first cut at :ChildOf

[catagits/Catalyst-Runtime.git] / lib / Catalyst / Manual / Intro.pod

=head1 NAME

Catalyst::Manual::Intro - Introduction to Catalyst

=head1 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 L<Catalyst::Manual::About>.
For a systematic step-by-step introduction to writing an application
with Catalyst, see L<Catalyst::Manual::Tutorial>.

=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. 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. Under Catalyst, it is always possible to do things in a
different way. However, this does mean that it is always possible to do
things in a different way. Other web frameworks are simpler to use and
easy to get up and running, 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.

=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 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 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 * B<Model>

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

=item * B<View>

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

=item * B<Controller>

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

=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, 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 method for web applications.

=head3 Flexibility

Catalyst is much more flexible than many other frameworks. We'll talk
more about this later, but rest assured you can use your favorite Perl
modules with Catalyst.

=over 4

=item * B<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 handles these components in a very flexible way. 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, and 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 * B<Reuseable 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 * B<Unrestrained URL-to-Action Dispatching>

Catalyst allows you to dispatch any URLs to any application L</Actions>,
even through regular expressions! Unlike most other frameworks, it
doesn't require mod_rewrite or class and method names in URLs.

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

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

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

=item * B<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.

=over 4

=item * B<Building Block Interface>

Components interoperate very smoothly. For example, Catalyst
automatically makes a L</Context> object available to 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 * B<Component Auto-Discovery>

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

=item * B<Pre-Built Components for Popular Modules>

See L<Catalyst::Model::DBIC::Schema> for L<DBIx::Class>, or
L<Catalyst::View::TT> for L<Template Toolkit|Template>.

=item * B<Built-in Test Framework>

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

=item * B<Helper Scripts>

Catalyst provides helper scripts to quickly generate running starter
code for components and unit tests. See L<Catalyst::Helper>.

=back

=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 Task::Catalyst'

=head3 Setup

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

=head3 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.)


=over 4

=item http://localhost:3000/

=item http://localhost:3000/library/login/

=back

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, and extend Catalyst.

    package MyApp;

    use strict;
    use Catalyst qw/-Debug/;

    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 #####, below),
to avoid namespace collisions.

=over 4

=item * B<name>

The name of your application.

=back

Optionally, you can specify a B<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
C<$context-E<gt>config-E<gt>{$param_name}>.

###### We need a short section on configuration here.

=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. 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 C<$context> for
clarity, but most Catalyst developers just call it C<$c>:

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

The Context contains several important objects:

=over 4

=item * L<Catalyst::Request>

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

The request object contains all kinds of request-specific information, 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::Response>

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

The response is like the request, but contains just response-specific
information.

    $c->res->body('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 * B<Stash>

    $c->stash
    $c->stash->{foo} = 'bar';
    $c->stash->{baz} = {baz => 'qox'};
    $c->stash->{fred} = [qw/ wilma pebbles/];

and so on.

=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:

    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 more persistent data, use a session.

=head3 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.

=over 4

=item * B<Application Wide Actions>

Actions which are called at the root level of the application
(e.g. http://localhost:3000/ ) go in MyApp::Controller::Root, like
this:

    package MyApp::Controller::Root;
    use base 'Catalyst::Controller';
    # Sets the actions in this controller to be registered with no prefix
    # so they function identically to actions created in MyApp.pm
    __PACKAGE__->config->{namespace} = '';
    sub default : Private {
        my ( $self, $context ) = @_;
        $context->response->body('Catalyst rocks!');
    }
    1;


=back

For most applications, Catalyst requires you to define only one config
parameter:

=head4 Action types

Catalyst supports several types of actions:

=over 4

=item * B<Literal> (B<Path> actions)

    package MyApp::Controller::My::Controller;
    sub bar : Path('foo/bar') { }

Literal C<Path> actions will act relative to their current
namespace. The above example matches only
http://localhost:3000/my/controller/foo/bar. If you start your path with
a forward slash, it will match from the root. Example:

    package MyApp::Controller::My::Controller;
    sub bar : Path('/foo/bar') { }

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

    package MyApp::Controller::My::Controller;
    sub bar : Path { }

By leaving the C<Path> definition empty, it will match on the namespace
root. The above code matches http://localhost:3000/my/controller.

=item * B<Regex>

    sub bar : Regex('^item(\d+)/order(\d+)$') { }

Matches any URL that matches the pattern in the action key, e.g.
http://localhost:3000/item23/order42. The '' around the regexp is
optional, but perltidy likes it. :)

Regex matches act globally, i.e. without reference to the namespace from
which it is called, so that a C<bar> method in the
C<MyApp::Controller::Catalog::Order::Process> namespace won't match any
form of C<bar>, C<Catalog>, C<Order>, or C<Process> unless you
explicitly put this in the regex. To achieve the above, you should
consider using a C<LocalRegex> action.

