We will finish the common Gang of Four design patterns with my favorite: the Observer pattern, also known as publish/subscriber or publish/listener. With the State pattern, we defined our first event-driven architecture, but with the Observer pattern we will really reach a new level of abstraction.
The idea behind the Observer pattern is simple--to subscribe to some event that will trigger some behavior on many subscribed types. Why is this so interesting? Because we uncouple an event from its possible handlers.
For example, imagine a login button. We could code that when the user clicks the button, the button color changes, an action is executed, and a form check is performed in the background. But with the Observer pattern, the type that changes the color will subscribe to the event of the clicking of the button. The type that checks the form and the type that performs an action will subscribe to this event too.