1Catalyst::Manual::ExtenUdsienrgCCaotnatlryisbtu(t3e)d PeCraltaDloycsutm:e:nMtaantuiaoln::ExtendingCatalyst(3)
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6 Catalyst::Manual::ExtendingCatalyst - Extending The Framework
7
9 This document will provide you with access points, techniques and best
10 practices to extend the Catalyst framework, or to find more elegant
11 ways to abstract and use your own code.
12 The design of Catalyst is such that the framework itself should not get
14 in your way. There are many entry points to alter or extend Catalyst's
15 behaviour, and this can be confusing. This document is written to help
16 you understand the possibilities, current practices and their
17 consequences.
18 Please read the "BEST PRACTICES" section before deciding on a design,
20 especially if you plan to release your code to CPAN. The Catalyst
21 developer and user communities, which you are part of, will benefit
22 most if we all work together and coordinate.
23 If you are unsure on an implementation or have an idea you would like
25 to have RFC'ed, it surely is a good idea to send your questions and
26 suggestions to the Catalyst mailing list (See "SUPPORT" in Catalyst)
27 and/or come to the "#catalyst" channel on the "irc.perl.org" network.
28 You might also want to refer to those places for research to see if a
29 module doing what you're trying to implement already exists. This might
30 give you a solution to your problem or a basis for starting.
31
33 During Catalyst's early days, it was common to write plugins to provide
34 functionality application wide. Since then, Catalyst has become a lot
35 more flexible and powerful. It soon became a best practice to use some
36 other form of abstraction or interface, to keep the scope of its
37 influence as close as possible to where it belongs.
38 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 "Namespaces".
42 Quick Checklist
44 Use the "CatalystX::*" namespace if you can!
45 If your extension isn't a Model, View, Controller, Plugin, Engine,
46 or Log, it's best to leave it out of the "Catalyst::" namespace.
47 Use <CatalystX::> instead.
48 Don't make it a plugin unless you have to!
50 A plugin should be careful since it's overriding Catalyst
51 internals. If your plugin doesn't really need to muck with the
52 internals, make it a base Controller or Model.
53 Also, if you think you really need a plugin, please instead
55 consider using a Moose::Role.
56 There's a community. Use it!
58 There are many experienced developers in the Catalyst community,
59 there's always the IRC channel and the mailing list to discuss
60 things.
61 Add tests and documentation!
63 This gives a stable basis for contribution, and even more
64 importantly, builds trust. The easiest way is a test application.
65 See Catalyst::Manual::Tutorial::Testing for more information.
66 Namespaces
68 While some core extensions (engines, plugins, etc.) have to be placed
69 in the "Catalyst::*" namespace, the Catalyst core would like to ask
70 developers to use the "CatalystX::*" namespace if possible.
71 Please do not invent components which are outside the well known
73 "Model", "View", "Controller" or "Plugin" namespaces!
74 When you try to put a base class for a "Model", "View" or "Controller"
76 directly under your "MyApp" directory as, for example,
77 "MyApp::Controller::Foo", you will have the problem that Catalyst will
78 try to load that base class as a component of your application. The
79 solution is simple: Use another namespace. Common ones are
80 "MyApp::Base::Controller::*" or "MyApp::ControllerBase::*" as examples.
81 Can it be a simple module?
83 Sometimes you want to use functionality in your application that
84 doesn't require the framework at all. Remember that Catalyst is just
85 Perl and you always can just "use" a module. If you have application
86 specific code that doesn't need the framework, there is no problem in
87 putting it in your "MyApp::*" namespace. Just don't put it in "Model",
88 "Controller" or "View", because that would make Catalyst try to load
89 them as components.
90 Writing a generic component that only works with Catalyst is wasteful
92 of your time. Try writing a plain perl module, and then a small bit of
93 glue that integrates it with Catalyst. See
94 Catalyst::Model::DBIC::Schema for a module that takes the approach.
95 The advantage here is that your "Catalyst" DBIC schema works perfectly
96 outside of Catalyst, making testing (and command-line scripts) a
97 breeze. The actual Catalyst Model is just a few lines of glue that
98 makes working with the schema convenient.
