ios app 启动过程

1. 系统调用app的main函数
2. main函数调用UIApplicationMain.
3. UIApplicationMain创建shared application instance, UIApplication默认的instance.
4. UIApplicationMain读取Info.plist找到主nib文件, 加载nib，把shared application instance 设为nib的owner.
5. 通过nib文件，创建app的独立UIWindows object.
6. 通过nib，实例化了程序的AppDelegate object.
7. app内部启动结束，application:didFinishLaunchingWith- Options: 被设定成 wAppDelegate instance.
8. AppDelegate向UIWindow instance发makeKeyAndVisible消息, app界面展示给用户. app准备好接收用户的操作指令.