Observer pattern

When to use

If you want to perform an action when something changes / is called on a second object. You want to clearly separate the two objects as the behaviour is not something which should all be encapsulated in the one object.

You want to clearly separate the source of the news from the consumer

e.g. you update a spreadsheet cell. It should also trigger formatting changes etc., but that should not be defined in the update method itself.

class Employee
  def initialize(a,b)
    @a = a
    @b = b
    @observers = []
  end

  def add_observer(observer)
    @observers << observer
  end

  def update
    #make some change
    @observers.each { |observer| observer.update(self) }
  end
end

Ruby also has the Observable library

###watch out###

• watch out for calling the observer too frequently. Make sure something has changed
• watch out for inconsistent state (e.g. salary and title changing, and different observers for each one.

ActiveRecord uses this for after_create etc. Also with convention over configuration, ActiveRecord knows that ModelObserver is out there to observe Models... should play with this.