=head1 NAME

Catalyst::Manual::Intro - Introduction to Catalyst

=head1 DESCRIPTION

This is a brief overview of why and how to use Catalyst. It explains how Catalyst works and shows how to quickly get a simple application up and running.

=head2 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 L<Maypole>, upon which it was originally based.

=head3 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 tat handles one concern without affecting code that handles the others. Catalyst promotes re-use of existing Perl modules that already handle common web application concerns well.

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

=over 4

=item * Model

Access and modify content (data). L<Class::DBI>, L<Plucene>, L<Net::LDAP>...

=item * View

Present content to the user. L<Template Toolkit|Template>, L<Mason|HTML::Mason>...

=item * Controller

Control the whole request phase, check parameters, dispatch actions, flow control. Catalyst!

=back

If you're unfamiliar with MVC and design patterns, you may want to check out the original book on the subject, I<Design Patterns>, by Gamma, Helm, Johson and Vlissides, a.k.a. the Gang of Four (GoF). Or just search the web. Many, many web application frameworks follow MVC, including all those listed above.

=head3 Flexibility

Catalyst is much more flexible than many other frameworks.

=over 4

=item * Multiple Models, Views and Controllers

To build a Catalyst application, you handle each type of concern inside special modules called L</Components>. Often this code will be very simple, just calling out to Perl modules like those listed above under L</MVC>. Catalyst is very flexible about these Components. Use as many Models, Views and Controllers as you like, using as many different Perl modules as you like, all in the same application. Want to manipulate multiple databases, plus retrieve some data via LDAP? No problem. Want to present data from the same Model using L<Template Toolkit|Template> and L<PDF::Template>? Easy.

=item * Re-Useable Components

Not only does Catalyst promote the re-use of already-existing Perl modules, it also allows you to re-use your Catalyst components in multiple Catalyst applications.

=item * Unrestrained URL-to-Action Dispatching

Catalyst allows you to dispatch any URLs to any application L<Actions>, even via regular expressions! Unlike some other frameworks, it doesn't require you to put class and method names in your URLs.

With Catalyst you register your actions and address them directly. For example:

    MyApp->action( 'hello' => sub {
        my ( $self, $context ) = @_;
        $context->response->output('Hello World!');
    });

Now http://localhost:3000/hello prints "Hello World!".

=item * Support for CGI, mod_perl, Apache::Request

Use L<Catalyst::Engine::Apache> or L<Catalyst::Engine::CGI>.

=back

=head3 Simplicity

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

=item * Building Block Interface

Components interoperate very smoothly. For example, Catalyst automatically makes a L<Context> object available in every component. Via the context, you can access the request object, share data between components, and control the flow of your application. Building a Catalyst application feels a lot like snapping together toy building blocks, and everything just works.

=item * Component Auto-Discovery

No need to C<use> all of your components. Catalyst automatically finds and loads them.

=item * Pre-Built Components for Popular Modules

See L<Catalyst::Model::CDBI> for L<Class::DBI>, or L<Catalyst::View::TT> for L<Template Toolkit|Template>. You can even get an instant web database front end with L<Catalyst::Model::CDBI::CRUD>.

=item * Builtin Test Framework

Catalyst comes with a builtin, lightweight http server and test framework, making it easy to test applications from the command line.

=item * Helper Scripts

Catalyst provides helper scripts to quickly generate running starter code for components and unit tests.

=head2 Quickstart

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

=head3 Install

    $ perl -MCPAN -e 'install Bundle::Catalyst'

=head3 Setup

    $ catalyst My::App
    $ cd My-App
    $ bin/create controller My::Controller

=head3 Run

    $ bin/server

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

=over 4

=item http://localhost:3000/

=item http://localhost:3000/my_controller/

=back

Dead easy!

=head2 How It Works

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

=head3 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, define application-wide actions and extend Catalyst.

    package MyApp;

    use strict;
    use Catalyst qw/-Debug/;

    MyApp->config(
        name => 'My Application',
        root => '/home/joeuser/myapp/root',

        # You can put whatever you want in here:
        # my_param_name => $my_param_value,
    );

    MyApp->action( '!default' => sub {
        my ( $self, $context ) = @_;
        $context->response->output('Catalyst rockz!');
    });

    1;

For most applications, Catalyst requires you to define only two config parameters:

=over 4

=item * name

Name of your application.

