动画:将视图的alpha与表示层的不透明度同步

如果目标是捕捉 opacity 当应用程序进入后台时,你可以为其添加一个观察者 UIApplicationDidEnterBackground ,捕捉不透明度,取消动画,并设置 alpha . E、 g.在Swift中:

class ViewController: UIViewController {

    @IBOutlet weak var viewToAnimate: UIView!

    private var observer: NSObjectProtocol!

    override func viewDidLoad() {
        super.viewDidLoad()

        observer = NotificationCenter.default.addObserver(forName: .UIApplicationDidEnterBackground, object: nil, queue: .main) { [weak self] notification in
            if let opacity = self?.viewToAnimate.layer.presentation()?.opacity {
                self?.viewToAnimate.layer.removeAllAnimations()
                self?.viewToAnimate.alpha = CGFloat(opacity)
            }
        }
    }

    deinit {
        NotificationCenter.default.removeObserver(observer)
    }

    // I'm just doing a basic animation, but the idea is the same whatever animation you're doing

    @IBAction func didTapButton(_ sender: Any) {
        UIView.animate(withDuration: 10) {
            self.viewToAnimate.alpha = 0
        }
    }

}

如果你的目标是即使应用程序被终止也要记住它,那么你需要将其保存在持久存储中。但是如果你的目标只是设定 当应用程序挂起和/或在后台运行时,上述内容就足够了。