Observer Pattern

Social media helps us immensely in understanding the observer pattern. If you are registered on Twitter then whenever you follow someone, you are essentially asking Twitter to send you (the observer) tweet updates of the person (the subject) you followed. The pattern consists of two actors, the observer who is interested in the updates and the subject who generates the updates.

A subject can have many observers and is a one to many relationship. However, an observer is free to subscribe to updates from other subjects too. You can subscribe to news feed from a Facebook page, which would be the subject and whenever the page has a new post, the subscriber would see the new post.

The pattern is formally defined as a one to many dependency between objects so that when one object changes state all the dependents are notified.