NAME

Catalyst::Manual::Intro - Introduction to Catalyst

DESCRIPTION

This is a brief introduction to Catalyst. It explains the most important features of how Catalyst works and shows how to get a simple application up and running quickly. For an introduction (without code) to Catalyst itself, and why you should be using it, see the Catalyst::Manual::About manpage. For a systematic step-by-step introduction to writing an application with Catalyst, see the Catalyst::Manual::Tutorial manpage.

What is Catalyst?

Catalyst is an elegant web application framework, extremely flexible yet extremely simple. It's similar to Ruby on Rails, Spring (Java), and Maypole, upon which it was originally based. Its most important design philosphy is to provide easy access to all the tools you need to develop web applications, with few restrictions on how you need to use these tools. However, this does mean that it is always possible to do things in a different way. Other web frameworks are initially simpler to use, but achieve this by locking the programmer into a single set of tools. Catalyst's emphasis on flexibility means that you have to think more to use it. We view this as a feature. For example, this leads to Catalyst being more suited to system integration tasks than other web frameworks.

MVC

Catalyst follows the Model-View-Controller (MVC) design pattern, allowing you to easily separate concerns, like content, presentation, and flow control, into separate modules. This separation allows you to modify code that handles one concern without affecting code that handles the others. Catalyst promotes the re-use of existing Perl modules that already handle common web application concerns well.

Here's how the Model, View, and Controller map to those concerns, with examples of well-known Perl modules you may want to use for each.

If you're unfamiliar with MVC and design patterns, you may want to check out the original book on the subject, Design Patterns, by Gamma, Helm, Johnson, and Vlissides, also known as the Gang of Four (GoF). Many, many web application frameworks are based on MVC, which is becoming a popular design paradigm for the world wide web.

Flexibility

Catalyst is much more flexible than many other frameworks. Rest assured you can use your favorite Perl modules with Catalyst.

Simplicity

The best part is that Catalyst implements all this flexibility in a very simple way.

Quickstart

Here's how to install Catalyst and get a simple application up and running, using the helper scripts described above.

Install

Installation of Catalyst can be a time-consuming and frustrating effort, due to its large number of dependencies. The easiest way to get up and running is to use Matt Trout's cat-install script, from http://www.shadowcatsystems.co.uk/static/cat-install, and then install the Catalyst::Devel manpage.

    # perl cat-install
    # perl -MCPAN -e 'install Catalyst::Devel'

Setup

    $ catalyst.pl MyApp
    # output omitted
    $ cd MyApp
    $ script/myapp_create.pl controller Library::Login

Run

    $ script/myapp_server.pl

Now visit these locations with your favorite browser or user agent to see Catalyst in action:

(NOTE: Although we create a controller here, we don't actually use it. Both of these URLs should take you to the welcome page.)

http://localhost:3000/
http://localhost:3000/library/login/

How It Works

Let's see how Catalyst works, by taking a closer look at the components and other parts of a Catalyst application.

Components

Catalyst has an uncommonly flexible component system. You can define as many Models, Views, and Controllers as you like. As discussed previously, the general idea is that the View is responsible for the output of data to the user (typically via a web browser, but a View can also generate PDFs or e-mails, for example); the Model is responsible for providing data (typically from a relational database); and the Controller is responsible for interacting with the user and deciding how user input determines what actions the application takes.

In the world of MVC, there are frequent discussions and disagreements about the nature of each element - whether certain types of logic belong in the Model or the Controller, etc. Catalyst's flexibility means that this decision is entirely up to you, the programmer; Catalyst doesn't enforce anything. See the Catalyst::Manual::About manpage for a general discussion of these issues.

All components must inherit from the Catalyst::Base manpage, which provides a simple class structure and some common class methods like config and new (constructor).

    package MyApp::Controller::Catalog;
    use strict;
    use base 'Catalyst::Base';
    __PACKAGE__->config( foo => 'bar' );
    1;

You don't have to use or otherwise register Models, Views, and Controllers. Catalyst automatically discovers and instantiates them when you call setup in the main application. All you need to do is put them in directories named for each Component type. You can use a short alias for each one.

In older versions of Catalyst, the recommended practice (and the one automatically created by helper scripts) was to name the directories M/, V/, and C/. Though these still work, we now recommend the use of the full names.

