Observer Pattern


Definition 1

The observer pattern is a software design pattern in which an object, called the subject, maintains a list of its dependents, called observers, and notifies them automatically of any state changes, usually by calling one of their methods. It is mainly used to implement distributed event handling systems. The Observer pattern is also a key part in the familiar model–view–controller (MVC) architectural pattern. The observer pattern is implemented in numerous programming libraries and systems, including almost all GUI toolkits.

Structure 2


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


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


  • stores state of interest to ConcreteObserver objects.
  • sends a notification to its observers when its state changes.


  • 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.

Example Code


Real Implementations

Broadcast Receiver 3 4 on Android


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