Define a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically.
Dependents, Publish-Subscribe
Use the Observer pattern in any of the following situations:
- When an abstraction has two aspects, one dependent on the other. Encapsulating these aspects in separate objects lets you vary and reuse them independently
- When a change to one object requires changing others, and you don't know how many objects need to be changed
- When an object should be able to notify other objects without making assumptions about who these objects are. In other words, you don't want these objects tightly coupled
- Subject
- knows its observers. Any number of Observer objects may observe a subject
- provides an interface for attaching and detaching Observer objects
- Observer
- defines an updating interface for objects that should be notified of changes in a subject
- ConcreteSubject
- stores state of interest to ConcreteObserver objects
- sends a notification to its observers when its state changes
- 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
Mediator: By encapsulating complex update semantics, the ChangeManager acts as mediator between subjects and observers.
Singleton: The ChangeManager may use the Singleton pattern to make it unique and globally accessible.