99 If you want the thinnest interface possible, take a look at
101 Catalyst::Model::Adaptor.
102 Using Moose roles to apply method modifiers
104 Rather than having a complex set of base classes which you have to
105 mixin via multiple inheritance, if your functionality is well
106 structured, then it's possible to use the composability of Moose roles,
107 and method modifiers to hook onto to provide functionality.
108 These can be applied to your models/views/controllers, and your
110 application class, and shipped to CPAN. Please see
111 Catalyst::Manual::CatalystAndMoose for specific information about using
112 Roles in combination with Catalyst, and Moose::Manual::Roles for more
113 information about roles in general.
114 Inheritance and overriding methods
116 When overriding a method, keep in mind that some day additional
117 arguments may be provided to the method, if the last parameter is not a
118 flat list. It is thus better to override a method by shifting the
119 invocant off of @_ and assign the rest of the used arguments, so you
120 can pass your complete arguments to the original method via @_:
121 use MRO::Compat; ...
123 sub foo {
125 my $self = shift;
126 my ($bar, $baz) = @_; # ... return
127 $self->next::method(@_);
128 }
129 If you would do the common
131 my ($self, $foo, $bar) = @_;
133 you'd have to use a much uglier construct to ensure that all arguments
135 will be passed along and the method is future proof:
136 $self->next::method(@_[ 1 .. $#_ ]);
138 Tests and documentation
140 When you release your module to the CPAN, proper documentation and at
141 least a basic test suite (which means more than pod or even just
142 "use_ok", sorry) gives people a good base to contribute to the module.
143 It also shows that you care for your users. If you would like your
144 module to become a recommended addition, these things will prove
145 invaluable.
146 If you're just getting started, try using CatalystX::Starter to
148 generate some example tests for your module.
149 Maintenance
151 In planning to release a module to the community (Catalyst or CPAN and
152 Perl), you should consider if you have the resources to keep it up to
153 date, including fixing bugs and accepting contributions.
154 If you're not sure about this, you can always ask in the proper
156 Catalyst or Perl channels if someone else might be interested in the
157 project, and would jump in as co-maintainer.
158 A public repository can further ease interaction with the community.
160 Even read only access enables people to provide you with patches to
161 your current development version. subversion, SVN and SVK, are broadly
162 preferred in the Catalyst community.
163 If you're developing a Catalyst extension, please consider asking the
165 core team for space in Catalyst's own subversion repository. You can
166 get in touch about this via IRC or the Catalyst developers mailing
167 list.
168 The context object
170 Sometimes you want to get a hold of the context object in a component
171 that was created on startup time, where no context existed yet. Often
172 this is about the model reading something out of the stash or other
173 context information (current language, for example).
174 If you use the context object in your component you have tied it to an
176 existing request. This means that you might get into problems when you
177 try to use the component (e.g. the model - the most common case)
178 outside of Catalyst, for example in cronjobs.
179 A stable solution to this problem is to design the Catalyst model
181 separately from the underlying model logic. Let's take
182 Catalyst::Model::DBIC::Schema as an example. You can create a schema
183 outside of Catalyst that knows nothing about the web. This kind of
184 design ensures encapsulation and makes development and maintenance a
185 whole lot easier. The you use the aforementioned model to tie your
186 schema to your application. This gives you a "MyApp::DBIC" (the name is
187 of course just an example) model as well as "MyApp::DBIC::TableName"
188 models to access your result sources directly.
189 By creating such a thin layer between the actual model and the Catalyst
191 application, the schema itself is not at all tied to any application
192 and the layer in-between can access the model's API using information
193 from the context object.
194 A Catalyst component accesses the context object at request time with
196 "ACCEPT_CONTEXT($c, @args)" in Catalyst::Component.
197
199 The application has to interact with the extension with some
200 configuration. There is of course again more than one way to do it.