=item * B<LocalRegex>

    sub bar : LocalRegex('^widget(\d+)$') { }

LocalRegex actions act locally. If you were to use C<bar> in
C<MyApp::Controller::Catalog>, the above example would match urls like
http://localhost:3000/catalog/widget23.

If you omit the "C<^>" from your regex, then it will match any depth
from the controller and not immediately off of the controller name. The
following example differs from the above code in that it will match
http://localhost:3000/catalog/foo/widget23 as well.

    package MyApp::Controller::Catalog;
    sub bar : LocalRegex('widget(\d+)$') { }

For both LocalRegex and Regex actions, if you use capturing parentheses
to extract values within the matching URL, those values are available in
the C<$c-E<gt>req-E<gt>captures> array. In the above example, "widget23"
would capture "23" in the above example, and
C<$c-E<gt>req-E<gt>captures-E<gt>[0]> would be "23". If you want to pass
arguments at the end of your URL, you must use regex action keys. See
L</URL Path Handling> below.

=item * B<Top-level> (B<Global>)

    package MyApp::Controller::Foo;
    sub foo : Global { }

Matches http://localhost:3000/foo. The function name is mapped
directly to the application base.  You can provide an equivalent
function in this case  by doing the following:

    package MyApp::Controller::Root
    sub foo : Local { }

=item * B<Namespace-Prefixed> (B<Local>)

    package MyApp::Controller::My::Controller; 
    sub foo : Local { }

Matches http://localhost:3000/my/controller/foo. 

This action type 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 * B<Private>

    sub foo : Private { }

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>. Note that, as
discussed there, when forwarding from another component, you must use
the absolute path to the method, so that a private C<bar> method in your
C<MyApp::Controller::Catalog::Order::Process> controller must, if called
from elsewhere, be reached with
C<$c-E<gt>forward('/catalog/order/process/bar')>.

=item * B<Args>

Args is not an action type per se, but an action modifier - it adds a match
restriction to any action it's provided to, requiring only as many path parts
as are specified for the action to be valid - for example in
MyApp::Controller::Foo,

  sub bar :Local

would match any URL starting /foo/bar/. To restrict this you can do

  sub bar :Local :Args(1)

to only match /foo/bar/*/

=item * B<PathPart>, B<Captures> and B<ChildOf>

Matt is an idiot and hasn't documented this yet.

=back

B<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 C<forward>s.

=head4 Built-in Private Actions

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

=over 4

=item * B<default : Private>

Called when no other action matches. Could be used, for example, for
displaying a generic frontpage for the main app, or an error page for
individual controllers.

If C<default> isn't acting how you would expect, look at using a
L</Literal> C<Path> action (with an empty path string). The difference is
that C<Path> takes arguments relative from the namespace and C<default>
I<always> takes arguments relative from the root, regardless of what
controller it's in.

=item * B<index : Private>

C<index> is much like C<default> except that it takes no arguments
and it is weighted slightly higher in the matching process. It is
useful as a static entry point to a controller, e.g. to have a static
welcome page. Note that it's also weighted higher than Path.

=item * B<begin : Private>

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

=item * B<end : Private>

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

=back

=head4 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 C<MyApp::Controller::Catalog::begin> exists, it will be
run in place of C<MyApp::begin> if you're in the C<catalog> namespace,
and C<MyApp::Controller::Catalog::Order::begin> would override this in
turn.

In addition to the normal built-in actions, you have a special action
for making chains, C<auto>. Such C<auto> actions will be run after any
C<begin>, but before your action is processed. Unlike the other
built-ins, C<auto> actions I<do not> override each other; they will be
called in turn, starting with the application class and going through to
the I<most> specific class. I<This is the reverse of the order in which
the normal built-ins override each other>.

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

=over 4

=item for a request for C</foo/foo>

  MyApp::begin
  MyApp::auto
  MyApp::Controller::Foo::default # in the absence of MyApp::Controller::Foo::Foo
  MyApp::end

=item for a request for C</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

=back

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

=over 4

=item for a request for C</foo/bar/foo> where first C<auto> returns
false

  MyApp::Controller::Foo::Bar::begin
  MyApp::auto
  MyApp::Controller::Foo::Bar::end

=back

An example of why one might use this is an authentication action: you
could set up a C<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.

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

=head4 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 '$' anchor 
must be used. For example, suppose you want to handle C</foo/$bar/$baz>, 
where C<$bar> and C<$baz> may vary:

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

But what if you also defined actions for C</foo/boo> and C</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 C<@_>.

