A simple observer implementation for POROs (Plain Old Ruby Object)
and ActiveRecord objects.

Add this line to your application's Gemfile:

gem "voltage"

And then execute:

$ bundle

Or install it yourself as:

$ gem install voltage

You can use Voltage with PORO (Plain Old Ruby Object) and ActiveRecord.

All you have to do is including the Voltage module. Then you can add listeners and trigger events.

class Status
  include Voltage

  def ready!
    emit(:ready)
  end
end

status = Status.new
status.before(:ready) { puts "Before the ready event!" }
status.on(:ready) { puts "I'm ready!" }
status.after(:ready) { puts "After the ready event!" }
status.ready!
#=> Before the ready event!
#=> I'm ready!
#=> After the ready event!

You can also pass objects that implement methods like before_*, on_* and after_*.

class MyListener
  def before_ready
    puts "Before the ready event!"
  end

  def on_ready
    puts "I'm ready!"
  end

  def after_ready
    puts "After the ready event!"
  end
end

Status.new
  .add_listener(MyListener.new)
  .ready!
#=> Before the ready event!
#=> I'm ready!
#=> After the ready event!

Executed blocks don't switch context. You always have to emit the object you're interested in. The follow example uses emit(:output, self) to send the Contact instance to all listeners.

class Contact
  include Voltage

  attr_reader :name, :email

  def initialize(name, email)
    @name, @email = name, email
  end

  def output!
    emit(:output, self)
  end
end

contact = Contact.new('John Doe', 'john@example.org')
contact.on(:output) {|contact| puts contact.name, contact.email }
contact.output!
#=> John Doe
#=> john@example.org

You can provide arguments while emitting a voltage:

class Arguments
  include Voltage
end

class MyListener
  def on_args(a, b)
    puts a, b
  end
end

Arguments.new
  .on(:args) {|a, b| puts a, b }
  .add_listener(MyListener.new)
  .emit(:args, 1, 2)

You can use Voltage with ActiveRecord, which will give you some default events like :create, :update, :remove and :validation.

class Thing < ActiveRecord::Base
  include Voltage.active_record

  validates_presence_of :name
end

thing = Thing.new(:name => "Stuff")
thing.on(:create) {|thing| puts thing.updated_at, thing.name }
thing.on(:update) {|thing| puts thing.updated_at, thing.name }
thing.on(:remove) {|thing| puts thing.destroyed? }
thing.on(:validation) {|thing| p thing.errors.full_messages }

thing.save!
#=> 2013-01-26 10:32:39 -0200
#=> Stuff

thing.update_attributes(:name => "Updated stuff")
#=> 2013-01-26 10:33:11 -0200
#=> Updated stuff

thing.update_attributes(:name => nil)
#=> ["Name can't be blank"]

thing.destroy
#=> true

These are the available events:

• before(:create): triggered before creating the record (record is valid).
• on(:create): triggered after before(:create) event.
• after(:create): triggered after the on(:create) event.
• before(:update): triggered before updating the record (record is valid).
• on(:update): triggered when the before(:update) event.
• after(:update): triggered after the on(:update) event.
• before(:remove): triggered before removing the record.
• on(:remove): triggered after the before(:remove).
• after(:remove): triggered after the on(:remove) event.
• before(:validation): triggered before validating record.
• on(:validation): triggered when record is invalid.
• after(:validation): triggered after validating record.

Although there's no special code for Rails, here's just an example of how you can use it:

class UsersController < ApplicationController
  def create
    @user = User.new(user_params)

    Signup.new(@user)
      .on(:success) { redirect_to login_path, notice: 'Welcome to MyApp!' }
      .on(:failure) { render :new }
      .call
  end
end

If you're using plain ActiveRecord, just do something like the following:

class UsersController < ApplicationController
  def create
    @user = User.new(user_params)
    @user
      .on(:create) { redirect_to login_path, notice: 'Welcome to MyApp!' }
      .on(:validation) { render :new }
      .save
  end
end

You can include Voltage.call instead, so you can have a common interface for your observable object. This will add the .call() method to the target class, which will delegate attributes to the observable's initialize method and call its call method.

class Contact
  include Voltage.call

  attr_reader :name, :email

  def initialize(name, email)
    @name, @email = name, email
  end

  def call
    emit(:output, self)
  end
end

Contact.call('John', 'john@example.com') do |o|
  o.on(:output) {|contact| puts contact }
end

Notice that you don't have to explicit call the instance's call method; Contact.call will initialize the object with all the provided parameters and call Contact#call after the block has been executed.

Voltage::Mock can be helpful for most test situations where you don't want to bring other mock libraries.

require "voltage/mock"

class SomeTest < Minitest::Test
  def test_some_test
    mock = Voltage::Mock.new

    # Using listener
    sum = Sum.new
    sum.add_listener(mock)

    # Calling `mock.on(event_name)` is required because
    # the handler doesn't receive the event name, just the
    # arguments.
    sum = Sum.new
    sum.on(:result, &mock.on(:result))

    # Using with Voltage.call
    Sum.call(1, 2, &mock)

    assert mock.received?(:result)
    assert mock.received?(:result, times: 1)
    assert mock.received?(:result, with: [3])
    assert mock.received?(:result, with: ->(result) { result == 3 } )
  end
end

1. Fork it
2. Create your feature branch (git checkout -b my-new-feature)
3. Commit your changes (git commit -am 'Add some feature')
4. Push to the branch (git push origin my-new-feature)
5. Create new Pull Request

Copyright (c) 2013 Nando Vieira

MIT License

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, sub-license, 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.