如果同时开始动画，则 UIPageViewController 转换会出现错误

Question

我在使用 UIPageViewController 的应用程序中有一个奇怪的行为。我的应用程序的布局是一个 PageViewController（类似相机胶卷），底部有一个广告横幅。

横幅的容器开始时是隐藏的，当广告加载时，我设置了 isHidden=false 动画。

我的问题是，当横幅进入屏幕时，它会中断 UIPageViewController 转换（如果正在进行），如本视频所示：

我做了一个新项目，只需几行就可以很容易地重现错误，你可以在 GITHUB 中检查它：你只需要向 "Next" 按钮发送垃圾邮件，直到横幅被加载。它也可以通过滑动 PageViewController 来复制，但更难复制。

完整的示例代码是：

class TestViewController: UIViewController,UIPageViewControllerDelegate,UIPageViewControllerDataSource {
    @IBOutlet weak var constraintAdviewHeight: NSLayoutConstraint!
    weak var pageViewController : UIPageViewController?

    @IBOutlet weak var containerAdView: UIView!
    @IBOutlet weak var adView: UIView!
    @IBOutlet weak var containerPager: UIView!

    var currentIndex = 0;
    var clickEnabled = true

    override func viewDidLoad() {
        super.viewDidLoad()
        let pageVC = UIPageViewController(transitionStyle: .scroll, navigationOrientation: .horizontal, options: nil)
        pageViewController = pageVC
        pageVC.delegate = self
        pageVC.dataSource = self
        addChildViewController(pageVC)
        pageVC.didMove(toParentViewController: self)
        containerPager.addSubview(pageVC.view)
        pageVC.view.translatesAutoresizingMaskIntoConstraints = true
        pageVC.view.frame = containerPager.bounds
        pushViewControllerForCurrentIndex()
    }
    override func viewDidAppear(_ animated: Bool) {
        super.viewDidAppear(animated)
        self.containerAdView.isHidden = true

        DispatchQueue.main.asyncAfter(deadline: .now()+4) {
            self.simulateBannerLoad()
        }

    }

    @IBAction func buttonClicked(_ sender: Any) {
        guard clickEnabled else {return}
        currentIndex -= 1;
        pushViewControllerForCurrentIndex()

    }

    @IBAction func button2Clicked(_ sender: Any) {
        guard clickEnabled else {return}
        currentIndex += 1;
        pushViewControllerForCurrentIndex()
    }


    private func simulateBannerLoad(){
        constraintAdviewHeight.constant = 50
        pageViewController?.view.setNeedsLayout()
        UIView.animate(withDuration: 0.3,
                       delay: 0, options: .allowUserInteraction, animations: {
                        self.containerAdView.isHidden = false
                        self.view.layoutIfNeeded()
                        self.pageViewController?.view.layoutIfNeeded()
        })

    }

    //MARK: data source

    func pageViewController(_ pageViewController: UIPageViewController, viewControllerAfter viewController: UIViewController) -> UIViewController? {
        return getViewControllerForIndex(currentIndex+1)
    }

    func pageViewController(_ pageViewController: UIPageViewController, viewControllerBefore viewController: UIViewController) -> UIViewController? {
        return getViewControllerForIndex(currentIndex-1)
    }

    func getViewControllerForIndex(_ index:Int) -> UIViewController? {
        guard (index>=0) else {return nil}
        let vc :UIViewController = UIStoryboard(name: "Main", bundle: .main).instantiateViewController(withIdentifier: "pageTest")
        vc.view.backgroundColor = (index % 2 == 0) ? .red : .green
        return vc
    }

    func pushViewControllerForCurrentIndex() {
        guard let vc = getViewControllerForIndex(currentIndex) else {return}
        print("settingViewControllers start")
        clickEnabled = false
        pageViewController?.setViewControllers([vc], direction: .forward, animated: true, completion: { finished in
            print("setViewControllers finished")
            self.clickEnabled = true
        })
    }
}

注意： 另一个不需要的效果是当错误发生时最后一个完成块没有被调用，所以它使按钮处于禁用状态：

    func pushViewControllerForCurrentIndex() {
        guard let vc = getViewControllerForIndex(currentIndex) else {return}
        print("settingViewControllers start")
        clickEnabled = false
        pageViewController?.setViewControllers([vc], direction: .forward, animated: true, completion: { finished in
            print("setViewControllers finished")
            self.clickEnabled = true
        })
    }

Note2: 横幅加载事件是我无法手动控制的。由于用于显示广告的库，它在主线程中随时可能发生的回调。在示例项目中，这是通过 DispatchQueue.main.asyncAfter: 调用

模拟的

我该如何解决？谢谢

Answer 1

怎么了？

布局会中断动画。

如何预防？

pageViewController 动画时不改变布局。

加载广告时：确认pageViewController是否正在动画，如果是，等动画完成后再更新，或者update

示例：

    private func simulateBannerLoad(){
        if clickEnabled {
            self.constraintAdviewHeight.constant = 50
        } else {
            needUpdateConstraint = true
        }
    }
    var needUpdateConstraint = false
    var clickEnabled = true {
        didSet {
            if clickEnabled && needUpdateConstraint {
                self.constraintAdviewHeight.constant = 50
                needUpdateConstraint = false
            }
        }
    }

Answer 2

对我们来说，这是由于设备旋转导致布局传递与新视图控制器的动画同时发生。在这种情况下，我们不知道如何停止布局传递。

这对我们有用：

let pageController = UIPageViewController()

func updatePageController() {
    pageController.setViewControllers(newViewControllers, direction: .forward, animated: true, completion: {[weak self] _ in
        // There is a bug with UIPageViewController where a layout pass during an animation
        // to a new view controller can cause the UIPageViewController to display the old and new view controller
        // In our case, we can compare the view controller `children` count against the the `viewControllers` count
        // In other cases, we may need to examine `children` more closely against `viewControllers` to see if there discrepancies.
        // Since this is on the animation callback we need to dispatch to the main thread to work around another bug: 
        DispatchQueue.main.async { [weak self] in
            guard let self = self else {return}
            
            if self.pageController.children.count != self.pageController.viewControllers?.count {
                self.pageController.setViewControllers(self.pageController.viewControllers, direction: .forward, animated: false, completion: nil)
            }
        }
    })
}

如果同时开始动画，则 UIPageViewController 转换会出现错误

UIPageViewController transition bugged if animation starts in the meanwhile

ios

swift

uipageviewcontroller