201 Attributes
203 You can specify any valid Perl attribute on Catalyst actions you like.
204 (See "Syntax of Attribute Lists" in attributes for a description of
205 what is valid.) These will be available on the Catalyst::Action
206 instance via its "attributes" accessor. To give an example, this
207 action:
208 sub foo : Local Bar('Baz') {
210 my ($self, $c) = @_;
211 my $attributes = $self->action_for('foo')->attributes;
212 $c->res->body($attributes->{Bar}[0] );
213 }
214 will set the response body to "Baz". The values always come in an array
216 reference. As you can see, you can use attributes to configure your
217 actions. You can specify or alter these attributes via "Component
218 Configuration", or even react on them as soon as Catalyst encounters
219 them by providing your own component base class.
220 Component Configuration
222 At creation time, the class configuration of your component (the one
223 available via "$self->config") will be merged with possible
224 configuration settings from the applications configuration (either
225 directly or via config file). This is done by Catalyst, and the
226 correctly merged configuration is passed to your component's
227 constructor (i.e. the new method).
228 Ergo, if you define an accessor for each configuration value that your
230 component takes, then the value will be automatically stored in the
231 controller object's hash reference, and available from the accessor.
232 The "config" accessor always only contains the original class
234 configuration and you MUST NEVER call "$self->config" to get your
235 component configuration, as the data there is likely to be a subset of
236 the correct config.
237 For example:
239 package MyApp
241 use Moose;
242 extends 'Catalyst';
244 ...
246 __PACKAGE__->config(
248 'Controller::Foo' => { some_value => 'bar' },
249 );
250 ...
252 package MyApp::Controller::Foo;
254 use Moose;
255 use namespace::autoclean;
256 BEGIN { extends 'Catalyst::Controller' };
257 has some_value ( is => 'ro', required => 1 );
259 sub some_method {
261 my $self = shift;
262 return "the value of 'some_value' is " . $self->some_value;
263 }
264 ...
266 my $controller = $c->controller('Foo');
268 warn $controller->some_value;
269 warn $controller->some_method;
270
272 This part contains the technical details of various implementation
273 methods. Please read the "BEST PRACTICES" before you start your
274 implementation, if you haven't already.
275 Action classes
277 Usually, your action objects are of the class Catalyst::Action. You
278 can override this with the "ActionClass" attribute to influence
279 execution and/or dispatching of the action. A widely used example of
280 this is Catalyst::Action::RenderView, which is used in every newly
281 created Catalyst application in your root controller:
282 sub end : ActionClass('RenderView') { }
284 Usually, you want to override the "execute" and/or the "match" method.
286 The execute method of the action will naturally call the methods code.
287 You can surround this by overriding the method in a subclass:
288 package Catalyst::Action::MyFoo;
290 use Moose;
291 use namespace::autoclean;
292 use MRO::Compat;
293 extends 'Catalyst::Action';
294 sub execute {
296 my $self = shift;
297 my ($controller, $c, @args) = @_;
298 # put your 'before' code here
299 my $r = $self->next::method(@_);
300 # put your 'after' code here
301 return $r;
302 }
303 1;
304 We are using MRO::Compat to ensure that you have the next::method call,
306 from Class::C3 (in older perls), or natively (if you are using perl
307 5.10) to re-dispatch to the original "execute" method in the
308 Catalyst::Action class.
309 The Catalyst dispatcher handles an incoming request and, depending upon
311 the dispatch type, will call the appropriate target or chain. From
312 time to time it asks the actions themselves, or through the controller,
313 if they would match the current request. That's what the "match" method
314 does. So by overriding this, you can change on what the action will
315 match and add new matching criteria.
