Life cycle of View 生命週期
loadView：用程式碼產生畫面

viewDidLoad：讀入畫面之後執行
第一個載入的方法，可以按照自己希望App一開始呈現的畫面來設定。
只有在一開始執行App時呼叫一次。

viewWillAppear：畫面即將顯示到螢幕上

viewDidAppear：畫面已經顯示到螢幕上
當畫面呈現給使用者時，就呼叫此方法，每次進入畫面時都會呼叫。

viewWillDisappear：畫面即將離開螢幕
veiwDidDisappear：畫面已經離開螢幕

ViewController.swift

override func viewDidLoad() {
        super.viewDidLoad()
        print("view: did load")
    }
    
    override func viewWillAppear(_ animated: Bool) {
        super.viewWillAppear(animated)
        print("view: will appear")
    }
    
    override func viewDidAppear(_ animated: Bool) {
        super.viewDidAppear(animated)
        print("view: did appear")
    }
    
    override func viewWillDisappear(_ animated: Bool) {
        super.viewWillDisappear(animated)
        print("view: will disapper")
    }

    override func viewDidDisappear(_ animated: Bool) {
        super.viewDidAppear(animated)
        print("view: did disappear")
    }

}

AppDelegate.swift

func application(_ application: UIApplication,     didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool{
    print("app: did finishing launching")
    return true
}

func applicationWillResignActive(_ application: UIApplication){
    print("app: will resign active")
}

func applicationDidEnterBackground(_ application: UIApplication){
    print("app: did enter background")
}

func applicationWillEnterForeground(_ application: UIApplication){
    print("app: did enter foreground")
}

func applicationDidBecomeActive(_ application: UIApplication){
    print("app: did become active")
}

func applicationWillTerminate(_ application: UIApplication){
    print("app: will terminate")
}