The GoF describe two design patterns named the State pattern, and the Mediator pattern. The State pattern uses a set of concrete classes that are derived from a base class to describe a particular state. As an example, consider creating an enum to describe the states that a door can be in. At first glance, a door could be either Open or Closed. In this case, a simple if...else control flow will likely take care of any logic we wish to apply.
Consider, however, what happens to our control flow and logic if we needed a Locked and Unlocked state, or if it were a sliding door for a SlightlyAjar, HalfOpen, AlmostFullyOpen, and FullyOpen state. The State design pattern allows us to easily define these states, and adjust our logic based on the current state of an object.
If we think about our application a little, we know that our screens will be in one or another particular...