# ABSTRACT: A guide to extending Dancer2 via engines and plugins

package Dancer2::Manual::Extending;

__END__

=pod

=encoding UTF-8

=head1 NAME

Dancer2::Manual::Extending - A guide to extending Dancer2 via engines and plugins

=head1 VERSION

version 2.1.0

=head1 Name

Dancer2::Extending - How to Extend Dancer2 by Writing Your Own Engines, Handlers, and Plugins

=head1 Introduction

Dancer2 is designed to be flexible and extensible. Whether you need a
custom template engine, session engine, serializer, or file handler,
Dancer2 provides a clean API that makes it easy to extend its core
functionality.

In this document, we’ll cover how to write your own engines, handlers,
and plugins for Dancer2. The goal is to give you the tools you need to
build custom components that integrate seamlessly into your Dancer2
applications.

=head1 Writing Your Own Engine

In Dancer2, engines are used to handle various aspects of your
application, such as templates, sessions, serializers, and loggers. Each
engine follows a standard interface, making it easy to plug in new
implementations.

Engines can live in the Dancer2 namespace:

    set serializer => 'JSON';
    set template => 'Mason';

(these resolve to L<Dancer2::Serializer::JSON> and L<Dancer2::Template::Mason>
respectively)

Or you can provide the fully qualified namespace using the C<+> indicator:

    set serializer => `+My::Awesome::Serializer`;
    set template => `+My::Template::DWIM`;

The latter is similar to how L<Plack> handles this.

=head2 Writing a Template Engine

A custom template engine must implement the L<Dancer2::Core::Role::Template>
role. This role defines the basic methods that any template engine needs
to implement.

=head3 Example: Custom Template Engine

    package Dancer2::Template::MyTemplateEngine;
    use Moo;
    with 'Dancer2::Core::Role::Template';

    # Define the required methods
    # ...

    sub render {
        my ($self, $template, $tokens) = @_;

        # Your rendering logic here
        return "<html><body>Welcome to Danceyland, $tokens->{'visitor_name'}!</body></html>";
    }

    1;

In this example, C<render> is the method responsible for processing the
template and returning the final output.

To use your custom engine, configure it in your C<config.yml> file:

    template: "MyTemplateEngine"

=head2 Writing a Session Engine

Session engines handle storing and retrieving session data. All session
engines must implement the L<Dancer2::Core::Role::SessionFactory> role.

=head3 Example: Custom Session Engine

    package Dancer2::Session::MySessionEngine;
    use Moo;
    with 'Dancer2::Core::Role::SessionFactory';

    sub retrieve {
        my ($self, $id) = @_;
        # Fetch session data based on session ID
    }

    sub create {
        my ($self, $data) = @_;
        # Create a new session
    }

    sub destroy {
        my ($self, $id) = @_;
        # Destroy the session identified by $id
    }

    sub flush {
        my ($self, $id, $data) = @_;
        # Save session data
    }

    1;

This engine defines the basic methods to manage session data.

=head2 Writing a Serializer Engine

Serializer engines handle serializing and deserializing data between
Perl data structures and different formats like JSON, YAML, or XML.

All serializer engines must implement the L<Dancer2::Core::Role::Serializer>
role.

=head3 Example: Custom Serializer Engine

    package Dancer2::Serializer::MySerializer;
    use Moo;
    with 'Dancer2::Core::Role::Serializer';

    sub serialize {
        my ($self, $content) = @_;
        # Convert $content to a custom format
    }

    sub deserialize {
        my ($self, $content) = @_;
        # Convert custom format back to Perl data structure
    }

    sub content_type {
        return 'application/custom';
    }

    1;

This engine defines the methods needed to serialize and deserialize data.

=head2 Writing a Logger Engine

Logger engines are responsible for handling log messages in your
application. All logger engines must implement the
L<Dancer2::Core::Role::Logger> role.

=head3 Example: Custom Logger Engine

    package Dancer2::Logger::MyLogger;
    use Moo;
    with 'Dancer2::Core::Role::Logger';

    sub log {
        my ($self, $level, $message) = @_;
        # Custom logging logic here
        warn "[$level] $message";
    }

    1;

This logger will output log messages to C<warn>, and you can configure
it in your C<config.yml> file:

    logger: "MyLogger"

=head1 Writing Your Own File Handler

File handlers in Dancer2 are responsible for serving static files from
disk. If you need custom logic for serving files (such as restricting
access or adding dynamic features), you can create your own file handler
by consuming the L<Dancer2::Core::Role::Handler> role.

=head2 Example: Custom File Handler

    package Dancer2::Handler::MyFileHandler;
    use Moo;
    with 'Dancer2::Core::Role::Handler';

    sub methods { qw(GET) }

    sub regexp { '/static/:file' }

    sub code {
        return sub {
            my $app = shift;
            my $file = $app->request->params->{file};

            # Add custom logic for serving files
            my $file_path = "/path/to/static/files/$file";
            if (-e $file_path) {
                $app->send_file($file_path);
            } else {
                $app->send_error('File not found', 404);
            }
        };
    }

    sub register {
        my ($self, $app) = @_;
        $app->add_route(
            method => $_,
            regexp => $self->regexp,
            code   => $self->code,
        ) for $self->methods;
    }

    1;

This handler will serve static files from the specified directory, but
it also checks if the file exists before serving it.

=head1 Writing A Dancer2 Plugin

Plugins in Dancer2 are reusable components that extend your app's
functionality. They are often used for tasks like authentication,
database management, or form validation.

All plugins must consume the L<Dancer2::Core::Role::Plugin> role.

