Giving partial access to a class using an interface
The solution is an interface. While it is possible to pass a reference of GameEngine from GameEngine to GameState, this isn't desirable. What we need is a way to give GameState direct but limited control. If it had a full reference to GameEngine, it is likely that as the project progressed, it would end up creating problems because GameState has too much access to GameEngine. For example, what if GameState decided to pause the game at the wrong time?
Interface refresher
If you think back to Chapter 8, Object-Oriented Programming, an interface is a class without any method bodies. A class can implement an interface and when it does, it must provide the body (including the code) for that method or methods. Furthermore, when a class implements an interface, it is an object of that type. When a class is a specific type, it can be used polymorphically as that type even if it is other types as well. Here are some examples of...