316 For example, the action class below will make the action only match on
318 Mondays:
319 package Catalyst::Action::OnlyMondays;
321 use Moose;
322 use namespace::autoclean;
323 use MRO::Compat;
324 extends 'Catalyst::Action';
325 sub match {
327 my $self = shift;
328 return 0 if ( localtime(time) )[6] == 1;
329 return $self->next::method(@_);
330 }
331 1;
332 And this is how we'd use it:
334 sub foo: Local ActionClass('OnlyMondays') {
336 my ($self, $c) = @_;
337 $c->res->body('I feel motivated!');
338 }
339 If you are using action classes often or have some specific base
341 classes that you want to specify more conveniently, you can implement a
342 component base class providing an attribute handler.
343 It is not possible to use multiple action classes at once, however
345 Catalyst::Controller::ActionRole allows you to apply Moose Roles to
346 actions.
347 For further information on action classes and roles, please refer to
349 Catalyst::Action and Catalyst::Manual::Actions.
350 Component base classes
352 Many Catalyst::Plugin that were written in Catalyst's early days should
353 really have been just controller base classes. With such a class, you
354 could provide functionality scoped to a single controller, not
355 polluting the global namespace in the context object.
356 You can provide regular Perl methods in a base class as well as actions
358 which will be inherited to the subclass. Please refer to "Controllers"
359 for an example of this.
360 You can introduce your own attributes by specifying a handler method in
362 the controller base. For example, to use a "FullClass" attribute to
363 specify a fully qualified action class name, you could use the
364 following implementation. Note, however, that this functionality is
365 already provided via the "+" prefix for action classes. A simple
366 sub foo : Local ActionClass('+MyApp::Action::Bar') { ... }
368 will use "MyApp::Action::Bar" as action class.
370 package MyApp::Base::Controller::FullClass;
372 use Moose;
373 use namespace::autoclean;
374 BEGIN { extends 'Catalyst::Controller'; }
375 sub _parse_FullClass_attr {
377 my ($self, $app_class, $action_name, $value, $attrs) = @_;
378 return( ActionClass => $value );
379 }
380 1;
381 Note that the full line of arguments is only provided for completeness
383 sake. We could use this attribute in a subclass like any other Catalyst
384 attribute:
385 package MyApp::Controller::Foo;
387 use Moose;
388 use namespace::autoclean;
389 BEGIN { extends 'MyApp::Base::Controller::FullClass'; }
390 sub foo : Local FullClass('MyApp::Action::Bar') { ... }
392 1;
394 Controllers
396 Many things can happen in controllers, and it often improves
397 maintainability to abstract some of the code out into reusable base
398 classes.
399 You can provide usual Perl methods that will be available via your
401 controller object, or you can even define Catalyst actions which will
402 be inherited by the subclasses. Consider this controller base class:
403 package MyApp::Base::Controller::ModelBase;
405 use Moose;
406 use namespace::autoclean;
407 BEGIN { extends 'Catalyst::Controller'; }
409 sub list : Chained('base') PathPart('') Args(0) {
411 my ($self, $c) = @_;
412 my $model = $c->model( $self->{model_name} );
413 my $condition = $self->{model_search_condition} || {};
414 my $attrs = $self->{model_search_attrs} || {};
415 $c->stash(rs => $model->search($condition, $attrs);
416 }
417 sub load : Chained('base') PathPart('') CaptureArgs(1) {
419 my ($self, $c, $id) = @_;
420 my $model = $c->model( $self->{model_name} );
421 $c->stash(row => $model->find($id));
422 }
423 1;
424 This example implements two simple actions. The "list" action chains to
426 a (currently non-existent) "base" action and puts a result-set into the
427 stash taking a configured "model_name" as well as a search condition
428 and attributes. This action is a chained endpoint. The other action,
429 called " load " is a chain midpoint that takes one argument. It takes
430 the value as an ID and loads the row from the configured model. Please
431 not that the above code is simplified for clarity. It misses error
432 handling, input validation, and probably other things.
