The Perl Toolchain Summit needs more sponsors. If your company depends on Perl, please support this very important event.
 / 
Dist-Zilla-Plugin-Beam-Connector-0.001003
River stage one • 1 direct dependent • 6 total dependents
++
/ Dist::Zilla::Plugin::Beam::Connector

NAME

Dist::Zilla::Plugin::Beam::Connector - Connect events to listeners in Dist::Zilla plugins.

VERSION

version 0.001003

SYNOPSIS

  [Some::PluginA / PluginA]
  [Some::PluginB / PluginB]

  [Beam::Connector]
  ; PluginA emitting event 'foo' passes the event to PluginB
  on   = plugin:PluginA#foo    =>   plugin:PluginB#handle_foo
  on   = plugin:PluginA#bar    =>   plugin:PluginB#handle_bar
  ; Load 'beam.yml' as a Beam::Wire container
  container = beam.yml
  ; Handle Dist::Zilla plugin events with arbitrary classes
  ; loaded by Beam::Wire
  on   = plugin:PluginA#foo    =>   container:servicename#handle_foo
  on   = plugin:PluginA#bar    =>   container:otherservicename#handle_bar

DESCRIPTION

This module aims to allow Dist::Zilla to use plugins using Beam::Event and Beam::Emitter, and perhaps reduce the need for massive amounts of composition and role application proliferating CPAN.

This is in lieu of a decent dependency injection system, and is presently relying on Dist::Zilla to load and construct the plugins itself, and then you just connect the plugins together informally, without necessitating each plugin be specifically tailored to the recipient.

Hopefully, this may also give scope for non-dzil plugins being loadable into memory some day, and allowing message passing of events to those plugins. ( Hence, the plugin: prefix )

A Real World Example of what a future could look like?

  [GatherDir]

  [Test::Compile]

  [Beam::Connector]
  on = plugin:GatherDir#collect => plugin:Test::Compile#generate_test

GatherDir in this example would build a mutable tree of files, attach them to an event ::GatherDir::Tree, and pass that event to Test::Compile#generate_test, which would then add ( or remove, or mutate ) any files in that tree.

Tree state mutation then happens in order of prescription, in the order given by the various on declarations.

Thus, a single plugin can be in 2 places in the same logical stage.

  [Beam::Connector]
  on = plugin:GatherDir#collect => plugin:Test::Compile#generate_test
  ; lots more collectors here
  on = plugin:GatherDir#collect => plugin:Test::Compile#finalize_test

Whereas presently, order of affect is either governed by:

  • phase - where you can add but not remove or mutate, mutate but not add or remove, remove, but not add or mutate

  • plugin order - where a single plugin cant be both early in a single phase and late

If that example is not convincing enough for you, consider all the different ways there are presently for implementing [MakeMaker]. If you're following the standard logic its fine, but as soon as you set out of the box, you have a few things you're going to have to do instead:

  • Subclass MakeMaker in some way

  • Re-implement MakeMaker in some way

  • Fuss a lot with phase ordering and then inject code in the File that MakeMaker generates.

These approaches all work, but they're an open door to everyone re-implementing the same thing thousands of times over.

  [MakeMaker]

  [DynamicPrereqs]
  -phases = none

  [Beam::Connector]
  on = plugin:MakeMaker#collect_augments => plugin:DynamicPrereqs#inject_augments

MakeMaker here can just create an event, pass it to DynamicPrereqs, DynamicPrereqs can inject its desired content into the event, and then MakeMaker can integrate the injected events at "wherever" the right place for them is.

This is much superior to scraping the generated text file and injecting events at a given place based on a RegEx match.

PARAMETERS

container

Allows loading an arbitrary Beam::Wire container specification, initializing the relevant objects lazily, and connecting them to relevant events emitted by dzil plugins.

  [Beam::Connector]
  container = inc/dist_beam.yml

The value can be a path to any file name that Beam::Wire->new( file => ... ) understands, (which itself is any file name that Config::Any->load_files understands).

Items in loaded container can then be referred to by their identifiers to the on parameter in the form

  container:${name}#${method}

For example:

  [Beam::Connector]
  container = inc/dist_beam.yml
  on = plugin:GatherDir#gather_files => container:file_gatherer#on_gather_files

This would register the object called file_gatherer inside the container to be a recipient of any events called gather_files emitted by the plugin named GatherDir

on

Defines a connection between an event emitter and a listener.

The general syntax is:

  on = emitterspec => listenerspec

Where emitterspec and listenerspec are of the form

  objectnamespace:objectname#connector

objectnamespace

There are presently two defined object name-spaces.

  • plugin: Resolves objectname to a Dist::Zilla plugin by its name identifier

  • container: Resolves objectname to an explicitly named object inside an associated container

connector

For an emitter, the connector property identifies the name of the event that is expected to be emitted by that emitter

For a listener, the connector property identifies the name of a method that is expected to receive the event.

WRITING EVENT EMITTERS

Adding support for hookable events in new and existing Dist::Zilla plugins is relatively straight-forward, and uses Beam::Emitter

  # Somewhere after `use Moose`
  with "Beam::Emitter";

And your class is now ready to broadcast events, and plugins are now able to hook events. Even though they don't exist yet.

