MooX::Role::Pluggable - A plugin pipeline for your Moo-based class


  # A simple pluggable dispatcher:
  package MyDispatcher;
  use MooX::Role::Pluggable::Constants;
  use Moo;
  with 'MooX::Role::Pluggable';

  sub BUILD {
    my ($self) = @_;

    # (optionally) Configure our plugin pipeline
      reg_prefix => 'Plug_',
      ev_prefix  => 'Event_',
      types      => {
        NOTIFY  => 'N',
        PROCESS => 'P',

  around '_pluggable_event' => sub {
    # This override redirects internal events (errors, etc) to ->process()
    my ($orig, $self) = splice @_, 0, 2;
    $self->process( @_ )

  sub process {
    my ($self, $event, @args) = @_;

    # Dispatch to 'P_' prefixed "PROCESS" type handlers.
    # _pluggable_process will automatically strip a leading 'ev_prefix'
    # (see the call to _pluggable_init above); that lets us easily
    # dispatch errors to our P_plugin_error handler below without worrying
    # about our ev_prefix ourselves:
    my $retval = $self->_pluggable_process( PROCESS =>

    unless ($retval == EAT_ALL) {
      # The pipeline allowed the event to continue.
      # A dispatcher might re-dispatch elsewhere, etc.

  sub shutdown {
    my ($self) = @_;
    # Unregister all of our plugins.

  sub P_plugin_error {
    # Since we re-dispatched errors in our _pluggable_event handler,
    # we could handle exceptions here and then eat them, perhaps:
    my ($self, undef) = splice @_, 0, 2;

    # Arguments are references:
    my $plug_err  = ${ $_[0] };
    my $plug_obj  = ${ $_[1] };
    my $error_src = ${ $_[2] };

    # ...


  # A Plugin object.
  package MyPlugin;

  use MooX::Role::Pluggable::Constants;

  sub new { bless {}, shift }

  sub Plug_register {
    my ($self, $core) = @_;

    # Subscribe to events:
    $core->subscribe( $self, 'PROCESS',

    # Log that we're here, do some initialization, etc ...

    return EAT_NONE

  sub Plug_unregister {
    my ($self, $core) = @_;
    # Called when this plugin is unregistered
    # ... do some cleanup, etc ...
    return EAT_NONE

  sub P_my_event {
    # Handle a dispatched "PROCESS"-type event:
    my ($self, $core) = splice @_, 0, 2;

    # Arguments are references and can be modified:
    my $arg = ${ $_[0] };

    # ... do some work ...

    # Return an EAT constant to control event lifetime
    # EAT_NONE allows this event to continue through the pipeline
    return EAT_NONE

  # An external package that interacts with our dispatcher;
  # this is just a quick and dirty example to show external
  # plugin manipulation:

  package MyController;
  use Moo;

  has dispatcher => (
    is      => 'rw',
    default => sub {  MyDispatcher->new()  },

  sub BUILD {
    my ($self) = @_;
    $self->dispatcher->plugin_add( 'MyPlugin', MyPlugin->new );

  sub do_stuff {
    my $self = shift;
    $self->dispatcher->process( 'my_event', @_ )


A Moo::Role for turning instances of your class into pluggable objects. Consumers of this role gain a plugin pipeline and methods to manipulate it, as well as a flexible dispatch system (see "_pluggable_process").

The logic and behavior is based almost entirely on Object::Pluggable (see "AUTHOR"). Some methods are the same; implementation & some interface differ. Dispatch is significantly faster -- see "Performance".

If you're using POE, also see MooX::Role::POE::Emitter, which consumes this role.