433 The class above is not very useful on its own, but we can combine it
435 with some custom actions by sub-classing it:
436 package MyApp::Controller::Foo;
438 use Moose;
439 use namespace::autoclean;
440 BEGIN { extends 'MyApp::Base::Controller::ModelBase'; }
442 __PACKAGE__->config( model_name => 'DB::Foo',
444 model_search_condition=> { is_active => 1 },
445 model_search_attrs => { order_by => 'name' },
446 );
447 sub base : Chained PathPart('foo') CaptureArgs(0) { }
449 sub view : Chained('load') Args(0) {
451 my ($self, $c) = @_;
452 my $row = $c->stash->{row};
453 $c->res->body(join ': ', $row->name,
454 $row->description); }
455 1;
456 This class uses the formerly created controller as a base class. First,
458 we see the configurations that were used in the parent class. Next
459 comes the "base" action, where everything chains off of.
460 Note that inherited actions act like they were declared in your
462 controller itself. You can therefor call them just by their name in
463 "forward"s, "detaches" and "Chained(..)" specifications. This is an
464 important part of what makes this technique so useful.
465 The new "view" action ties itself to the "load" action specified in the
467 base class and outputs the loaded row's "name" and "description"
468 columns. The controller "MyApp::Controller::Foo" now has these publicly
469 available paths:
470 /foo
472 Will call the controller's "base", then the base classes "list"
473 action.
474 /foo/$id/view
476 First, the controller's "base" will be called, then it will "load"
477 the row with the corresponding $id. After that, "view" will display
478 some fields out of the object.
479 Models and Views
481 If the functionality you'd like to add is really a data-set that you
482 want to manipulate, for example internal document types, images, files,
483 it might be better suited as a model.
484 The same applies for views. If your code handles representation or
486 deals with the applications interface and should be universally
487 available, it could be a perfect candidate for a view.
488 Please implement a "process" method in your views. This method will be
490 called by Catalyst if it is asked to forward to a component without a
491 specified action. Note that "process" is not a Catalyst action but a
492 simple Perl method.
493 You are also encouraged to implement a "render" method corresponding
495 with the one in Catalyst::View::TT. This has proven invaluable, because
496 people can use your view for much more fine-grained content generation.
497 Here is some example code for a fictional view:
499 package Catalyst::View::MyView;
501 use Moose;
502 use namespace::autoclean;
503 extends 'Catalyst::View';
505 sub process {
507 my ($self, $c) = @_;
508 my $template = $c->stash->{template};
509 my $content = $self->render($c, $template, $c->stash);
510 $c->res->body( $content );
511 }
512 sub render {
514 my ($self, $c, $template, $args) = @_;
515 # prepare content here
516 return $content;
517 }
518 1;
519 Plugins
521 The first thing to say about plugins is that if you're not sure if your
522 module should be a plugin, it probably shouldn't. It once was common to
523 add features to Catalyst by writing plugins that provide accessors to
524 said functionality. As Catalyst grew more popular, it became obvious
525 that this qualifies as bad practice.
526 By designing your module as a Catalyst plugin, every method you
528 implement, import or inherit will be available via your applications
529 context object. A plugin pollutes the global namespace, and you should
530 be only doing that when you really need to.
531 Often, developers design extensions as plugins because they need to get
533 hold of the context object. Either to get at the stash or
534 request/response objects are the widely spread reasons. It is, however,
535 perfectly possible to implement a regular Catalyst component (read:
536 model, view or controller) that receives the current context object via
537 "ACCEPT_CONTEXT($c, @args)" in Catalyst::Component.
538 When is a plugin suited to your task? Your code needs to be a plugin to
540 act upon or alter specific parts of Catalyst's request lifecycle. If
541 your functionality needs to change some "prepare_*" or "finalize_*"
542 stages, you won't get around a plugin.
543 Note, if you just want to hook into such a stage, and run code before,
545 or after it, then it is recommended that you use Moose's method
546 modifiers to do this.
547 Another valid target for a plugin architecture are things that really
549 have to be globally available, like sessions or authentication.
550 Please do not release Catalyst extensions as plugins only to provide
552 some functionality application wide. Design it as a controller base
553 class or another better suited technique with a smaller scope, so that
554 your code only influences those parts of the application where it is
555 needed, and namespace clashes and conflicts are ruled out.
