your 'after' code here
308 We are using L<MRO::Compat> to ensure that you have the next::method
309 call, from L<Class::C3> (in older perls), or natively (if you are using
310 perl 5.10) to re-dispatch to the original C<execute> method in the
311 L<Catalyst::Action> class.
313 The Catalyst dispatcher handles an incoming request and, depending
314 upon the dispatch type, will call the appropriate target or chain.
315 From time to time it asks the actions themselves, or through the
316 controller, if they would match the current request. That's what the
317 C<match> method does. So by overriding this, you can change on what
318 the action will match and add new matching criteria.
320 For example, the action class below will make the action only match on
323 package Catalyst::Action::OnlyMondays;
327 use base 'Catalyst::Action';
331 return 0 if ( localtime(time) )[6] == 1;
332 return $self->next::method(@_);
336 And this is how we'd use it:
338 sub foo: Local ActionClass('OnlyMondays') {
340 $c->res->body('I feel motivated!');
343 If you are using action classes often or have some specific base
344 classes that you want to specify more conveniently, you can implement
345 a component base class providing an attribute handler.
347 For further information on action classes, please refer to
348 L<Catalyst::Action> and L<Catalyst::Manual::Actions>.
350 =head2 Component base classes
352 Many L<Catalyst::Plugin> that were written in Catalyst's early days
353 should really have been just controller base classes. With such a
354 class, you could provide functionality scoped to a single controller,
355 not polluting the global namespace in the context object.
357 You can provide regular Perl methods in a base class as well as
358 actions which will be inherited to the subclass. Please refer to
359 L</Controllers> for an example of this.
361 You can introduce your own attributes by specifying a handler method
362 in the controller base. For example, to use a C<FullClass> attribute
363 to specify a fully qualified action class name, you could use the
364 following implementation. Note, however, that this functionality is
365 already provided via the C<+> prefix for action classes. A simple
367 sub foo : Local ActionClass('+MyApp::Action::Bar') { ... }
369 will use C<MyApp::Action::Bar> as action class.
371 package MyApp::Base::Controller::FullClass; use strict; use base
372 'Catalyst::Controller';
374 sub _parse_FullClass_attr {
375 my ($self, $app_class, $action_name, $value, $attrs) = @_;
376 return( ActionClass => $value );
380 Note that the full line of arguments is only provided for completeness
381 sake. We could use this attribute in a subclass like any other
384 package MyApp::Controller::Foo;
386 use base 'MyApp::Base::Controller::FullClass';
388 sub foo : Local FullClass('MyApp::Action::Bar') { ... }
394 Many things can happen in controllers, and it often improves
395 maintainability to abstract some of the code out into reusable base
398 You can provide usual Perl methods that will be available via your
399 controller object, or you can even define Catalyst actions which will
400 be inherited by the subclasses. Consider this controller base class:
402 package MyApp::Base::Controller::ModelBase;
404 use base 'Catalyst::Controller';
406 sub list : Chained('base') PathPart('') Args(0) {
408 my $model = $c->model( $self->{model_name} );
409 my $condition = $self->{model_search_condition} || {};
410 my $attrs = $self->{model_search_attrs} || {};
411 $c->stash(rs => $model->search($condition, $attrs);
414 sub load : Chained('base') PathPart('') CaptureArgs(1) {
415 my ($self, $c, $id) = @_;
416 my $model = $c->model( $self->{model_name} );
417 $c->stash(row => $model->find($id));
421 This example implements two simple actions. The C<list> action chains
422 to a (currently non-existent) C<base> action and puts a result-set
423 into the stash taking a configured C<model_name> as well as a search
424 condition and attributes. This action is a
425 L<chained|Catalyst::DispatchType::Chained> endpoint. The other action,
426 called C< load > is a chain midpoint that takes one argument. It takes
427 the value as an ID and loads the row from the configured model. Please
428 not that the above code is simplified for clarity. It misses error
429 handling, input validation, and probably other things.
431 The class above is not very useful on its own, but we can combine it
432 with some custom actions by sub-classing it:
434 package MyApp::Controller::Foo;
436 use base 'MyApp::Base::Controller::ModelBase';
438 __PACKAGE__->config( model_name => 'DB::Foo',
439 model_search_condition=> { is_active => 1 },
440 model_search_attrs => { order_by => 'name' },
443 sub base : Chained PathPart('foo') CaptureArgs(0) { }
445 sub view : Chained('load') Args(0) {
447 my $row = $c->stash->{row};
448 $c->res->body(join ': ', $row->name,
449 $row->description); }
452 This class uses the formerly created controller as a base
453 class. First, we see the configurations that were used in the parent
454 class. Next comes the C<base> action, where everything chains off of.
456 Note that inherited actions act like they were declared in your
457 controller itself. You can therefor call them just by their name in
458 C<forward>s, C<detaches> and C<Chained(..)> specifications. This is an
459 important part of what makes this technique so useful.
461 The new C<view> action ties itself to the C<load> action specified in
462 the base class and outputs the loaded row's C<name> and C<description>
463 columns. The controller C<MyApp::Controller::Foo> now has these
464 publicly available paths:
470 Will call the controller's C<base>, then the base classes C<list>
475 First, the controller's C<base> will be called, then it will C<load>
476 the row with the corresponding C<$id>. After that, C<view> will
477 display some fields out of the object.
481 =head2 Models and Views
483 If the functionality you'd like to add is really a data-set that you
484 want to manipulate, for example internal document types, images,
485 files, it might be better suited as a model.
