An object, named the Subject maintains a list of its dependents, called observers. When a subject changes state, all registered observers are notified an updated automatically.
The subject notifies the observers by calling their update().
The responsibility of observers is to register and unregister themselves with a subject.
Subject and observers are loosely coupled. However, observers can be added and removed independently at run time.
This notification-registration interaction is also known as publish-subscribe.
The observer pattern can cause memory leaks, known as the lapsed listener problem. This can be prevented if the subject holds weak references to the observers.