Observer pattern

Observer is a behavioral design pattern. It specifies communication between objects: observable and observers. An observable is an object which notifies observers about the changes in its state.

Observer pattern is used when there is one-to-many relationship between objects such as if one object is modified, its dependent objects are to be notified automatically.

Implementation:

What problems can the Observer design pattern solve?

• A one-to-many dependency between objects should be defined without making the objects tightly coupled.
• It should be ensured that when one object changes state an open-ended number of dependent objects are updated automatically.
• It should be possible that one object can notify an open-ended number of other objects.

What solution does the Observer design pattern describe?

• Define Subject and Observer objects.
• so that when a subject changes state, all registered observers are notified and updated automatically (and probably asynchronously).

The sole responsibility of a subject is to maintain a list of observers and to notify them of state changes by calling their update() operation.

The responsibility of observers is to register (and unregister) themselves on a subject (to get notified of state changes) and to update their state (synchronize their state with subject's state) when they are notified.

This makes subject and observers loosely coupled. Subject and observers have no explicit knowledge of each other. Observers can be added and removed independently at run-time.

This notification-registration interaction is also known as publish-subscribe.