556 The implementation is pretty easy. Your plugin will be inserted in the
558 application's inheritance list, above Catalyst itself. You can by this
559 alter Catalyst's request lifecycle behaviour. Every method you declare,
560 every import in your package will be available as method on the
561 application and the context object. As an example, let's say you want
562 Catalyst to warn you every time uri_for was called without an action
563 object as the first parameter, for example to test that all your
564 chained uris are generated from actions (a recommended best practice).
565 You could do this with this simple implementation (excuse the lame
566 class name, it's just an example):
567 package Catalyst::Plugin::UriforUndefWarning;
569 use strict;
570 use Scalar::Util qw/blessed/;
571 use MRO::Compat;
572 sub uri_for {
574 my $c = shift;
575 my $uri = $c->next::method(@_);
576 $c->log->warn( 'uri_for with non action: ', join(', ', @_), )
577 if (!blessed($_[0]) || !$_[0]->isa('Catalyst::Action'));
578 return $uri;
579 }
580 1;
582 This would override Catalyst's "uri_for" method and emit a "warn" log
584 entry containing the arguments to uri_for.
585 Please note this is not a practical example, as string URLs are fine
587 for static content etc.
588 A simple example like this is actually better as a Moose role, for
590 example:
591 package CatalystX::UriforUndefWarning;
593 use Moose::Role;
594 use namespace::autoclean;
595 after 'uri_for' => sub {
597 my ($c, $arg) = @_;
598 $c->log->warn( 'uri_for with non action: ', join(', ', @_), )
599 if (!blessed($_[0]) || !$_[0]->isa('Catalyst::Action'));
600 return $uri;
601 };
602 Note that Catalyst will load any Moose Roles in the plugin list, and
604 apply them to your application class.
605 Factory components with COMPONENT()
607 Every component inheriting from Catalyst::Component contains a
608 "COMPONENT" method. It is used on application startup by
609 "setup_components" to instantiate the component object for the Catalyst
610 application. By default, this will merge the components own
611 "config"uration with the application wide overrides and call the class'
612 "new" method to return the component object.
613 You can override this method and do and return whatever you want.
615 However, you should use Class::C3 (via MRO::Compat) to forward to the
616 original "COMPONENT" method to merge the configuration of your
617 component.
618 Here is a stub "COMPONENT" method:
620 package CatalystX::Component::Foo;
622 use Moose;
623 use namespace::autoclean;
624 extends 'Catalyst::Component';
626 sub COMPONENT {
628 my $class = shift;
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) = @_;
633 # Do things here before instantiation
635 $new = $class->next::method(@_);
636 # Do things to object after instantiation
637 return $new;
638 }
639 The arguments are the class name of the component, the class name of
641 the application instantiating the component, and a hash reference with
642 the controller's configuration.
643 You are free to re-bless the object, instantiate a whole other
645 component or really do anything compatible with Catalyst's expectations
646 on a component.
647 For more information, please see "COMPONENT($c,$arguments)" in
649 Catalyst::Component.
650 Applying roles to parts of the framework
652 CatalystX::RoleApplicator will allow you to apply Roles to the
653 following classes:
654 Request
656 Response
657 Engine
658 Dispatcher
659 Stats
660 These roles can add new methods to these classes, or wrap preexisting
662 methods.
663 The namespace for roles like this is "Catalyst::TraitFor::XXX::YYYY".
665 For an example of a CPAN component implemented in this manor, see
667 Catalyst::TraitFor::Request::BrowserDetect.
668
670 Catalyst, Catalyst::Manual::Actions, Catalyst::Component
671
673 Catalyst Contributors, see Catalyst.pm
674
676 This library is free software. You can redistribute it and/or modify it
677 under the same terms as Perl itself.
678perl v5.32.1 2021-01C-a2t6alyst::Manual::ExtendingCatalyst(3)