In our ongoing series about design patterns, we introduce the Decorator Pattern. The decorator pattern serves to add specific functionality to an existing class. The classic example of this is in GUI programming, when a widget needs a border, or graphic surrounding it. Not using the Decorator Pattern, one might just subclass out the widget class and add a border to the subclassed widget. This becomes more difficult however, when there are multiple classes of widgets. This becomes almost impossible when dealing with a collection of widgets.

So, enter the Decorator Pattern. (more…)

One of my favorite design patterns, is the Observer Pattern. The Observer Pattern is used far more often then you might realize. The Observer Pattern is quintessential, for instance, in most GUI applications. Most applications that respond to key strokes, mouse clicks, or network updates use Observer Pattern. The idea, is that some object in the application is observable, and it has other objects that observe it.

It’s a lot like a bunch of soldiers waiting for the order to charge. The general is observable, and the soldiers are observers. When the general gives an order, the soldiers react in a way appropriate to the order. This is the essence of the Observer Pattern. Here’s the class diagram of the situation : (more…)