ios - 仅当 style 设置为 'Scroll' 时，UIPageViewController 中的约束异常

Question

我使用 UIPageView 允许用户滚动浏览图像集合，当 UIPageViewController 设置为 Scroll 时，出现约束错误(没有共同祖先) code> 过渡样式，但设置为 Page Curl 时则不然。

到目前为止，我已经尝试将抛出错误的约束的 anchor (我已经准确地确定了罪魁祸首)更改为不同的东西(尝试了Safe Area和Superview)，但没有任何改变。我对 Swift 比较陌生，并且不太了解 UIPageViewController 的工作原理，因此很有可能我只是以错误的顺序调用事物而没有意识到这一点。我有点困惑，为什么错误仅发生在 Scroll 过渡样式而不是 Page Curl 上，因为在我看来，它们应该类似地工作...还对为什么约束 anchor 锚定到自己的 super View 时不在同一 View 层次结构中感到困惑？

这是 UIPageViewController 委托(delegate):

class ImageDetailPageViewController: UIPageViewController {

    // MARK: Properties

    var images: [UIImage]!
    var index: Int!

    var navbarHeight: CGFloat!

    fileprivate lazy var pages: [UIViewController] = {
        return getPages()
    }()

    // MARK: Methods

    override func viewDidLoad() {
        super.viewDidLoad()

        self.dataSource = self
        self.delegate = self

        self.view.translatesAutoresizingMaskIntoConstraints = false

        setViewControllers([pages[index]], direction: .forward, animated: true, completion: nil)
    }

    override var preferredStatusBarStyle: UIStatusBarStyle {
        return .lightContent
    }

    // MARK: Methods

    fileprivate func getPages() -> [ImageView] {
        var pages = [ImageView]()

        let storyboard = UIStoryboard(name: "ImageDetail", bundle: nil)

        for image in images {
            let imageDetail = storyboard.instantiateViewController(withIdentifier: "ImageDetail") as! ImageView
            imageDetail.image = image

            pages.append(imageDetail)
        }

        return pages
    }
}

// MARK: Extensions

extension ImageDetailPageViewController: UIPageViewControllerDataSource {
    func pageViewController(_ pageViewController: UIPageViewController, viewControllerBefore viewController: UIViewController) -> UIViewController? {
        guard let viewControllerIndex = pages.firstIndex(of: viewController) else {
            return nil
        }

        let previousIndex = viewControllerIndex - 1

        guard previousIndex >= 0 else {
            return pages.last
        }

        guard pages.count > previousIndex else {
            return nil
        }

        return pages[previousIndex]
    }

    func pageViewController(_ pageViewController: UIPageViewController, viewControllerAfter viewController: UIViewController) -> UIViewController? {       
        guard let viewControllerIndex = pages.firstIndex(of: viewController) else {
            return nil
        }

        let nextIndex = viewControllerIndex + 1

        guard nextIndex < pages.count else {
            return pages.first
        }

        guard pages.count > nextIndex else {
            return nil
        }

        return pages[nextIndex]
    }

    func presentationCount(for pageViewController: UIPageViewController) -> Int {
        return pages.count
    }

    func presentationIndex(for pageViewController: UIPageViewController) -> Int {
        return index
    }
}

extension ImageDetailPageViewController: UIPageViewControllerDelegate { }

页面 View 中的 View Controller (ImageDetail)非常简单。它由锚定到安全区域的UIScrollView和锚定到UIScrollView的UIImageView组成。

当我尝试滚动到页面 View 中的下一页时，应用程序崩溃，并抛出以下错误:

*** Terminating app due to uncaught exception 'NSGenericException', reason: 'Unable to activate constraint with anchors <NSLayoutYAxisAnchor:0x600001b72400 "UIImageView:0x7ff44c56caf0.top"> and <NSLayoutYAxisAnchor:0x600001b77e40 "UIScrollView:0x7ff44d164800.top"> because they have no common ancestor.  Does the constraint or its anchors reference items in different view hierarchies?  That's illegal.'

当设置为Page Curl时，它工作得很好。当在页面 View 之外自行加载时， View Controller 也可以完美呈现。

编辑:以下是 ImageDetail View 的代码:

class ImageView: UIViewController, UIScrollViewDelegate {

    // MARK: Properties

    @IBOutlet weak var scrollView: UIScrollView!
    @IBOutlet weak var imageView: UIImageView!
    @IBOutlet var zoomGestureRecognizer: UITapGestureRecognizer!