But that's not very useful in itself. You need to find good places in your code to write events, and construct little bundles of state, "messages" to pass around, and perhaps, allow modifying.

Designing an Event

You want to start off designing an event class that communicates the absolute minimum required to be useful.

Carrying too much state, or too much indirect state is the enemy.

For instance, it would generally be unwise to design an Event that you passed to something which carried a $zilla instance with it.

You want to make it as obscure as possible who is even sending the event, as the contents of the event should be usable in total isolation, because you have no idea where your events are going to get sent ( because that is outside the scope of your plugin ), and receivers have no solid expectations of where events are going to come from ( because that is dictated by the connector ).

Namespace and Indexing recommendations

It is presently recommended you define these events inline somewhere, either in the plugin that emits them, or in some shared container.

The recommended namespace scheme to follow is:

  Dist::Zilla::Event::

Preferably, structuring it similar to your plugin

  Dist::Zilla::Plugin::Thing::Dooer
  Dist::Zilla::Event::Thing::Dooer::BeforeDoingThing

This I'm sure you'll agree is much nicer than

  Dist::Zilla::Plugin::Thing::Dooer::BeforeDoingThingEvent # O_O
  Dist::Zilla::Plugin::BeforeDoingThingEvent               # Not a plugin

It is also recommended to NOT index said Event packages at present, as that would encourage people depending on the events at some point, which for this system, is likely unwanted toxicity.

Only people emitting events should be caring about loading the class.

Implementing an Event

Events themselves are quite straight forward: They're just objects, objects extending Beam::Event.

This is an example event definition: It will communicate a file name it intends to prepend lines to and pass a mutable, empty array for the event handler to inject lines into.

  package # hide from PAUSE
    Dist::Zilla::Event::Prepender::BeforePrepend;

  use Moose;  # or Moo, both work
  extends "Beam::Event"

  has 'filename' => (
      is       => 'ro',
      isa      => Str,
      required => 1,
  );
  has 'lines' => (
      is      => 'rw',
      isa     => ArrayRef[Str],
      lazy    => 1,
      default => sub { [] },
  );
  __PACKAGE__->meta->make_immutable;

See Using Custom Events in Beam::Emitter for details.

Emitting and Handling an Event

Once you have an Event class designed, gluing it into your code is also quite simple:

  # somewhere deep in your plugin

  my $event = $self->emit(
    'before_append',                                          # the "name" of the event, this corresponds to the "connector"
                                                              # property in Beam::Connector

    class => 'Dist::Zilla::Event::Prepender::BeforePrepend',  # The class to construct an instance of

    filename => 'lib/Foo.pm',                                 # attribute property of the Event object.
  );

An instance of class is created with the defined name, and is passed in-order to all the objects who subscribed to the before_append event, and then returned once they're done.

And then you can extract any of the state in the passed object and use it to do your work.

WRITING EVENT LISTENERS

Fortunately, the requirements for an Event Receiver is very low.

Receiving Events

If you're using the Dist::Zilla::Plugin/plugin: approach, all that is required is

  • A Valid Dist::Zilla plugin that registers in $zilla->plugins

  • Some method name of any description that can be passed an argument

For Example:

  package My::Plugin;

  use Moose;
  with 'Dist::Zilla::Role::Plugin';

  sub on_before_append {
    my ( $self, $event ) = @_;
    ...
  }

If you're using the Beam::Wire/container: approach, all that is required is:

  • A named object

  • Some method name of any description that can be passed an argument

For Example:

  package My::Listener;

  sub new { bless {}, $_[0] }

  sub on_before_append {
    my ( $self, $event ) = @_;
    ...
  }

These listeners will do nothing on their own, but have events routed to them by relevant Beam configuration.

Identifying and Handling Events

Your method will be called with one argument: The event.

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

  }

What sort of events you receive of course depends on who sent them.

You can then filter them the same way as you would with any Perl Object, via ->isa etc,

  sub on_whatever {
    my ( $self, $event ) = @_;
    if ( $event->isa('Dist::Zilla::Plugin::Prepender::AppenderEvent') ) {

    }
  }

But you can identify events by other means, via the ->name property.

  sub on_whatever {
    my ( $self, $event ) = @_;
    if ( q[before_append] eq $event->name ) ) {

    }
  }

You can then read the data of the event, or potentially modify it in-place, to communicate data back to the sender of the event.

  sub on_whatever {
    my ( $self, $event ) = @_;
    if ( q[before_append] eq $event->name ) ) {
      push @{$event->lines}, 'use Moose;' if $event->filename =~ /\bMooseX\b/; # Rediculous example I know.
    }
  }

But you don't need to return anything from the sub, return values are entirely ignored.

FOOTNOTE

Beam::Event and Beam::Emitter have some tools for controlling intra-event flow, however, their usage is not 100% clear and their API may be subject to change in future.

So I have deleted the relevant instruction on this and it will be resurrected when I'm more sure about how it should be instructed.

AUTHOR

Kent Fredric <kentnl@cpan.org>

COPYRIGHT AND LICENSE

This software is copyright (c) 2017 by Kent Fredric <kentfredric@gmail.com>.

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