    # Prefix for registration events.
    # Defaults to 'plugin_' ('plugin_register' / 'plugin_unregister')
    reg_prefix   => 'plugin_',

    # Prefix for dispatched internal events
    #  (add, del, error, register, unregister ...)
    # Defaults to 'plugin_ev_'
    event_prefix => 'plugin_ev_',

    # Map type names to prefixes;
    #  Event types can be named arbitrarily. Their respective prefix is
    #  prepended when dispatching events of that type.
    #  Here are the defaults:
    types => {
      NOTIFY  => 'N',
      PROCESS => 'P',

A consumer can call _pluggable_init to set up pipeline-related options appropriately; this should be done prior to loading plugins or dispatching to "_pluggable_process". If it is not called, the defaults (as shown above) are used.

types => can be either an ARRAY of event types (which will be used as prefixes):

  types => [ qw/ IncomingEvent OutgoingEvent / ],

... or a HASH mapping an event type to a prefix:

  types => {
    Incoming => 'I',
    Outgoing => 'O',

A trailing _ is automatically appended to event type prefixes when events are dispatched via "_pluggable_process"; thus, an event destined for our 'Incoming' type shown above will be dispatched to appropriate I_ handlers:

  # Dispatched to 'I_foo' method in plugins registered for Incoming 'foo':
  $self->_pluggable_process( Incoming => 'foo', 'bar', 'baz' );

reg_prefix/event_prefix are not automatically munged in any way.

An empty string is a valid value for reg_prefix/event_prefix.



Shuts down the plugin pipeline, unregistering/unloading all known plugins.


  # In our consumer:
  sub _pluggable_event {
    my ($self, $event, @args) = @_;
    # Dispatch out, perhaps.

_pluggable_event is called for internal notifications, such as plugin load/unload and error reporting (see "Internal Events") -- it can be overriden in your consuming class to do something useful with the dispatched event and any arguments passed.

The $event given will be prefixed with the configured event_prefix.

(It's not strictly necessary to implement a _pluggable_event handler; errors will also warn.)


A plugin is any blessed object that is registered with your Pluggable object via "plugin_add"; during registration, plugins usually subscribe to some events via "subscribe".

See "plugin_add" regarding loading plugins.


Subscribe a plugin to some pluggable events.

  $self->subscribe( $plugin_obj, $type, @events );

Registers a plugin object to receive @events of type $type.

This is typically called from within the plugin's registration handler (see "plugin_register"):

  # In a plugin:
  sub plugin_register {
    my ($self, $core) = @_;

    $core->subscribe( $self, PROCESS =>

    $core->subscribe( $self, NOTIFY => 'all' );


Subscribe to all to receive all events -- but note that subscribing many plugins to 'all' events is less performant during calls to "_pluggable_process" than many subscriptions to specific events.


Unsubscribe a plugin from subscribed events.

Carries the same arguments as "subscribe".

The plugin is still loaded and registered until "plugin_del" is called, even if there are no current event subscriptions.


Defined in your plugin(s) and called at load time.

(Note that 'plugin_' is just a default register method prefix; it can be changed prior to loading plugins. See "_pluggable_init" for details.)

The plugin_register method is called on a loaded plugin when it is added to the pipeline; it is passed the plugin object ($self), the Pluggable object, and any arguments given to "plugin_add" (or similar registration methods).

Normally one might call a "subscribe" from here to start receiving events after load-time:

  # In a plugin:
  sub plugin_register {
    my ($self, $core, @args) = @_;
    $core->subscribe( $self, 'NOTIFY', @events );


Defined in your plugin(s) and called at load time.

(Note that 'plugin_' is just a default register method prefix; it can be changed prior to loading plugins. See "_pluggable_init" for details.)

The unregister counterpart to "plugin_register", called when the plugin object is removed from the pipeline (via "plugin_del" or "_pluggable_destroy").

  # In a plugin:
  sub plugin_unregister {
    my ($self, $core) = @_;

Carries the same arguments as "plugin_register".



  # In your consumer's dispatch method:
  my $eat = $self->_pluggable_process( $type, $event, \@args );
  return 1 if $eat == EAT_ALL;

The _pluggable_process method handles dispatching.

If $event is prefixed with our event prefix (see "_pluggable_init"), the prefix is stripped prior to dispatch (to be replaced with a type prefix matching the specified $type).

