This chapter covers the Observer pattern.
Define a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically.
In this pattern, there are many observers (objects) that are observing a particular subject (also an object). Observers want to be notified when there is a change made inside the subject. So, they register for that subject. When they lose interest in the subject, they simply unregister from the subject. Sometimes this model is called a Publisher-Subscriber (Pub-Sub) model ...