生命周期

    // 加载视图时会调用该方法
    override func loadView() {
        super.loadView()
        print("加载视图")
    }
    
    // 当加载视图结束时调用该方法
    override func viewDidLoad() {
        super.viewDidLoad()
        print("加载视图结束")
        // 监听应用进入前后台
        NotificationCenter.default.addObserver(self, selector: #selector(appWillEnterForeground), name: UIApplication.willEnterForegroundNotification, object: nil)
        NotificationCenter.default.addObserver(self, selector: #selector(appDidEnterBackground), name: UIApplication.didEnterBackgroundNotification, object: nil)
    }
    
    // 视图将要显示时调用该方法
    override func viewWillAppear(_ animated: Bool) {
        super.viewWillAppear(animated)
        print("视图即将显示")
    }
    
    // 当视图已经显示时调用该方法
    override func viewDidAppear(_ animated: Bool) {
        super.viewDidAppear(animated)
        print("视图已经显示")
    }
    
    // 当视图将要消失时调用该方法
    override func viewWillDisappear(_ animated: Bool) {
        super.viewWillDisappear(animated)
        print("视图即将消失")
    }
    
    // 当时图已经消失时调用该方法
    override func viewDidDisappear(_ animated: Bool) {
        super.viewDidDisappear(animated)
        print("视图已经消失")
    }
    
    // 当接收到内存警告时会执行这个方法
    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
    }
    
    // 将进入前台通知
    @objc func appWillEnterForeground(){
        print("周期 ---将进入前台通知")
    }
    //应用程序确实进入了后台
    @objc func appDidEnterBackground(){
        print("周期 ---应用程序确实进入了后台")
    }