Mediator

(object behavioral pattern)

GoF definition (p. 273):

"Define an object that encapsulates how a set of objects interact. Mediator promotoes loose coupling by keeping objects from referring to each other explicitly, and it lets you vary their interaction independently."

The mediator serves as an intermediary that keeps objects the work together from referring to each other explicitly – they only refer to the mediator, which alone has knowledge of the group.

Pros

• Promotes loose coupling between colleagues.
• Abstracts how colleagues collaborate – encapsulating how objects interact in a separate object allows you to focus on how objects interact apart from their individual behavior.

Cons

• As more behavior is delegated to the mediator, it can become unwieldy and difficult to maintain.

Example

Colleagues

class Tweeter
  attr_accessor :email

  def initialize(email)
    @messages = []
    @mediator = TweeterWatcher.new
    @email = email
  end

  def post_message(message)
    @messages.push(message)
    @mediator.message_posted(self)
  end

  def latest_message
    @messages.last
  end
end

class TweeterMailer
  def mail(args)
    puts "TO: #{args[:to]} MESSAGE: #{args[:message]}"
  end
end

Mediator

class TweeterWatcher
  def initialize
    @mailer = TweeterMailer.new
  end

  def message_posted(tweeter)
    @mailer.mail({
      to: tweeter.email,
      message: tweeter.latest_message
    })
    # Maybe do some other stuff - update followers, parse for hashtags, etc.
    # This "collaboration" behavior is encapsulated within the mediator so the
    # colleagues can focus on their own focused behaviors.
  end
end

Implimentation

my_tweeter_account = Tweeter.new("[email protected]")
my_tweeter_account.post_message("Hello, world")