Quick Checklist

Use the "CatalystX::*" namespace if you can!

If your extension isn't a Model, View, Controller, Plugin, Engine, or Log, it's best to leave it out of the "Catalyst::" namespace. Use <CatalystX::> instead.

Don't make it a plugin unless you have to!

A plugin should be careful since it's overriding Catalyst internals. If your plugin doesn't really need to muck with the internals, make it a base Controller or Model.

Also, if you think you really need a plugin, please instead consider using a Moose::Role.

There's a community. Use it!

There are many experienced developers in the Catalyst community, there's always the IRC channel and the mailing list to discuss things.

Add tests and documentation!

This gives a stable basis for contribution, and even more importantly, builds trust. The easiest way is a test application. See Catalyst::Manual::Tutorial::Testing for more information.

Namespaces

Please do not invent components which are outside the well known "Model", "View", "Controller" or "Plugin" namespaces!

When you try to put a base class for a "Model", "View" or "Controller" directly under your "MyApp" directory as, for example, "MyApp::Controller::Foo", you will have the problem that Catalyst will try to load that base class as a component of your application. The solution is simple: Use another namespace. Common ones are "MyApp::Base::Controller::*" or "MyApp::ControllerBase::*" as examples.

Can it be a simple module?

Writing a generic component that only works with Catalyst is wasteful of your time. Try writing a plain perl module, and then a small bit of glue that integrates it with Catalyst. See Catalyst::Model::DBIC::Schema for a module that takes the approach. The advantage here is that your "Catalyst" DBIC schema works perfectly outside of Catalyst, making testing (and command-line scripts) a breeze. The actual Catalyst Model is just a few lines of glue that makes working with the schema convenient.

If you want the thinnest interface possible, take a look at Catalyst::Model::Adaptor.

Using Moose roles to apply method modifiers

These can be applied to your models/views/controllers, and your application class, and shipped to CPAN. Please see Catalyst::Manual::CatalystAndMoose for specific information about using Roles in combination with Catalyst, and Moose::Manual::Roles for more information about roles in general.

Inheritance and overriding methods

  use MRO::Compat; ...

  sub foo {
    my $self = shift;
    my ($bar, $baz) = @_; # ...  return
    $self->next::method(@_);
  }

If you would do the common

  my ($self, $foo, $bar) = @_;

you'd have to use a much uglier construct to ensure that all arguments will be passed along and the method is future proof:

  $self->next::method(@_[ 1 .. $#_ ]);

Tests and documentation

If you're just getting started, try using CatalystX::Starter to generate some example tests for your module.

Maintenance

If you're not sure about this, you can always ask in the proper Catalyst or Perl channels if someone else might be interested in the project, and would jump in as co-maintainer.

A public repository can further ease interaction with the community. Even read only access enables people to provide you with patches to your current development version. subversion, SVN and SVK, are broadly preferred in the Catalyst community.

If you're developing a Catalyst extension, please consider asking the core team for space in Catalyst's own subversion repository. You can get in touch about this via IRC or the Catalyst developers mailing list.

The context object

If you use the context object in your component you have tied it to an existing request. This means that you might get into problems when you try to use the component (e.g. the model - the most common case) outside of Catalyst, for example in cronjobs.

A stable solution to this problem is to design the Catalyst model separately from the underlying model logic. Let's take Catalyst::Model::DBIC::Schema as an example. You can create a schema outside of Catalyst that knows nothing about the web. This kind of design ensures encapsulation and makes development and maintenance a whole lot easier. The you use the aforementioned model to tie your schema to your application. This gives you a "MyApp::DBIC" (the name is of course just an example) model as well as "MyApp::DBIC::TableName" models to access your result sources directly.

By creating such a thin layer between the actual model and the Catalyst application, the schema itself is not at all tied to any application and the layer in-between can access the model's API using information from the context object.

A Catalyst component accesses the context object at request time with "ACCEPT_CONTEXT($c, @args)" in Catalyst::Component.

