Observer

• Also known as: Publisher / Subscriber; Dependents
• A way of notifying change to a number of classes
• Define a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically

Subject

• Knows its observers. Any number of Observer objects may observe a subject
• Provides an interface for attaching and detaching Observer objects

ConcreteSubject

• Stores state of interest to ConcreteObserver
• Sends a notification to its observers when its state changes

Observer

• Defines an updating interface for objects that should be notified of changes in a subject

ConcreteObserver

• Maintains a reference to a ConcreteSubject object
• Stores state that should stay consistent with the subject's
• Implements the Observer updating interface to keep its state consistent with the subject's