    // imageView contraints
    @IBOutlet var imageViewBottomConstraint: NSLayoutConstraint!
    @IBOutlet var imageViewLeadingConstraint: NSLayoutConstraint!
    @IBOutlet var imageViewTopConstraint: NSLayoutConstraint!
    @IBOutlet var imageViewTrailingConstraint: NSLayoutConstraint!

    var image: UIImage!

    // MARK: Overrides

    override func viewDidLoad() {
        super.viewDidLoad()

        // set the image
        if let image = image {
            imageView.image = image
        } else {
            // log error
            os_log("Error. Image Detail opened without an image loaded.", log: OSLog.default, type: .error)

            // dismiss view
            self.dismiss(animated: false, completion: nil)
        }

        // assume scrollView delegate
        scrollView.delegate = self
        scrollView.maximumZoomScale = 2

        // set imageView snapshot mode
        imageView.snapshotView(afterScreenUpdates: true)
    }

    override func viewWillLayoutSubviews() {
        super.viewWillLayoutSubviews()

        updateMinZoomScaleForSize(view.compatibleSafeAreaLayoutGuide.layoutFrame.size)
        updateConstraintsForSize(view.compatibleSafeAreaLayoutGuide.layoutFrame.size)
        scrollView.setZoomScale(scrollView.minimumZoomScale, animated: false)
    }

    // MARK: Methods

    func viewForZooming(in scrollView: UIScrollView) -> UIView? {
        return imageView
    }

    func scrollViewDidZoom(_ scrollView: UIScrollView) {
        updateConstraintsForSize(view.compatibleSafeAreaLayoutGuide.layoutFrame.size)
    }

    fileprivate func updateConstraintsForSize(_ size: CGSize) {
        let yOffset = max(0, (view.compatibleSafeAreaLayoutGuide.layoutFrame.height - imageView.frame.height) / 2)
        imageViewTopConstraint.constant = yOffset
        imageViewBottomConstraint.constant = yOffset

        let xOffset = max(0, (view.compatibleSafeAreaLayoutGuide.layoutFrame.width - imageView.frame.width) / 2)
        imageViewLeadingConstraint.constant = xOffset
        imageViewTrailingConstraint.constant = xOffset

        // activate constraints
        imageViewTopConstraint.isActive = true
        imageViewBottomConstraint.isActive = true
        imageViewLeadingConstraint.isActive = true
        imageViewTrailingConstraint.isActive = true

        view.layoutIfNeeded()
    }

    fileprivate func updateMinZoomScaleForSize(_ size: CGSize) {
        let widthScale = view.compatibleSafeAreaLayoutGuide.layoutFrame.width / imageView.bounds.width
        let heightScale = view.compatibleSafeAreaLayoutGuide.layoutFrame.height / imageView.bounds.height
        let minScale = min(widthScale, heightScale)

        scrollView.minimumZoomScale = minScale
        scrollView.zoomScale = minScale
    }

    fileprivate func zoomRectForScale(_ scale: CGFloat, center: CGPoint) -> CGRect {
        // create and size view window
        var zoomRect = CGRect.zero
        zoomRect.size.height = imageView.frame.size.height
        zoomRect.size.width = imageView.frame.size.width

        // center on tapped point
        let newCenter = imageView.convert(center, from: view)
        zoomRect.origin.x = newCenter.x - (zoomRect.size.width / 2.0)
        zoomRect.origin.y = newCenter.y - (zoomRect.size.height / 2.0)

        // return rect
        return zoomRect
    }

    // MARK: Actions

    @IBAction func tapToZoom(_ sender: UITapGestureRecognizer) {
        if scrollView.zoomScale == scrollView.minimumZoomScale {
            scrollView.zoom(to: zoomRectForScale(scrollView.maximumZoomScale, center: zoomGestureRecognizer.location(in: zoomGestureRecognizer.view)), animated: true)
        } else {
            scrollView.setZoomScale(scrollView.minimumZoomScale, animated: true)
        }
    }
}

以及相应的 Storyboard: Storyboard screenshot

Answer 1

知道这一行 imageView.snapshotView(afterScreenUpdates: true) 明确告诉渲染 UIImageView 的外观，如果失败，则快照可能不可见内容。

我不确定您想要实现什么目标，但注释掉该行肯定会解决您的问题。如果您需要任何帮助来呈现您想要的 UI 或其他内容，请告诉我。

阅读官方documentation了解更多详情。