=item * root

Path to additional files like templates, images or other static data.

=back

However, 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}.

=head3 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 L<Components> together. 

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

    MyApp->action( 'hello' => sub {
        my ( $self, $c ) = @_;
        $c->res->output('Hello World!');
    });

The Context contains several important objects:

=over 4

=item * L<Catalyst::Request>

    $c->request
    $c->req # alias

The request contains all kind of request specific informations like query parameters, cookies, uploads, headers and more.

    $c->req->params->{foo};
    $c->req->cookies->{sessionid};
    $c->req->headers->content_type;
    $c->req->base;

=item * L<Catalyst::Reponse>

    $c->response
    $c->res # alias

The response is like the request but contais just response specific informations.

    $c->res->output('Hello World');
    $c->res->status(404);
    $c->res->redirect('http://oook.de');

=item * L<Catalyst::Config>

    $c->config

    $c->config->root;
    $c->config->name;

=item * L<Catalyst::Log>

    $c->log

    $c->log->debug('Something happened');
    $c->log->info('Something you should know');

=item * Stash

    $c->stash

    $c->stash->{foo} = 'bar';

=back

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 example:

    MyApp->action(

        'hello' => sub {
            my ( $self, $c ) = @_;
            $c->stash->{message} = 'Hello World!';
            $c->forward('!show-message');
        },

        '!show-message' => sub {
            my ( $self, $c ) = @_;
            $c->res->output( $c->stash->{message} );
        },

    );

=head3 Actions

To define a Catalyst action, register it into your applicaton with the C<action> method. C<action> accepts a key-value pair, where the key represents one or more URLs or application states and the value is a code reference, the action to execute in reponse to the URL(s) or application state(s).

Catalyst supports several ways to define Actions:

=over 4

=item * Literal

    $c->action( 'foo/bar' => sub { } );

Matches only http://localhost:3000/foo/bar.

=item * Regex

    $c->action( '^/foo(\d+)/bar(\d+)$/' => sub { } );

Matches any URL that matches the pattern in the action key, e.g. http://localhost:3000/foo23/bar42. The pattern must be enclosed with forward slashes, i.e. '/$pattern/'.

If you use capturing parantheses to extract values within the matching URL (23, 42 in the above example), those values are available in the $c->req->snippets array. Be sure to use ^ and $ if your action has arguments.

=item * Namespace-Prefixed

    package MyApp::Controller::My::Controller; 
    $c->action( '?foo' => sub { } );

Matches http://localhost:3000/my_controller/foo. The action key must be prefixed with '?'.

Prefixing the action key with '?' indicates that the matching URL must be prefixed with a modified form of the component's class (package) name. This modified class name excludes the parts that have a pre-defined meaning in Catalyst ("MyApp::Controller" in the above example), replaces "::" with "_" and converts the name to lower case. See L</Components> for a full explanation of the pre-defined meaning of Catalyst component class names.

=item * Private

    $c->action( '!foo' => sub { } );

Matches no URL, and cannot be executed by requesting a URL that corresponds to the action key. Private actions can be executed only inside a Catalyst application, by calling the C<forward> method:

    $c->forward('!foo');

See L</Flow Control> for a full explanation of C<forward>.

=back

=head4 Builtin Private Actions

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

=over 4

=item * !default 

Called when no other action matches.

=item * !begin

Called at the beginning of a request, before any matching actions are called.

=item * !end

Called at the end of a request, after all matching actions are called.

=item * !?default, !?begin and !?end

Like their equivalents above but used to overload them from Controllers.
So each Controller can have their own !?default, !?begin and !?end.

=back

=head3 Flow Control

Control the application flow with the C<forward> method, which accepts the key of an action to execute.

    MyApp->action(

        'hello' => sub {
            my ( $self, $c ) = @_;
            $c->stash->{message} = 'Hello World!';
            $c->forward('!check-message');
        },

        '!check-message' => sub {
            my ( $self, $c ) = @_;
            return unless $c->stash->{message};
            $c->forward('!show-message');
        },

        '!show-message' => sub {
            my ( $self, $c ) = @_;
            $c->res->output( $c->stash->{message} );
        },

    );

You can also forward to classes and methods.