Views

To show how to define views, we'll use an already-existing base class for the Template Toolkit, the Catalyst::View::TT manpage. All we need to do is inherit from this class:

    package MyApp::View::TT;
    use strict;
    use base 'Catalyst::View::TT';
    1;

(You can also generate this automatically by using the helper script:

    script/myapp_create.pl view TT TT

where the first TT tells the script that the name of the view should be TT, and the second that it should be a Template Toolkit view.)

This gives us a process() method and we can now just do $c->forward('MyApp::View::TT') to render our templates. The base class makes process() implicit, so we don't have to say $c->forward(qw/MyApp::View::TT process/).

    sub hello : Global {
        my ( $self, $c ) = @_;
        $c->stash->{template} = 'hello.tt';
    }
    sub end : Private {
        my ( $self, $c ) = @_;
        $c->forward( $c->view('TT') );
    }

You normally render templates at the end of a request, so it's a perfect use for the global end action.

In practice, however, you would use a default end action as supplied by the Catalyst::Action::RenderView manpage.

Also, be sure to put the template under the directory specified in $c->config->{root}, or you'll end up looking at the debug screen.

Models

Models are providers of data. This data could come from anywhere - a search engine index, a spreadsheet, the file system - but typically a Model represents a database table. The data source does not intrinsically have much to do with web applications or Catalyst - it could just as easily be used to write an offline report generator or a command-line tool.

To show how to define models, again we'll use an already-existing base class, this time for the DBIx::Class manpage: the Catalyst::Model::DBIC::Schema manpage. We'll also need the DBIx::Class::Schema::Loader manpage.

But first, we need a database.

    -- myapp.sql
    CREATE TABLE foo (
        id INTEGER PRIMARY KEY,
        data TEXT
    );
    CREATE TABLE bar (
        id INTEGER PRIMARY KEY,
        foo INTEGER REFERENCES foo,
        data TEXT
    );
    INSERT INTO foo (data) VALUES ('TEST!');
    % sqlite /tmp/myapp.db < myapp.sql

Now we can create a DBIC::Schema model for this database.

    script/myapp_create.pl model MyModel DBIC::Schema MySchema create=static 'dbi:SQLite:/tmp/myapp.db'

the DBIx::Class::Schema::Loader manpage automatically loads table layouts and relationships, and converts them into a static schema definition MySchema, which you can edit later.

Use the stash to pass data to your templates.

We add the following to MyApp/Controller/Root.pm

    sub view : Global {
        my ( $self, $c, $id ) = @_;
        
        $c->stash->{item} = $c->model('MyModel::Foo')->find($id);
    }
    1;
    
    sub end : Private {
        my ( $self, $c ) = @_;
        
        $c->stash->{template} ||= 'index.tt';
        $c->forward( $c->view('TT') );
    }

We then create a new template file ``root/index.tt'' containing:

    The Id's data is [% item.data %]

Models do not have to be part of your Catalyst application; you can always call an outside module that serves as your Model:

    # in a Controller
    sub list : Local {
      my ( $self, $c ) = @_;
      
      $c->stash->{template} = 'list.tt';
      
      use Some::Outside::Database::Module;
      my @records = Some::Outside::Database::Module->search({
        artist => 'Led Zeppelin',
        });
      
      $c->stash->{records} = \@records;
    }

But by using a Model that is part of your Catalyst application, you gain several things: you don't have to use each component, Catalyst will find and load it automatically at compile-time; you can forward to the module, which can only be done to Catalyst components. Only Catalyst components can be fetched with $c->model('SomeModel').

Happily, since many people have existing Model classes that they would like to use with Catalyst (or, conversely, they want to write Catalyst models that can be used outside of Catalyst, e.g. in a cron job), it's trivial to write a simple component in Catalyst that slurps in an outside Model:

    package MyApp::Model::DB;
    use base qw/Catalyst::Model::DBIC::Schema/;
    __PACKAGE__->config(
        schema_class => 'Some::DBIC::Schema',
        connect_info => ['dbi:SQLite:foo.db', '', '', {AutoCommit=>1}]
    );
    1;

and that's it! Now Some::DBIC::Schema is part of your Cat app as MyApp::Model::DB.

Within Catalyst, the common approach to writing a model for your application is wrapping a generic model (e.g. the DBIx::Class::Schema manpage, a bunch of XMLs, or anything really) with an object that contains configuration data, convenience methods, and so forth. Thus you will in effect have two models - a wrapper model that knows something about Catalyst and your web application, and a generic model that is totally independent of these needs.