Arguments should be passed as a reference to an array. During dispatch, references to the provided arguments are passed to relevant plugin subroutines following the automatically-prepended plugin and Pluggable consumer objects (respectively); this allows for argument modification as an event is passed along the plugin pipeline:

  my @args = qw/baz bar/;
  $self->_pluggable_process( NOTIFY => foo => \@args );

  # In a plugin:
  sub N_foo {
    # Remove automatically-provided plugin and consumer objects from @_
    my ($self, $core) = splice @_, 0, 2;

    # Dereference expected scalars
    my $bar = ${ $_[0] };
    my $num = ${ $_[1] };

    # Increment actual second argument before pipeline dispatch continues
    ++${ $_[1] };


Dispatch Process

Your Pluggable consuming class typically provides syntax sugar to dispatch different types or "classes" of events:

  sub process {
    # Dispatch to 'PROCESS'-type events
    my ($self, $event, @args) = @_;
    my $eat = $self->_pluggable_process( PROCESS => $event, \@args );
    # ... possibly take further action based on $eat return value, see below

  sub notify {
    # Dispatch to 'NOTIFY'-type events
    my ($self, $event, @args) = @_;
    my $eat = $self->_pluggable_process( NOTIFY => $event, \@args );
    # ...

Event types and matching prefixes can be arbitrarily named to provide event dispatch flexibility. For example, the dispatch process for $event 'foo' of $type 'NOTIFY' performs the following actions:

  $self->_pluggable_process( NOTIFY => foo => \@args );

  # - Prepend the known prefix for the specified type and '_'
  #   'foo' -> 'N_foo'
  # - Attempt to dispatch to $self->N_foo()
  # - If no such method, attempt to dispatch to $self->_default()
  #   (When using _default, the method we were attempting to call is prepended
  #   to arguments.)
  # - If the event was not eaten by the Pluggable consumer (see below), call
  #   $plugin->N_foo() for subscribed plugins sequentially until event is eaten
  #   or no relevant plugins remain.

"Eaten" means a handler returned an EAT_* constant from MooX::Role::Pluggable::Constants indicating that the event's lifetime should terminate.

If our consuming class provides a method or '_default' that returns:

    EAT_ALL:    skip plugin pipeline, return EAT_ALL
    EAT_CLIENT: continue to plugin pipeline
                return EAT_ALL if plugin returns EAT_PLUGIN later
    EAT_PLUGIN: skip plugin pipeline entirely
                return EAT_NONE unless EAT_CLIENT was seen previously
    EAT_NONE:   continue to plugin pipeline

If one of our plugins in the pipeline returns:

    EAT_ALL:    skip further plugins, return EAT_ALL
    EAT_CLIENT: continue to next plugin, set pending EAT_ALL
                (EAT_ALL will be returned when plugin processing finishes)
    EAT_PLUGIN: return EAT_ALL if previous sub returned EAT_CLIENT
                else return EAT_NONE
    EAT_NONE:   continue to next plugin

This functionality (derived from Object::Pluggable) provides fine-grained control over event lifetime.

Higher-level layers (see MooX::Role::POE::Emitter for an example) can check for an EAT_ALL return value from "_pluggable_process" to determine whether to continue operating on a particular event (re-dispatch elsewhere, for example).

Plugins can use EAT_CLIENT to indicate that an event should be eaten after plugin processing is complete, EAT_PLUGIN to stop plugin processing, and EAT_ALL to indicate that the event should not be dispatched further.

Plugin Management Methods

These plugin pipeline management methods will set $@, warn via Carp, and return an empty list on error (unless otherwise noted). See "plugin_error" regarding errors raised during plugin registration and dispatch.


  $self->plugin_add( $alias, $plugin_obj, @args );

Add a plugin object to the pipeline.

Returns the same values as "plugin_pipe_push".


  $self->plugin_del( $alias_or_plugin_obj, @args );

Remove a plugin from the pipeline.

Takes either a plugin alias or object. Returns the removed plugin object.


  my $plug_obj = $self->plugin_get( $alias );
        my ($plug_obj, $plug_alias) = $self->plugin_get( $alias_or_plugin_obj );

In scalar context, returns the plugin object belonging to the specified alias.

In list context, returns the object and alias, respectively.


  my @loaded = $self->plugin_alias_list;

Returns the list of loaded plugin aliases.