=head2 Example: Custom Dancer2 Plugin

    package Dancer2::Plugin::MyPlugin;
    use Dancer2::Plugin;

    register 'greet' => sub {
        my ($dsl, $name) = @_;
        return "Hello, $name! Welcome to Danceyland!";
    };

    register_plugin;

In this example, a C<greet> keyword is created, and can be called within
any Dancer2 route to display a greeting. To use the plugin, just include
it in your app:

    use Dancer2::Plugin::MyPlugin;

    get '/hello/:name' => sub {
        return greet( route_parameters->get('name') );
    };

=head1 Custom Session Storage Mechanisms

Dancer2 allows you to define your own custom session storage mechanism
by implementing the L<Dancer2::Core::Role::SessionFactory> role. This
is useful if you need to store session data in a format or storage medium
that isn’t supported out-of-the-box, such as a proprietary database or
external service.

=head2 Example: Custom Session Storage

Here’s how you can create a custom session storage engine that stores
session data in a simple hash in memory. In practice, you’d likely store
data in a more robust medium like a database or external cache.

    package Dancer2::Session::MySessionStorage;
    use Moo;
    with 'Dancer2::Core::Role::SessionFactory';

    # Simple in-memory session store (not suitable for production use)
    my %session_store;

    sub retrieve {
        my ($self, $id) = @_;
        return $session_store{$id};
    }

    sub create {
        my ($self, $data) = @_;
        my $session_id = $self->generate_session_id;
        $session_store{$session_id} = $data;
        return $session_id;
    }

    sub destroy {
        my ($self, $id) = @_;
        delete $session_store{$id};
    }

    sub flush {
        my ($self, $id, $data) = @_;
        $session_store{$id} = $data;
    }

    1;

=head3 Using the Custom Session Storage

To use your custom session storage mechanism, configure it in your
F<config.yml> file:

    session: "MySessionStorage"

This simple example uses an in-memory hash to store sessions, but you
can adapt it to any storage medium, such as Redis, MongoDB, or a
relational database. Be sure to implement locking mechanisms and
persistence for production usage.

=head1 Custom Middleware

Middleware is code that runs before or after the core request handler in
Dancer2. It allows you to intercept, modify, or even short-circuit the
request/response cycle before it reaches your app’s route handlers or
after they return a response.

=head2 How Middleware Works

When a request is received, middleware runs in a predefined order:

=over 4

=item 1.

Before the app handler is executed (C<before> stage).

=item 2.

The app's route handlers process the request.

=item 3.

After the app handler processes the request and returns a response
(C<after> stage).

=back

Middleware gives you the ability to modify the request before it's passed
to the app handler, or to change the response after it has been generated.
You can also prevent the app handler from running altogether and return a
custom response directly from the middleware.

=head2 Example: Custom Middleware

Here’s an example of custom middleware that runs before and after the app
handler. It logs the incoming request, processes the app’s routes, and
then modifies the response by appending a footer message.

    package Dancer2::Middleware::MyCustomMiddleware;
    use Plack::Middleware;
    use parent 'Plack::Middleware';

    sub call {
        my ($self, $env) = @_;

        # BEFORE the app handler: Log the request method and path
        warn "Incoming request: $env->{REQUEST_METHOD} $env->{PATH_INFO}";

        # Call the app handler (if you want the app to continue processing)
        my $res = $self->app->($env);

        # AFTER the app handler: Modify the response (e.g., add a footer)
        return sub {
            my $respond = shift;
            my $writer = $respond->(
                [
                    $res->[0],    # HTTP status code
                    $res->[1],    # HTTP headers
                    $res->[2],    # HTTP body
                ]
            );

            # Append a footer to the response body
            $writer->write("\n-- Thank you for visiting Danceyland!");
            $writer->close;
        };
    }

    1;

=head2 Using the Custom Middleware

To enable this middleware, modify your F<app.psgi> file to use it with
Dancer2’s built-in support for Plack middleware:

    use Dancer2;
    use MyApp;

    builder {
        enable 'Dancer2::Middleware::MyCustomMiddleware';
        MyApp->to_app;
    };

=head3 How Middleware Can Control the Flow

In the example above, the middleware logs the request before it reaches
the app and modifies the response after the app handler runs. However,
if you don’t want to call the app handler (for example, to short-circuit
the request and return a custom response), you can skip the call to
C<< $self->app->($env) >> and return a response directly from the
middleware:

    sub call {
        my ($self, $env) = @_;

        # If the request matches a certain condition, bypass the app handler
        if ($env->{PATH_INFO} =~ m{/secret}) {
            return [403, ['Content-Type' => 'text/plain'], ["Forbidden"]];
        }

        # Otherwise, continue to the app handler
        my $res = $self->app->($env);

        # Modify the response as needed
        ...
    }

In this case, if the request path contains C</secret>, the middleware
returns a 403 Forbidden response, skipping the app handler entirely.

=head3 Order of Operations

Middleware executes in the following order:

=over 4

=item *

Any code in the C<before> stage is executed before your app processes the
request. You can modify or reject the request here.

=item *

Your app processes the request and generates a response.

=item *

Any code in the C<after> stage is executed after the response has been
generated. You can modify or append to the response here before it’s
sent to the client.

=back

Custom middleware provides great flexibility for pre- and post-processing
requests and responses in your Dancer2 app.

=head1 AUTHOR

Dancer Core Developers

=head1 COPYRIGHT AND LICENSE

This software is copyright (c) 2026 by Alexis Sukrieh.

This is free software; you can redistribute it and/or modify it under
the same terms as the Perl 5 programming language system itself.

=cut
