Wisper

A micro library providing Ruby objects with Publish-Subscribe capabilities

• Decouple core business logic from external concerns in Hexagonal style architectures
• Use as an alternative to ActiveRecord callbacks and Observers in Rails apps
• Connect objects based on context without permanence
• React to events synchronously or asynchronously

Note: Wisper was originally extracted from a Rails codebase but is not dependant on Rails.

Installation

Add this line to your application's Gemfile:

gem 'wisper'

Usage

Any class with the Wisper::Publisher module included can broadcast events to subscribed listeners. Listeners subscribe, at runtime, to the publisher.

Publishing

class CancelOrder
  include Wisper::Publisher

  def call(order_id)
    order = Order.find_by_id(order_id)

    # business logic...

    if order.cancelled?
      broadcast(:cancel_order_successful, order.id)
    else
      broadcast(:cancel_order_failed, order.id)
    end
  end
end

When a publisher broadcasts an event it can include number of arguments.

The broadcast method is also aliased as publish.

You can also include Wisper.publisher instead of Wisper::Publisher.

Subscribing

Objects

Any object can be subscribed as a listener.

cancel_order = CancelOrder.new

cancel_order.subscribe(OrderNotifier.new)

cancel_order.call(order_id)

The listener would need to implement a method for every event it wishes to receive.

class OrderNotifier
  def cancel_order_successful(order_id)
    order = Order.find_by_id(order_id)
       
    # notify someone ...    
  end
end

Blocks

Blocks can be subscribed to single events and can be chained.

cancel_order = CancelOrder.new

cancel_order.on(:cancel_order_successful) { |order_id| ... }
            .on(:cancel_order_failed)     { |order_id| ... }
            
cancel_order.call(order_id)

Handling Events Asynchronously

cancel_order.subscribe(OrderNotifier.new, async: true)

Wisper has various adapters for asynchronous event handling, please refer to wisper-celluloid, wisper-sidekiq or wisper-activejob.

Depending on the adapter used the listener may need to be a class instead of an object.

ActionController

class CancelOrderController < ApplicationController
 
  def create
    cancel_order = CancelOrder.new

    cancel_order.subscribe(OrderMailer,        async: true)
    cancel_order.subscribe(ActivityRecorder,   async: true)
    cancel_order.subscribe(StatisticsRecorder, async: true)

    cancel_order.on(:cancel_order_successful) { |order_id| redirect_to order_path(order_id) }
    cancel_order.on(:cancel_order_failed)     { |order_id| render action: :new }

    cancel_order.call(order_id)
  end
end

ActiveRecord

If you wish to publish directly from ActiveRecord models you can broadcast events from callbacks:

class Order < ActiveRecord::Base
  include Wisper::Publisher
  
  after_commit     :publish_creation_successful, on: :create
  after_validation :publish_creation_failed,     on: :create

  private

  def publish_creation_successful
    broadcast(:order_creation_successful, self)
  end

  def publish_creation_failed
    broadcast(:order_creation_failed, self) if errors.any?
  end
end

There are more examples in the Wiki.

Global Listeners

Global listeners receive all broadcast events which they can respond to.

This is useful for cross cutting concerns such as recording statistics, indexing, caching and logging.

Wisper.subscribe(MyListener.new)

In a Rails app you might want to add your global listeners in an initalizer.

Global listeners are threadsafe.

Scoping by publisher class

You might want to globally subscribe a listener to publishers with a certain class.

Wisper.subscribe(MyListener.new, scope: :MyPublisher)

This will subscribe the listener to all instances of MyPublisher and its subclasses.

Alternatively you can also do exactly the same with a publisher class itself:

MyPublisher.subscribe(MyListener.new)

Temporary Global Listeners

You can also globally subscribe listeners for the duration of a block.

Wisper.subscribe(MyListener.new, OtherListener.new) do
  # do stuff
end

Any events broadcast within the block by any publisher will be sent to the listeners.

This is useful for capturing events published by objects to which you do not have access in a given context.

Temporary Global Listeners are threadsafe.

Subscribing to selected events

By default a listener will get notified of all events it can respond to. You can limit which events a listener is notified of by passing an string, symbol, array or regular expression to on:

post_creater.subscribe(PusherListener.new, on: :create_post_successful)

Prefixing broadcast events

If you would prefer listeners to receive events with a prefix, for example on, you can do so by passing a string or symbol to prefix:.

post_creater.subscribe(PusherListener.new, prefix: :on)

If post_creater were to broadcast the event post_created the subscribed listeners would receive on_post_created. You can also pass true which will use the default prefix, "on".

Mapping an event to a different method

By default the method called on the listener is the same as the event broadcast. However it can be mapped to a different method using with:.

report_creator.subscribe(MailResponder.new, with: :successful)

This is pretty useless unless used in conjuction with on:, since all events will get mapped to :successful. Instead you might do something like this:

report_creator.subscribe(MailResponder.new, on:   :create_report_successful,
                                            with: :successful)

If you pass an array of events to on: each event will be mapped to the same method when with: is specified. If you need to listen for select events and map each one to a different method subscribe the listener once for each mapping:

report_creator.subscribe(MailResponder.new, on:   :create_report_successful,
                                            with: :successful)

report_creator.subscribe(MailResponder.new, on:   :create_report_failed,
                                            with: :failed)

You could also alias the method within your listener, as such alias successful create_report_successful.

RSpec

Please see wisper-rspec.

Clearing Global Listeners

If you use global listeners in non-feature tests you might want to clear them in a hook to prevent global subscriptions persisting between tests.

after { Wisper.clear }

Compatibility

Tested with MRI 1.9.x, MRI 2.0.0, JRuby (1.9 and 2.0 mode) and Rubinius (1.9 mode).

See the build status for details.

Running Specs

rspec spec

There is both a Rakefile and Guardfile, if you like you prefer to run the specs using guard-rspec or rake.

License

(The MIT License)

Copyright (c) 2013 Kris Leech

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the 'Software'), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED 'AS IS', WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.