Attributes

  sub foo : Local Bar('Baz') {
      my ($self, $c) = @_;
      my $attributes = $self->action_for('foo')->attributes;
      $c->res->body($attributes->{Bar}[0] );
  }

will set the response body to "Baz". The values always come in an array reference. As you can see, you can use attributes to configure your actions. You can specify or alter these attributes via "Component Configuration", or even react on them as soon as Catalyst encounters them by providing your own component base class.

Component Configuration

Ergo, if you define an accessor for each configuration value that your component takes, then the value will be automatically stored in the controller object's hash reference, and available from the accessor.

The "config" accessor always only contains the original class configuration and you MUST NEVER call "$self->config" to get your component configuration, as the data there is likely to be a subset of the correct config.

For example:

  package MyApp
  use Moose;

  extends 'Catalyst';

  ...

  __PACKAGE__->config(
    'Controller::Foo' => { some_value => 'bar' },
  );

  ...

  package MyApp::Controller::Foo;
  use Moose;
  use namespace::autoclean;
  BEGIN { extends 'Catalyst::Controller' };

  has some_value ( is => 'ro', required => 1 );

  sub some_method {
      my $self = shift;
      return "the value of 'some_value' is " . $self->some_value;
  }

  ...

  my $controller = $c->controller('Foo');
  warn $controller->some_value;
  warn $controller->some_method;

Action classes

  sub end : ActionClass('RenderView') { }

Usually, you want to override the "execute" and/or the "match" method. The execute method of the action will naturally call the methods code. You can surround this by overriding the method in a subclass:

  package Catalyst::Action::MyFoo;
  use Moose;
  use namespace::autoclean;
  use MRO::Compat;
  extends 'Catalyst::Action';

  sub execute {
      my $self = shift;
      my ($controller, $c, @args) = @_;
      # put your 'before' code here
      my $r = $self->next::method(@_);
      # put your 'after' code here
      return $r;
  }
  1;

We are using MRO::Compat to ensure that you have the next::method call, from Class::C3 (in older perls), or natively (if you are using perl 5.10) to re-dispatch to the original "execute" method in the Catalyst::Action class.

The Catalyst dispatcher handles an incoming request and, depending upon the dispatch type, will call the appropriate target or chain. From time to time it asks the actions themselves, or through the controller, if they would match the current request. That's what the "match" method does. So by overriding this, you can change on what the action will match and add new matching criteria.

For example, the action class below will make the action only match on Mondays:

  package Catalyst::Action::OnlyMondays;
  use Moose;
  use namespace::autoclean;
  use MRO::Compat;
  extends 'Catalyst::Action';

  sub match {
      my $self = shift;
      return 0 if ( localtime(time) )[6] == 1;
      return $self->next::method(@_);
   }
  1;

And this is how we'd use it:

  sub foo: Local ActionClass('OnlyMondays') {
      my ($self, $c) = @_;
      $c->res->body('I feel motivated!');
  }

If you are using action classes often or have some specific base classes that you want to specify more conveniently, you can implement a component base class providing an attribute handler.

It is not possible to use multiple action classes at once, however Catalyst::Controller::ActionRole allows you to apply Moose Roles to actions.

For further information on action classes and roles, please refer to Catalyst::Action and Catalyst::Manual::Actions.

Component base classes

You can provide regular Perl methods in a base class as well as actions which will be inherited to the subclass. Please refer to "Controllers" for an example of this.

You can introduce your own attributes by specifying a handler method in the controller base. For example, to use a "FullClass" attribute to specify a fully qualified action class name, you could use the following implementation. Note, however, that this functionality is already provided via the "+" prefix for action classes. A simple

  sub foo : Local ActionClass('+MyApp::Action::Bar') { ... }

will use "MyApp::Action::Bar" as action class.

  package MyApp::Base::Controller::FullClass;
  use Moose;
  use namespace::autoclean;
  BEGIN { extends 'Catalyst::Controller'; }

  sub _parse_FullClass_attr {
      my ($self, $app_class, $action_name, $value, $attrs) = @_;
      return( ActionClass => $value );
  }
  1;

