This chapter covers the Observer pattern.
GoF Definition
Define a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically.
Concept
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 ...
