How can we control actions through buttons?

In Chapter 3, Improving on the Decorator Pattern with the Component Object Model, we implemented game objects. Now that we have them, it seems trivial to create buttons on the screen. In fact, in genres such as real-time strategy, there is no difference between clickable buttons and game objects. The player can click on any unit or building and give them orders.

At first thought, our buttons could just be game objects. They both have a position, scale, and texture, and that texture will be drawn to the screen. Depending on the game, you might draw your buttons using orthographic projection while the objects will be drawn using perspective projection. However, the differences go deeper than that.

At its core, a button has an action that needs to be performed when it is clicked or selected. This behavior is usually simple; it doesn't require creating an entire state machine class. It does however, require a little thought so we don't end up hardcoding...