Note that the full line of arguments is only provided for completeness sake. We could use this attribute in a subclass like any other Catalyst attribute:

  package MyApp::Controller::Foo;
  use Moose;
  use namespace::autoclean;
  BEGIN { extends 'MyApp::Base::Controller::FullClass'; }

  sub foo : Local FullClass('MyApp::Action::Bar') { ... }

  1;

Controllers

You can provide usual Perl methods that will be available via your controller object, or you can even define Catalyst actions which will be inherited by the subclasses. Consider this controller base class:

  package MyApp::Base::Controller::ModelBase;
  use Moose;
  use namespace::autoclean;

  BEGIN { extends 'Catalyst::Controller'; }

  sub list : Chained('base') PathPart('') Args(0) {
      my ($self, $c) = @_;
      my $model = $c->model( $self->{model_name} );
      my $condition = $self->{model_search_condition} || {};
      my $attrs = $self->{model_search_attrs} || {};
      $c->stash(rs => $model->search($condition, $attrs);
  }

  sub load : Chained('base') PathPart('') CaptureArgs(1) {
      my ($self, $c, $id) = @_;
      my $model = $c->model( $self->{model_name} );
      $c->stash(row => $model->find($id));
  }
  1;

This example implements two simple actions. The "list" action chains to a (currently non-existent) "base" action and puts a result-set into the stash taking a configured "model_name" as well as a search condition and attributes. This action is a chained endpoint. The other action, called " load " is a chain midpoint that takes one argument. It takes the value as an ID and loads the row from the configured model. Please not that the above code is simplified for clarity. It misses error handling, input validation, and probably other things.

The class above is not very useful on its own, but we can combine it with some custom actions by sub-classing it:

  package MyApp::Controller::Foo;
  use Moose;
  use namespace::autoclean;

  BEGIN { extends 'MyApp::Base::Controller::ModelBase'; }

  __PACKAGE__->config( model_name => 'DB::Foo',
                       model_search_condition=> { is_active => 1 },
                       model_search_attrs => { order_by => 'name' },
                   );

  sub base : Chained PathPart('foo') CaptureArgs(0) { }

  sub view : Chained('load') Args(0) {
      my ($self, $c) = @_;
      my $row = $c->stash->{row};
      $c->res->body(join ': ', $row->name,
      $row->description); }
  1;

This class uses the formerly created controller as a base class. First, we see the configurations that were used in the parent class. Next comes the "base" action, where everything chains off of.

Note that inherited actions act like they were declared in your controller itself. You can therefor call them just by their name in "forward"s, "detaches" and "Chained(..)" specifications. This is an important part of what makes this technique so useful.

The new "view" action ties itself to the "load" action specified in the base class and outputs the loaded row's "name" and "description" columns. The controller "MyApp::Controller::Foo" now has these publicly available paths:

/foo

Will call the controller's "base", then the base classes "list" action.

/foo/$id/view

First, the controller's "base" will be called, then it will "load" the row with the corresponding $id. After that, "view" will display some fields out of the object.

Models and Views

The same applies for views. If your code handles representation or deals with the applications interface and should be universally available, it could be a perfect candidate for a view.

Please implement a "process" method in your views. This method will be called by Catalyst if it is asked to forward to a component without a specified action. Note that "process" is not a Catalyst action but a simple Perl method.

You are also encouraged to implement a "render" method corresponding with the one in Catalyst::View::TT. This has proven invaluable, because people can use your view for much more fine-grained content generation.

Here is some example code for a fictional view:

  package Catalyst::View::MyView;
  use Moose;
  use namespace::autoclean;

  extends 'Catalyst::View';

  sub process {
      my ($self, $c) = @_;
      my $template = $c->stash->{template};
      my $content = $self->render($c, $template, $c->stash);
      $c->res->body( $content );
  }

  sub render {
      my ($self, $c, $template, $args) = @_;
      # prepare content here
      return $content;
  }
  1;

Plugins

By designing your module as a Catalyst plugin, every method you implement, import or inherit will be available via your applications context object. A plugin pollutes the global namespace, and you should be only doing that when you really need to.