    MyApp->action(

        'hello' => sub {
            my ( $self, $c ) = @_;
            $c->forward(qw/MyApp::M::Hello say_hello/);
        },

        'bye' => sub {
            my ( $self, $c ) = @_;
            $c->forward('MyApp::M::Hello');
        },

    );

    package MyApp::M::Hello;

    sub say_hello {
        my ( $self, $c ) = @_;
        $c->res->output('Hello World!');
    }

    sub process {
        my ( $self, $c ) = @_;
        $c->res->output('Goodbye World!');
    }

Note that C<forward> returns after processing.
Catalyst will automatically try to call process() if you omit the method.

=head3 Components

Again, Catalyst has an uncommonly flexible component system. You can define as many L<Models>, L<Views> and Controllers as you like.

All components are must inherit from L<Catalyst::Base>, which provides a simple class structure and some common class methods like C<config> and C<new> (constructor).

    package MyApp::Controller::MyController;

    use strict;
    use base 'Catalyst::Base';

    __PACKAGE__->config( foo => 'bar' );

    1;

You don't have to C<use> or otherwise register Models, Views and Controllers. Catalyst automatically discovers and instantiates them, at startup. All you need to do is put them in directories named for each Component type. Notice that you can use some very terse aliases for each one.

=over 4

=item * MyApp/Model/ 

=item * MyApp/M/

=item * MyApp/View/

=item * MyApp/V/

=item * MyApp/Controller/

=item * MyApp/C/

=back

=head4 Views

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

    package MyApp::V::TT;

    use strict;
    use base 'Catalyst::View::TT';

    1;

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

    MyApp->action(

        'hello' => sub {
            my ( $self, $c ) = @_;
            $c->stash->{template} = 'hello.tt';
        },

        '!end' => sub {
            my ( $self, $c ) = @_;
            $c->forward('MyApp::View::TT');
        },

    );

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

Also, be sure to put the template under the directory specified in C<$c-E<gt>config->{root}>, or you'll be forced to look at our eyecandy debug screen. ;)

=head4 Models

To show how to define models, again we'll use an already-existing base class, this time for L<Class::DBI>: L<Catalyst::Model::CDBI>.

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 CDBI component for this database.

    package MyApp::M::CDBI;

    use strict;
    use base 'Catalyst::Model::CDBI';

    __PACKAGE__->config(
        dsn           => 'dbi:SQLite:/tmp/myapp.db',
        relationships => 1
    );

    1;

Catalyst automatically loads table layouts and relationships. Use the stash to pass data to your templates.

    package MyApp;

    use strict;
    use Catalyst '-Debug';

    __PACKAGE__->config(
        name => 'My Application',
        root => '/home/joeuser/myapp/root'
    );

    __PACKAGE__->action(

        '!end' => sub {
            my ( $self, $c ) = @_;
            $c->stash->{template} ||= 'index.tt';
            $c->forward('MyApp::V::TT');
        },

        'view' => sub {
            my ( $self, $c, $id ) = @_;
            $c->stash->{item} = MyApp::M::CDBI::Foo->retrieve($id);
        }

    );

    1;

    The id is [% item.data %]

=head4 Controllers

Multiple Controllers are a good way to separate logical domains of your application and distribute tasks to different programmers in a teams.

    package MyApp::C::Login;

    MyApp->action(
        '?sign-in' => sub { },
        '?new-password' => sub { },
        '?sign-out' => sub { },
    );

    package MyApp::C::Catalog;

    MyApp->action(
        '?view' => sub { },
        '?list' => sub { },
    );

    package MyApp::C::Cart;

    MyApp->action(
        '?add' => sub { },
        '?update' => sub { },
        '?order' => sub { },
    );

=head3 Testing

Catalyst has a built in http server for testing! (Later, you can easily use a more powerful server, e.g. Apache/mod_perl, in a production environment).

Start your application on the command line...

    perl -I/home/joeuser/myapp/lib -MCatalyst::Test=MyApp -e1 3000

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

You can also do it all from the command line:

    perl -I/home/joeuser/myapp/lib -MCatalyst::Test=MyApp -e1 http://localhost/

Have fun!

=head1 AUTHOR

Sebastian Riedel, C<sri@oook.de> and David Naughton, C<naughton@umn.edu>

=head1 COPYRIGHT

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