Technically, within Catalyst a model is a component - an instance of the model's class belonging to the application. It is important to stress that the lifetime of these objects is per application, not per request.

While the model base class (the Catalyst::Model manpage) provides things like config to better integrate the model into the application, sometimes this is not enough, and the model requires access to $c itself.

Situations where this need might arise include:

From a style perspective it's usually considered bad form to make your model ``too smart'' about things - it should worry about business logic and leave the integration details to the controllers. If, however, you find that it does not make sense at all to use an auxillary controller around the model, and the model's need to access $c cannot be sidestepped, there exists a power tool called ACCEPT_CONTEXT.

Controllers

Multiple controllers are a good way to separate logical domains of your application.

    package MyApp::Controller::Login;
    use base qw/Catalyst::Controller/;
    sub login : Path("login") { }
    sub new_password : Path("new-password") { }
    sub logout : Path("logout") { }
    package MyApp::Controller::Catalog;
    use base qw/Catalyst::Controller/;
    sub view : Local { }
    sub list : Local { }
    package MyApp::Controller::Cart;
    use base qw/Catalyst::Controller/;
    sub add : Local { }
    sub update : Local { }
    sub order : Local { }

Note that you can also supply attributes via the Controller's config so long as you have at least one attribute on a subref to be exported (:Action is commonly used for this) - for example the following is equivalent to the same controller above:

    package MyApp::Controller::Login;
    use base qw/Catalyst::Controller/;
    __PACKAGE__->config(
      actions => {
        'sign_in' => { Path => 'sign-in' },
        'new_password' => { Path => 'new-password' },
        'sign_out' => { Path => 'sign-out' },
      },
    );
    sub sign_in : Action { }
    sub new_password : Action { }
    sub sign_out : Action { }

ACCEPT_CONTEXT

Whenever you call $c->component(``Foo'') you get back an object - the instance of the model. If the component supports the ACCEPT_CONTEXT method instead of returning the model itself, the return value of < $model-ACCEPT_CONTEXT( $c ) >> will be used.

This means that whenever your model/view/controller needs to talk to $c it gets a chance to do this when it's needed.

A typical ACCEPT_CONTEXT method will either clone the model and return one with the context object set, or it will return a thin wrapper that contains $c and delegates to the per-application model object.

A typical ACCEPT_CONTEXT method could look like this:

    sub ACCEPT_CONTEXT {
      my ( $self, $c, @extra_arguments ) = @_;
      bless { %$self, c => $c }, ref($self);
    }

effectively treating $self as a prototype object that gets a new parameter. @extra_arguments comes from any trailing arguments to $c->component( $bah, @extra_arguments ) (or $c->model(...), $c->view(...) etc).

The life time of this value is per usage, and not per request. To make this per request you can use the following technique:

Add a field to $c, like my_model_instance. Then write your ACCEPT_CONTEXT method to look like this:

    sub ACCEPT_CONTEXT {
      my ( $self, $c ) = @_;
      if ( my $per_request = $c->my_model_instance ) {
        return $per_request;
      } else {
        my $new_instance = bless { %$self, c => $c }, ref($self);
        Scalar::Util::weaken($new_instance->{c}); # or we have a circular reference
        $c->my_model_instance( $new_instance );
        return $new_instance;
      }
    }

Application Class

In addition to the Model, View, and Controller components, there's a single class that represents your application itself. This is where you configure your application, load plugins, and extend Catalyst.

    package MyApp;
    use strict;
    use Catalyst qw/-Debug/; # Add other plugins here, e.g.
                             # for session support
    MyApp->config(
        name => 'My Application',
        # You can put anything else you want in here:
        my_configuration_variable => 'something',
    );
    1;

In older versions of Catalyst, the application class was where you put global actions. However, as of version 5.66, the recommended practice is to place such actions in a special Root controller (see Actions, below), to avoid namespace collisions.

Optionally, you can specify a root parameter for templates and static data. If omitted, Catalyst will try to auto-detect the directory's location. You can define as many parameters as you want for plugins or whatever you need. You can access them anywhere in your application via $context->config->{$param_name}.