=head4 Parameter Processing

Parameters passed in the URL query string are handled with methods in
the L<Catalyst::Request> class. The C<param> method is functionally
equivalent to the C<param> method of C<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);

=head3 Flow Control

You control the application flow with the C<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 C<forward>, the control flow will
return to the method from which the C<forward> was issued.

A C<forward> is similar to a method call. The main differences are that
it wraps the call in an C<eval> to allow exception handling; it
automatically passes along the context object (C<$c> or C<$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 C<forward> does not create a new request, so your request object
(C<$c-E<gt>req>) will remain unchanged. This is a key difference between
using C<forward> and issuing a redirect.

You can pass new arguments to a C<forward> by adding them
in an anonymous array. In this case C<$c-E<gt>req-E<gt>args>
will be changed for the duration of the C<forward> only; upon
return, the original value of C<$c-E<gt>req-E<gt>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 C<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 C<detach> instead, which will execute
the C<detach>ed action and not return to the calling sub. In both cases,
Catalyst will automatically try to call process() if you omit the
method.

=head3 Components

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

All components 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::Catalog;

    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
when you call C<setup> in the main application. All you need to do is
put them in directories named for each Component type. Notice that you
can use a terse alias for each one.

=over 4

=item * B<MyApp/Model/> 

=item * B<MyApp/M/>

=item * B<MyApp/View/>

=item * B<MyApp/V/>

=item * B<MyApp/Controller/>

=item * B<MyApp/C/>

=back

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

=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::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 C<TT> tells the script that the name of the view should
be C<TT>, and the second that it should be a Template Toolkit view.)

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

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

Also, be sure to put the template under the directory specified in
C<$c-E<gt>config-E<gt>{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<DBIx::Class>: L<Catalyst::Model::DBIC::Schema>.
We'll also need L<DBIx::Class::Schema::Loader>.

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::SchemaLoader component for this database.

    script/myapp_create.pl model DBIC DBIC::SchemaLoader 'dbi:SQLite:/tmp/myapp.db'

L<DBIx::Class::Schema::Loader> automatically loads table layouts and
relationships. 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} = $c->model('DBIC::Foo')->find($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::DBIC::Module;
      my @records = Some::Outside::DBIC::Module->search({
        artist => 'sri',
        });
      
      $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 C<use> each component, Catalyst will
find and load it automatically at compile-time; you can C<forward> to
the module, which can only be done to Catalyst components; and only
Catalyst components can be fetched with
C<$c-E<gt>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 C<Some::DBIC::Schema> is part of your
Cat app as C<MyApp::Model::DB>.

=head4 Controllers

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

    package MyApp::Controller::Login;

    use base qw/Catalyst::Controller/;

    sub sign_in : Path("sign-in") { }
    sub new_password : Path("new-password") { }
    sub sign_out : Path("sign-out") { }

    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 { }

=head3 Models

Models are providers of data. This data could come from anywhere - a search
engine index, a database table, etc. Typically the data source does not have
much to do with web applications or Catalyst - it could be used to write an
offline report generator or a command line tool just the same.

The common approach to writing a Catalyst-style model for your application is
wrapping a generic model (e.g. L<DBIx::Class::Schema>, a bunch of XMLs, or
anything really) with an object that contains configuration data, convenience
methods, and so forth.

#### editor: move this part to =head3 Components somehow, right after this
#### section - this will require deeply rephrasing this paragraph.

Technically, within Catalyst a model is a B<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 (L<Catalyst::Model>) provides things like C<config>
and stuff to better integrate the model into the application, sometimes this is
not enough, and the model requires access to C<$c> itself.

Situations where this need might arise include:

=over 4

=item *

Interacting with another model

=item *

Using per-request data to control behavior

=item *

Using plugins in (for example L<Catalyst::Plugin::Cache>).

=back

From a style perspective usually it's bad 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<$c> cannot be sidestepped, there
exists a power tool called C<ACCEPT_CONTEXT>.

#### editor note: this part is "generic" - it also applies to views and
#### controllers.

=head3 ACCEPT_CONTEXT

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

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

A typical C<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<$c> and delegates to the per-application model object.

A typical C<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 B<prototype object> that gets a new parameter.
C<@extra_arguments> comes from any trailing arguments to
C<< $c->component( $bah, @extra_arguments ) >> (or C<< $c->model(...) >>,
C<< $c->view(...) >> etc).

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

Add a field to C<$c>, like C<my_model_instance>. Then write your
C<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;
                }
        }


=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 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/

Have fun!

=head1 SUPPORT

IRC:

    Join #catalyst on irc.perl.org.

Mailing-lists:

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

=head1 AUTHOR

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

=head1 COPYRIGHT

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