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昨天大概说了下什么是AppKit，今天详细说下，AppKit都包含哪些功能。官方的解释如下：

AppKit is a framework containing all the objects you need to implement your graphical, event-driven user interface: windows, panels, buttons, menus, scrollers, and text fields. AppKit handles all the details for you as it efficiently draws on the screen, communicates
with hardware devices and screen buffers, clears areas of the screen before drawing, and clips views. The number of classes in AppKit may seem daunting at first. However, most AppKit classes are support classes that you use indirectly. You also have the choice
at which level you use AppKit:

Use Interface Builder to create connections from user interface objects to your application objects. In this case, all you need to do is implement your application classes---implement those action and delegate methods. For example, implement the method
that is invoked when the user selects a menu item.
Control the user interface programmatically, which requires more familiarity with AppKit classes and protocols. For example, allowing the user to drag an icon from one window to another requires some programming and familiarity with the NSDragging... protocols.
Implement your own objects by subclassing NSView or other classes. When subclassing NSView you write your own drawing methods using graphics functions. Subclassing requires a deeper understanding of how AppKit works.

这里提到了你将来使用控件的几种level：

初级的创建控件，处理控件事件
高级一点的，用程序控制控件
再高级一点的，自定义控件