All applications rely on common interface elements to communicate with the user. By packaging these elements in a library, an operating system can make sure all applications look and behave the same way. And the more functionality the operating system provides "for free," the less work application developers need to do themselves.
Toward that end, Mac OS X provides a number of application frameworks, as shown in Figure 1-6, upon which programmers can build their applications: Cocoa, Carbon, and the Java JDK. These frameworks, described in more detail in the following sections, all provide the basic concepts essential for application design: how events are processed by the application, how window contents are organized and drawn, how controls are presented to the user, and so on.
It is important that all applications present their UI in a consistent manner, regardless of which application framework the program uses. In other words, all windows, menus, buttons, text fields, and so on should look and behave the same way on Mac OS X. These UI elements together on Mac OS X form a distinctive user experience that Apple calls the Aqua user interface. Consistency among apps is so important that Apple has published guidelines enumerating the proper way to use Aqua user interface elements; these guidelines are called the Apple Human Interface Guidelines.
Each of these application frameworks is appropriate in different situations. ...