5.2 Command in the Real World

A popular example associated with the Command pattern is tied to windowing environments. Good object-oriented GUI frameworks have used one form or another of the Command pattern for years.

Windowing-system writers need a generic way to transmit user actions (such as mouse clicks and keystrokes) to the application. When the user clicks a button, selects a menu option, or the like, the windowing system must notify the underlying application logic. From the windowing system's standpoint, the Options command under the Tools menu does not hold any special meaning. If it did, the application would have been locked into a very rigid framework. An effective way to decouple the windowing system from the application is to use Command objects for passing user actions. Commands serve as generic vehicles that transport actions from the windows to the application logic.

In the windowing example, the invokers are user interface elements (such as buttons, checkboxes, menu items, and widgets), and the receiver is the object responsible for reacting to user interface commands (for example, a dialog or the application itself).

Command objects constitute the lingua franca that the user interface and the application use. As discussed in the previous section, Command offers double-ended flexibility. First, you can plug in new kinds of user interface elements without changing the application logic. Such a program is known as skinnable because you can add new "skins" without changing the design of the product itself. Skins do not encompass any architecture—they only provide slots for Commands and knowledge to fire them appropriately. Second, you can easily reuse the same user interface elements across different applications.