487 The same applies for views. If your code handles representation or
488 deals with the applications interface and should be universally
489 available, it could be a perfect candidate for a view.
491 Please implement a C<process> method in your views. This method will
492 be called by Catalyst if it is asked to forward to a component without
493 a specified action. Note that C<process> is B<not a Catalyst action>
494 but a simple Perl method.
496 You are also encouraged to implement a C<render> method corresponding
497 with the one in L<Catalyst::View::TT>. This has proven invaluable,
498 because people can use your view for much more fine-grained content
501 Here is some example code for a fictional view:
503 package CatalystX::View::MyView;
505 use base 'Catalyst::View';
509 my $template = $c->stash->{template};
510 my $content = $self->render($c, $template, $c->stash);
511 $c->res->body( $content );
515 my ($self, $c, $template, $args) = @_;
516 # prepare content here
523 The first thing to say about plugins is that if you're not sure if
524 your module should be a plugin, it probably shouldn't. It once was
525 common to add features to Catalyst by writing plugins that provide
526 accessors to said functionality. As Catalyst grew more popular, it
527 became obvious that this qualifies as bad practice.
529 By designing your module as a Catalyst plugin, every method you
530 implement, import or inherit will be available via your applications
531 context object. A plugin pollutes the global namespace, and you
532 should be only doing that when you really need to.
534 Often, developers design extensions as plugins because they need to
535 get hold of the context object. Either to get at the stash or
536 request/response objects are the widely spread reasons. It is,
537 however, perfectly possible to implement a regular Catalyst component
538 (read: model, view or controller) that receives the current context
539 object via L<Catalyst::Component/"ACCEPT_CONTEXT($c, @args)">.
541 When is a plugin suited to your task? Your code needs to be a
542 plugin to act upon or alter specific parts of Catalyst's request
543 lifecycle. If your functionality needs to change some C<prepare_*> or
544 C<finalize_*> stages, you won't get around a plugin.
546 Note, if you just want to hook into such a stage, and run code before,
547 or after it, then it is recommended that you use L<Moose>s method modifiers
550 Another valid target for a plugin architecture are things that
551 B<really> have to be globally available, like sessions or
554 B<Please do not> release Catalyst extensions as plugins only to
555 provide some functionality application wide. Design it as a controller
556 base class or another suiting technique with a smaller scope, so that
557 your code only influences those parts of the application where it is
558 needed, and namespace clashes and conflicts are ruled out.
560 The implementation is pretty easy. Your plugin will be inserted in the
561 application's inheritance list, above Catalyst itself. You can by this
562 alter Catalyst's request lifecycle behaviour. Every method you
563 declare, every import in your package will be available as method on
564 the application and the context object. As an example, let's say you
565 want Catalyst to warn you every time uri_for was called without an action
566 object as the first parameter, for example to test that all your chained
567 uris are generated from actions (a recommended best practice).
568 You could do this with this simple
569 implementation (excuse the lame class name, it's just an example):
571 package Catalyst::Plugin::UriforUndefWarning;
573 use Scalar::Util qw/blessed/;
578 my $uri = $c->next::method(@_);
579 $c->log->warn( 'uri_for with non action: ', join(', ', @_), )
580 if (!blessed($_[0]) || !$_[0]->isa('Catalyst::Action'));
586 This would override Catalyst's C<uri_for> method and emit a C<warn>
587 log entry containing the arguments to uri_for.
589 Please note this is not a practical example, as string URLs are fine for
592 A simple example like this is actually better as a L<Moose> role, for example:
594 package CatalystX::UriforUndefWarning;
596 use namespace::clean -except => 'meta';
598 after 'uri_for' => sub {
600 $c->log->warn( 'uri_for with non action: ', join(', ', @_), )
601 if (!blessed($_[0]) || !$_[0]->isa('Catalyst::Action'));
605 =head2 Factory components with COMPONENT()
607 Every component inheriting from L<Catalyst::Component> contains a
608 C<COMPONENT> method. It is used on application startup by
609 C<setup_components> to instantiate the component object for the
610 Catalyst application. By default, this will merge the components own
611 C<config>uration with the application wide overrides and call the
612 class' C<new> method to return the component object.
614 You can override this method and do and return whatever you want.
615 However, you should use L<Class::C3> (via L<MRO::Compat>) to forward
616 to the original C<COMPONENT> method to merge the configuration of
619 Here is a stub C<COMPONENT> method:
621 package CatalystX::Component::Foo;
623 use base 'Catalyst::Component';
629 # Note: $app is like $c, but since the application isn't fully
630 # initialized, we don't want to call it $c yet. $config
631 # is a hashref of config options possibly set on this component.
632 my ($app, $config) = @_;
634 # Do things here before instantiation
635 $new = $class->next::method(@_);
636 # Do things to object after instantiation
640 The arguments are the class name of the component, the class name of
641 the application instantiating the component, and a hash reference with
642 the controller's configuration.
644 You are free to re-bless the object, instantiate a whole other
645 component or really do anything compatible with Catalyst's
646 expectations on a component.
648 For more information, please see
649 L<Catalyst::Component/"COMPONENT($c,$arguments)">.
653 L<Catalyst>, L<Catalyst::Manual::Actions>, L<Catalyst::Component>
657 Robert Sedlacek C<< <rs@474.at> >>
659 Jonathan Rockway C<< <jrockway@cpan.org> >>
661 =head1 LICENSE AND COPYRIGHT
663 This document is free, you can redistribute it and/or modify it under
664 the same terms as Perl itself.