Context

Catalyst automatically blesses a Context object into your application class and makes it available everywhere in your application. Use the Context to directly interact with Catalyst and glue your Components together. For example, if you need to use the Context from within a Template Toolkit template, it's already there:

    <h1>Welcome to [% c.config.name %]!</h1>

As illustrated in our URL-to-Action dispatching example, the Context is always the second method parameter, behind the Component object reference or class name itself. Previously we called it $context for clarity, but most Catalyst developers just call it $c:

    sub hello : Global {
        my ( $self, $c ) = @_;
        $c->res->body('Hello World!');
    }

The Context contains several important objects:

The last of these, the stash, is a universal hash for sharing data among application components. For an example, we return to our 'hello' action:

    sub hello : Global {
        my ( $self, $c ) = @_;
        $c->stash->{message} = 'Hello World!';
        $c->forward('show_message');
    }
    sub show_message : Private {
        my ( $self, $c ) = @_;
        $c->res->body( $c->stash->{message} );
    }

Note that the stash should be used only for passing data in an individual request cycle; it gets cleared at a new request. If you need to maintain persistent data, use a session. See the Catalyst::Plugin::Session manpage for a comprehensive set of Catalyst-friendly session-handling tools.

Actions

A Catalyst controller is defined by its actions. An action is a subroutine with a special attribute. You've already seen some examples of actions in this document. The URL (for example http://localhost.3000/foo/bar) consists of two parts, the base (http://localhost:3000/ in this example) and the path (foo/bar). Please note that the trailing slash after the hostname[:port] always belongs to base and not to the action.

Action types

Catalyst supports several types of actions:

Note: After seeing these examples, you probably wonder what the point is of defining names for regex and path actions. Every public action is also a private one, so you have one unified way of addressing components in your forwards.

Built-in Private Actions

In response to specific application states, Catalyst will automatically call these built-in private actions in your application class:

Built-in actions in controllers/autochaining

    Package MyApp::Controller::Foo;
    sub begin : Private { }
    sub default : Private { }
    sub auto : Private { }

You can define built-in private actions within your controllers as well. The actions will override the ones in less-specific controllers, or your application class. In other words, for each of the three built-in private actions, only one will be run in any request cycle. Thus, if MyApp::Controller::Catalog::begin exists, it will be run in place of MyApp::begin if you're in the catalog namespace, and MyApp::Controller::Catalog::Order::begin would override this in turn.

Here are some examples of the order in which the various built-ins would be called:

for a request for /foo/foo
  MyApp::begin
  MyApp::auto
  MyApp::Controller::Foo::default # in the absence of MyApp::Controller::Foo::Foo
  MyApp::end
for a request for /foo/bar/foo
  MyApp::Controller::Foo::Bar::begin
  MyApp::auto
  MyApp::Controller::Foo::auto
  MyApp::Controller::Foo::Bar::auto
  MyApp::Controller::Foo::Bar::default # for MyApp::Controller::Foo::Bar::foo
  MyApp::Controller::Foo::Bar::end

The auto action is also distinguished by the fact that you can break out of the processing chain by returning 0. If an auto action returns 0, any remaining actions will be skipped, except for end. So, for the request above, if the first auto returns false, the chain would look like this:

for a request for /foo/bar/foo where first auto returns false
  MyApp::Controller::Foo::Bar::begin
  MyApp::auto
  MyApp::Controller::Foo::Bar::end

An example of why one might use this is an authentication action: you could set up a auto action to handle authentication in your application class (which will always be called first), and if authentication fails, returning 0 would skip any remaining methods for that URL.

Note: Looking at it another way, auto actions have to return a true value to continue processing! You can also die in the auto action; in that case, the request will go straight to the finalize stage, without processing further actions.

URL Path Handling

You can pass variable arguments as part of the URL path, separated with forward slashes (/). If the action is a Regex or LocalRegex, the '$' anchor must be used. For example, suppose you want to handle /foo/$bar/$baz, where $bar and $baz may vary:

    sub foo : Regex('^foo$') { my ($self, $context, $bar, $baz) = @_; }

But what if you also defined actions for /foo/boo and /foo/boo/hoo?

    sub boo : Path('foo/boo') { .. }
    sub hoo : Path('foo/boo/hoo') { .. }

Catalyst matches actions in most specific to least specific order:

    /foo/boo/hoo
    /foo/boo
    /foo # might be /foo/bar/baz but won't be /foo/boo/hoo

So Catalyst would never mistakenly dispatch the first two URLs to the '^foo$' action.

If a Regex or LocalRegex action doesn't use the '$' anchor, the action will still match a URL containing arguments, however the arguments won't be available via @_.

Parameter Processing

Parameters passed in the URL query string are handled with methods in the the Catalyst::Request manpage class. The param method is functionally equivalent to the param method of CGI.pm and can be used in modules that require this.

