App entry： main

//set UIApplication object and delegation
int main() {
  return UIApplicationMain(xx, xx, UIApplication, AppDelegate);
}

Procedure

• Create UIApplication object
• Create APPDelegate object, and be the UIApplication delegation. it will listen the app life cycle and process memory warning
• Begin a main event loop, keep the app receiving and processing events
• Load info.plist
• if use main.storyboard
  • create UIWindow
  • load main main.storyboard, and set the initial controller as the rootController
  • set window.makeKeyAndVisible()
  • show the window

UIWindow

• fun: provide drawing support for the screen UIWindow. That is a special UIView container. every UIView showing into screen is the child View of UIWindow.

why need UIViewController ? instead of UIWindow

• we want rotation function
• see stackOverFlow

Key Window

• fun: a UIWindow can receive keyboard and non-touch event.
• Only one UIWindow is the key window in the given time.

The key window is the one that is designated to receive keyboard and other ooo-touch related events. 
Only one window at a time may be the key window.

custom setting UIWindow

func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplicationLaunchOptionsKey: Any]?) -> Bool {

        // create window and frame
        self.window = UIWindow(frame: UIScreen.main.bounds)

        // create rootController
        let rootVC = SubwayListerViewController(withTestFile: nil)
        let rootNav = UINavigationController(rootViewController: rootVC)

        // if setting window and rootController, as default rootController's view will be added into window
        // setting rootController, as default, we have rotation function
        window?.rootViewController = rootNav
        self.window?.makeKeyAndVisible()
        return true
    }

Window level realtionship

self.window.windowlebel = UIWindowAlert or UIWIndowNormal or UIWindowStatusBar