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今天具体讲下AppKit的Classes and Protocols，即类和协议，oc里的类和java里的类相当，协议则相当于java里的接口。

下面上一段官方关于类和协议的介绍：

AppKit is large; it comprises more than 125 classes and protocols. The classes all descend from the Foundation framework’s NSObject class. The following sections briefly describe some of the topics that AppKit addresses through its classes and protocols.

NSObject是所以oc类的基类，就像java里的Object。

这里再讲下APPKit中的NSApplication类，briefly describe如下：

Encapsulating an Application
Every application uses a single instance of NSApplication to control the main event loop, keep track of the application’s windows and menus, distribute events to the appropriate objects (that is, itself or one of its windows), set up autorelease pools, and receive notification of application-level events. An NSApplication object has a delegate (an object that you assign) that is notified when the application starts or terminates, is hidden or activated, should open a file selected by the user, and so forth. By setting the NSApplication object’s delegate and implementing the delegate methods, you customize the behavior of your application without having to subclass NSApplication.

总的来说，NSApplication相当于android里的Application，你可以通过实现它的代理方法，相当于Application里的生命周期方法，如onCreate，来实现自己app的一些业务逻辑。