Observer

The observer pattern is a software design pattern in which an object (called subject) maintains a list of its dependents (called observers), and notifies them automatically of any state changes, usually by calling one of their methods.

Ruby provides a simple mechanism to implement the Observer design pattern. The module Observable provides the logic to notify the subscriber of any changes in the Observable object.

For this to work, the observable has to assert it has changed and notify the observers.

Objects observing have to implement an update() method, which will be the callback for the Observer.

Let’s implement a small chat, where users can subscribe to users and when one of them write something, the subscribers get notified.

require "observer"

class Moderator
  include Observable

  def initialize(name)
    @name = name
  end

  def write
    message = "Computer says: No"
    changed
    notify_observers(message)
  end
end

class Warner
  def initialize(moderator, limit)
    @limit = limit
    moderator.add_observer(self)
  end
end

class Subscriber < Warner
  def update(message)
    puts "#{message}"
  end
end

moderator = Moderator.new("Rupert")
Subscriber.new(moderator, 1)
moderator.write
moderator.write

Producing the following output:

# Computer says: No
# Computer says: No

We’ve triggered the method write at the Moderator class twice, notifying its subscribers, in this case just one.

The more subscribers we add the more the changes will propagate.