As of version 1.002, the list is ordered to match actual plugin dispatch order. In prior versions, the list is unordered.


    old    => $alias_or_plugin_obj,
    alias  => $new_alias,
    plugin => $new_plugin_obj,
    # Optional:
    register_args   => [ ],
    unregister_args => [ ],

Replace an existing plugin object with a new one.

Returns the old (removed) plugin object.

Pipeline Methods


  $self->plugin_pipe_push( $alias, $plugin_obj, @args );

Add a plugin to the end of the pipeline; typically one would call "plugin_add" rather than using this method directly.


  my $plug = $self->plugin_pipe_pop( @unregister_args );

Pop the last plugin off the pipeline, passing any specified arguments to "plugin_unregister".

In scalar context, returns the plugin object that was removed.

In list context, returns the plugin object and alias, respectively.


  $self->plugin_pipe_unshift( $alias, $plugin_obj, @args );

Add a plugin to the beginning of the pipeline.

Returns the total number of loaded plugins (or an empty list on failure).


  $self->plugin_pipe_shift( @unregister_args );

Shift the first plugin off the pipeline, passing any specified args to "plugin_unregister".

In scalar context, returns the plugin object that was removed.

In list context, returns the plugin object and alias, respectively.


  my $idx = $self->plugin_pipe_get_index( $alias_or_plugin_obj );
  if ($idx < 0) {
    # Plugin doesn't exist

Returns the position of the specified plugin in the pipeline.

Returns -1 if the plugin does not exist.


    after  => $alias_or_plugin_obj,
    alias  => $new_alias,
    plugin => $new_plugin_obj,
    # Optional:
    register_args => [ ],

Add a plugin to the pipeline after the specified previously-existing alias or plugin object. Returns boolean true on success.


    before => $alias_or_plugin_obj,
    alias  => $new_alias,
    plugin => $new_plugin_obj,
    # Optional:
    register_args => [ ],

Similar to "plugin_pipe_insert_after", but insert before the specified previously-existing plugin, not after.


  $self->plugin_pipe_bump_up( $alias_or_plugin_obj, $count );

Move the specified plugin 'up' $count positions in the pipeline.

Returns -1 if the plugin cannot be bumped up any farther.


  $self->plugin_pipe_bump_down( $alias_or_plugin_obj, $count );

Move the specified plugin 'down' $count positions in the pipeline.

Returns -1 if the plugin cannot be bumped down any farther.

Internal Events

These events are dispatched to "_pluggable_event" prefixed with our pluggable event prefix; see "_pluggable_init".


Issued via "_pluggable_event" when an error occurs.

The arguments are, respectively: the error string, the offending object, and a string describing the offending object ('self' or 'plugin' with name appended).


Issued via "_pluggable_event" when a new plugin is registered.

Arguments are the new plugin alias and object, respectively.


Issued via "_pluggable_event" when a plugin is unregistered.

Arguments are the old plugin alias and object, respectively.


My motivation for writing this role was two-fold; I wanted Object::Pluggable behavior but without screwing up my class inheritance, and I needed a little bit more juice out of the pipeline dispatch process for a fast-paced daemon.

Dispatcher performance has been profiled and micro-optimized, but I'm most certainly open to further ideas ;-)

Some Benchmark runs. 30000 "_pluggable_process" calls with 20 loaded plugins dispatching one argument to one handler that does nothing except return EAT_NONE:

                      Rate    object-pluggable moox-role-pluggable
  object-pluggable    6173/s                  --                -38%
  moox-role-pluggable 9967/s                 61%

                       Rate    object-pluggable moox-role-pluggable
  object-pluggable     6224/s                  --                -38%
  moox-role-pluggable 10000/s                 61%                  --

                      Rate    object-pluggable moox-role-pluggable
  object-pluggable    6383/s                  --                -35%
  moox-role-pluggable 9868/s                 55%

(Benchmark script is available in the bench/ directory of the upstream repository; see


Jon Portnoy <>

Written from the ground up, but conceptually derived entirely from Object::Pluggable (c) Chris Williams, Apocalypse, Hinrik Orn Sigurosson and Jeff Pinyan.

Licensed under the same terms as Perl 5; please see the license that came with your Perl distribution for details.