Writing the plugin

The plugin must begin with

    use Dancer2::Plugin;

which will turn the package into a Moo class that inherits from Dancer2::Plugin. The base class provides the plugin with two attributes: "app", which is populated with the Dancer2 app object for which the plugin is being initialized for, and "config" which holds the plugin section of the application configuration.

Modifying the app at building time

If the plugin needs to tinker with the application -- add routes or hooks, for example -- it can do so within its "BUILD()" function.

    sub BUILD {
        my $plugin = shift;

        $plugin->app->add_route( ... );
    }

Adding keywords

Via "plugin_keywords"

Keywords that the plugin wishes to export to the Dancer2 app can be defined via the "plugin_keywords" keyword:

    plugin_keywords qw/
        add_smileys
        add_sad_kitten
    /;

Each of the keyword will resolve to the class method of the same name. When invoked as keyword, it'll be passed the plugin object as its first argument.

    sub add_smileys {
        my( $plugin, $text ) = @_;

        return join ' ', $text, $plugin->smiley;
    }

    # and then in the app

    get '/' => sub {
        add_smileys( "Hi there!" );
    };

You can also pass the functions directly to "plugin_keywords".

    plugin_keywords
        add_smileys => sub {
            my( $plugin, $text ) = @_;

            $text =~ s/ (?<= \. ) / $plugin->smiley /xeg;

            return $text;
        },
        add_sad_kitten => sub { ... };

Or a mix of both styles. We're easy that way:

    plugin_keywords
        add_smileys => sub {
            my( $plugin, $text ) = @_;

            $text =~ s/ (?<= \. ) / $plugin->smiley /xeg;

            return $text;
        },
        'add_sad_kitten';

    sub add_sad_kitten {
        ...;
    }

If you want several keywords to be synonyms calling the same function, you can list them in an arrayref. The first function of the list is taken to be the "real" method to link to the keywords.

    plugin_keywords [qw/ add_smileys add_happy_face /];

    sub add_smileys { ... }

Calls to "plugin_keywords" are cumulative.

Via the ":PluginKeyword" function attribute

For perl 5.12 and higher, keywords can also be defined by adding the ":PluginKeyword" attribute to the function you wish to export.

For Perl 5.10, the export triggered by the sub attribute comes too late in the game, and the keywords won't be exported in the application namespace.

    sub foo :PluginKeyword { ... }

    sub bar :PluginKeyword( baz quux ) { ... }

    # equivalent to

    sub foo { ... }
    sub bar { ... }

    plugin_keywords 'foo', [ qw/ baz quux / ] => \&bar;

For an attribute

You can also turn an attribute of the plugin into a keyword.

    has foo => (
        is => 'ro',
        plugin_keyword => 1,  # keyword will be 'foo'
    );

    has bar => (
        is => 'ro',
        plugin_keyword => 'quux',  # keyword will be 'quux'
    );

    has baz => (
        is => 'ro',
        plugin_keyword => [ 'baz', 'bazz' ],  # keywords will be 'baz' and 'bazz'
    );

Accessing the plugin configuration

The plugin configuration is available via the "config()" method.

    sub BUILD {
        my $plugin = shift;

        if ( $plugin->config->{feeling_polite} ) {
            $plugin->app->add_hook( Dancer2::Core::Hook->new(
                name => 'after',
                code => sub { $_[0]->content( $_[0]->content . " ... please?" ) }
            ));
        }
    }

Getting default values from config file

Since initializing a plugin with either a default or a value passed via the configuration file, like

    has smiley => (
        is => 'ro',
        default => sub {
            $_[0]->config->{smiley} || ':-)'
        }
    );

"Dancer2::Plugin" allows for a "from_config" key in the attribute definition. Its value is the plugin configuration key that will be used to initialize the attribute.

If it's given the value 1, the name of the attribute will be taken as the configuration key.

Nested hash keys can also be referred to using a dot notation.

If the plugin configuration has no value for the given key, the attribute default, if specified, will be honored.

If the key is given a coderef as value, it's considered to be a "default" value combo:

    has foo => (
        is => 'ro',
        from_config => sub { 'my default' },
    );


    # equivalent to
    has foo => (
        is => 'ro',
        from_config => 'foo',
        default => sub { 'my default' },
    );

For example:

    # in config.yml

    plugins:
        Polite:
            smiley: ':-)'
            greeting:
                casual: Hi!
                formal: How do you do?


