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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.