60. Observer

• Problem to solve:

  • Consider an object with state, referred to as a subject.

  • There may be a number of objects in the system who depend on the subject's state.

  • Therefore, dependent objects would benefit from becoming observers of the subject's state.

• Solution:

  1. Let Observers register with the Subject.

  2. When Subject undergoes its state change, all Observers are notified.

   

• The Observer pattern realizes a framework known as Publish/Subscribe, or Event Listener.