    # in the plugin

    has smiley => (             # will be ':-)'
        is          => 'ro',
        from_config => 1,
        default     => sub { ':-(' },
    );

    has casual_greeting => (    # will be 'Hi!'
        is          => 'ro',
        from_config => 'greeting.casual',
    );

    has apology => (            # will be 'sorry'
        is          => 'ro',
        from_config => 'apology',
        default     => sub { 'sorry' },
    )

    has closing => (            # will be 'See ya!'
        is => 'ro',
        from_config => sub { 'See ya!' },
    );

Config becomes immutable

The plugin's "config" attribute is loaded lazily on the first call to "config". After this first call "config" becomes immutable so you cannot do the following in a test:

    use Dancer2;
    use Dancer2::Plugin::FooBar;

    set plugins => {
        FooBar => {
            wibble => 1,  # this is OK
        },
    };

    flibble(45);          # plugin keyword called which causes config read
    
    set plugins => {
        FooBar => {
            wibble => 0,  # this will NOT change plugin config
        },
    };

Accessing the parent Dancer app

If the plugin is instantiated within a Dancer app, it'll be accessible via the method "app()".

    sub BUILD {
        my $plugin = shift;

        $plugin->app->add_route( ... );
    }

To use Dancer's DSL in your plugin:

    $self->dsl->debug( “Hi! I’m logging from your plugin!” );

See "DSL KEYWORDS" in Dancer2::Manual for a full list of Dancer2 DSL.

Using the plugin within the app

    use Dancer2::Plugin::Polite;

The plugin will assume that it's loading within a Dancer module and will automatically register itself against its "app()" and export its keywords to the local namespace. If you don't want this to happen, specify that you don't want anything imported via empty parentheses when "use"ing the module:

    use Dancer2::Plugin::Polite ();

Plugins using plugins

    package Dancer2::Plugin::SourPuss;
    
    use Dancer2::Plugin; 
    use Dancer2::Plugin::Polite; 
    
    sub my_keyword { my $smiley = smiley(); } 

    1;

This does not export "smiley()" into your application - it is only available from within your plugin. However, from the example above, you can wrap DSL from other plugins and make it available from your plugin.

Utilizing other plugins

    # MyApp.pm
    use Dancer2;
    use Dancer2::Plugin::Foo;
    use Dancer2::Plugin::Bar;

    # Dancer2::Plugin::Bar;
    ...

    sub my_keyword {
        my $self = shift;
        my $foo  = $self->find_plugin('Dancer2::Plugin::Foo')
            or $self->dsl->send_error('Could not find Foo');

        return $foo->foo_keyword(...);
    }

Hooks

    plugin_hooks 'my_hook', 'my_other_hook';

Hooks are prefixed with "plugin.plugin_name". So the plugin "my_hook" coming from the plugin "Dancer2::Plugin::MyPlugin" will have the hook name "plugin.myplugin.my_hook".

Hooks are executed within the plugin by calling them via the associated app.

    $plugin->execute_plugin_hook( 'my_hook' );

You can also call any other hook if you provide the full name using the "execute_hook" method:

    $plugin->app->execute_hook( 'core.app.route_exception' );

Or using their alias:

    $plugin->app->execute_hook( 'on_route_exception' );

Note: If your plugin consumes a plugin that declares any hooks, those hooks are added to your application, even though DSL is not.

Writing Test Gotchas

You'll usually get this one because you are defining both the plugin and app in your test file, and the runtime creation of Moo's attributes happens after the compile-time import voodoo dance.

To get around this nightmare, wrap your plugin definition in a "BEGIN" block.

    BEGIN {
        package Dancer2::Plugin::Foo;

        use Dancer2::Plugin;

            has bar => (
                is => 'ro',
                from_config => 1,
            );

            plugin_keywords qw/ bar /;

    }

    {
        package MyApp;

        use Dancer2;
        use Dancer2::Plugin::Foo;

        bar();
    }

You cannot overwrite a locally defined method (bar) with a reader

If you set an object attribute of your plugin to be a keyword as well, you need to call "plugin_keywords" after the attribute definition.

    package Dancer2::Plugin::Foo;

    use Dancer2::Plugin;

    has bar => (
        is => 'ro',
    );

    plugin_keywords 'bar';