Often, developers design extensions as plugins because they need to get hold of the context object. Either to get at the stash or request/response objects are the widely spread reasons. It is, however, perfectly possible to implement a regular Catalyst component (read: model, view or controller) that receives the current context object via "ACCEPT_CONTEXT($c, @args)" in Catalyst::Component.

When is a plugin suited to your task? Your code needs to be a plugin to act upon or alter specific parts of Catalyst's request lifecycle. If your functionality needs to change some "prepare_*" or "finalize_*" stages, you won't get around a plugin.

Note, if you just want to hook into such a stage, and run code before, or after it, then it is recommended that you use Moose's method modifiers to do this.

Another valid target for a plugin architecture are things that really have to be globally available, like sessions or authentication.

Please do not release Catalyst extensions as plugins only to provide some functionality application wide. Design it as a controller base class or another better suited technique with a smaller scope, so that your code only influences those parts of the application where it is needed, and namespace clashes and conflicts are ruled out.

The implementation is pretty easy. Your plugin will be inserted in the application's inheritance list, above Catalyst itself. You can by this alter Catalyst's request lifecycle behaviour. Every method you declare, every import in your package will be available as method on the application and the context object. As an example, let's say you want Catalyst to warn you every time uri_for was called without an action object as the first parameter, for example to test that all your chained uris are generated from actions (a recommended best practice). You could do this with this simple implementation (excuse the lame class name, it's just an example):

  package Catalyst::Plugin::UriforUndefWarning;
  use strict;
  use Scalar::Util qw/blessed/;
  use MRO::Compat;

  sub uri_for {
      my $c = shift;
      my $uri = $c->next::method(@_);
      $c->log->warn( 'uri_for with non action: ', join(', ', @_), )
        if (!blessed($_[0]) || !$_[0]->isa('Catalyst::Action'));
      return $uri;
  }

  1;

This would override Catalyst's "uri_for" method and emit a "warn" log entry containing the arguments to uri_for.

Please note this is not a practical example, as string URLs are fine for static content etc.

A simple example like this is actually better as a Moose role, for example:

  package CatalystX::UriforUndefWarning;
  use Moose::Role;
  use namespace::autoclean;

  after 'uri_for' => sub {
    my ($c, $arg) = @_;
    $c->log->warn( 'uri_for with non action: ', join(', ', @_), )
      if (!blessed($_[0]) || !$_[0]->isa('Catalyst::Action'));
    return $uri;
  };

Note that Catalyst will load any Moose Roles in the plugin list, and apply them to your application class.

Factory components with COMPONENT()

You can override this method and do and return whatever you want. However, you should use Class::C3 (via MRO::Compat) to forward to the original "COMPONENT" method to merge the configuration of your component.

Here is a stub "COMPONENT" method:

  package CatalystX::Component::Foo;
  use Moose;
  use namespace::autoclean;

  extends 'Catalyst::Component';

  sub COMPONENT {
      my $class = shift;
      # Note: $app is like $c, but since the application isn't fully
      # initialized, we don't want to call it $c yet.  $config
      # is a hashref of config options possibly set on this component.
      my ($app, $config) = @_;

      # Do things here before instantiation
      $new = $class->next::method(@_);
      # Do things to object after instantiation
      return $new;
  }

The arguments are the class name of the component, the class name of the application instantiating the component, and a hash reference with the controller's configuration.

You are free to re-bless the object, instantiate a whole other component or really do anything compatible with Catalyst's expectations on a component.

For more information, please see "COMPONENT($c,$arguments)" in Catalyst::Component.

Applying roles to parts of the framework

Request

Response

Engine

Dispatcher

Stats

These roles can add new methods to these classes, or wrap preexisting methods.

The namespace for roles like this is "Catalyst::TraitFor::XXX::YYYY".

For an example of a CPAN component implemented in this manor, see Catalyst::TraitFor::Request::BrowserDetect.