使用 NotificationCenter 訂閱各種事件

override func viewDidLoad() {
    super.viewDidLoad()
    //監聽Application事件
    NotificationCenter.default.addObserver(self, selector: #selector(eventNotification(_:)), name: NSNotification.Name.UIApplicationWillEnterForeground, object: nil);   
}
//取消監聽
deinit {
    NotificationCenter.default.removeObserver(self);
}
func eventNotification(_ notification: NSNotification){
    switch notification.name {
    case NSNotification.Name.UIApplicationWillEnterForeground:
        //...
        break;
    default:
        break;
    }
}