    # http://localhost:3000/catalog/view/?category=hardware&page=3
    my $category = $c->req->param('category');
    my $current_page = $c->req->param('page') || 1;
    # multiple values for single parameter name
    my @values = $c->req->param('scrolling_list');
    # DFV requires a CGI.pm-like input hash
    my $results = Data::FormValidator->check($c->req->params, \%dfv_profile);

Flow Control

You control the application flow with the forward method, which accepts the key of an action to execute. This can be an action in the same or another Catalyst controller, or a Class name, optionally followed by a method name. After a forward, the control flow will return to the method from which the forward was issued.

A forward is similar to a method call. The main differences are that it wraps the call in an eval to allow exception handling; it automatically passes along the context object ($c or $context); and it allows profiling of each call (displayed in the log with debugging enabled).

    sub hello : Global {
        my ( $self, $c ) = @_;
        $c->stash->{message} = 'Hello World!';
        $c->forward('check_message'); # $c is automatically included
    }
    sub check_message : Private {
        my ( $self, $c ) = @_;
        return unless $c->stash->{message};
        $c->forward('show_message');
    }
    sub show_message : Private {
        my ( $self, $c ) = @_;
        $c->res->body( $c->stash->{message} );
    }

A forward does not create a new request, so your request object ($c->req) will remain unchanged. This is a key difference between using forward and issuing a redirect.

You can pass new arguments to a forward by adding them in an anonymous array. In this case $c->req->args will be changed for the duration of the forward only; upon return, the original value of $c->req->args will be reset.

    sub hello : Global {
        my ( $self, $c ) = @_;
        $c->stash->{message} = 'Hello World!';
        $c->forward('check_message',[qw/test1/]);
        # now $c->req->args is back to what it was before
    }
    sub check_message : Private {
        my ( $self, $c ) = @_;
        my $first_argument = $c->req->args->[0]; # now = 'test1'
        # do something...
    }

As you can see from these examples, you can just use the method name as long as you are referring to methods in the same controller. If you want to forward to a method in another controller, or the main application, you will have to refer to the method by absolute path.

  $c->forward('/my/controller/action');
  $c->forward('/default'); # calls default in main application

Here are some examples of how to forward to classes and methods.

    sub hello : Global {
        my ( $self, $c ) = @_;
        $c->forward(qw/MyApp::Model::Hello say_hello/);
    }
    sub bye : Global {
        my ( $self, $c ) = @_;
        $c->forward('MyApp::Model::Hello'); # no method: will try 'process'
    }
    package MyApp::Model::Hello;
    sub say_hello {
        my ( $self, $c ) = @_;
        $c->res->body('Hello World!');
    }
    sub process {
        my ( $self, $c ) = @_;
        $c->res->body('Goodbye World!');
    }

Note that forward returns to the calling action and continues processing after the action finishes. If you want all further processing in the calling action to stop, use detach instead, which will execute the detached action and not return to the calling sub. In both cases, Catalyst will automatically try to call process() if you omit the method.

Testing

Catalyst has a built-in http server for testing or local deployment. (Later, you can easily use a more powerful server, for example Apache/mod_perl or FastCGI, in a production environment.)

Start your application on the command line...

    script/myapp_server.pl

...then visit http://localhost:3000/ in a browser to view the output.

You can also do it all from the command line:

    script/myapp_test.pl http://localhost/

Catalyst has a number of tools for actual regression testing of applications. The helper scripts will automatically generate basic tests that can be extended as you develop your project. To write your own comprehensive test scripts, the Test::WWW::Mechanize::Catalyst manpage is an invaluable tool.

For more testing ideas, see the Catalyst::Manual::Tutorial::Testing manpage.

Have fun!

SEE ALSO

SUPPORT

IRC:

    Join #catalyst on irc.perl.org.
    Join #catalyst-dev on irc.perl.org to help with development.

Mailing lists:

    http://lists.rawmode.org/mailman/listinfo/catalyst
    http://lists.rawmode.org/mailman/listinfo/catalyst-dev

AUTHOR

Sebastian Riedel, sri@oook.de David Naughton, naughton@umn.edu Marcus Ramberg, mramberg@cpan.org Jesse Sheidlower, jester@panix.com Danijel Milicevic, me@danijel.de Kieren Diment, kd@totaldatasolution.com Yuval Kogman, nothingmuch@woobling.org

COPYRIGHT

This program is free software. You can redistribute it and/or modify it under the same terms as Perl itself.