具有自定义流布局的UICollectionView-滚动时具有活动的平移手势时，单元会出现毛刺

Question

所以我要建立自己的拖放系统。 为此，我需要一种方法在用户将鼠标悬停在其中的collectionViews单元格之间创建“间隙”。

我现在正在尝试一些东西，并使用可移动单元格的自定义流程布局进行了基本演示。

我所做的演示包括一个简单的collectionView （使用IGListKit但这对这个问题无关紧要，我很确定）和一个UIPanGestureRecognizer ，它允许您在collectionView上平移以在手指下创建一个间隙。

我通过在每次pan gesture reconizer更改时使布局无效来实现此目的。 它以这种方式工作，但是当我在平移collectionView同时滚动时，单元格似乎会出现一点故障。 看起来像这样（看起来单元格的渲染跟不上）：

我很确定问题出在包含此调用的makeAGap函数内：

collectionView?.performBatchUpdates({
            self.invalidateLayout()
            self.collectionView?.layoutIfNeeded()
}, completion: nil)

如果我不为无效设置动画，像这样

self.invalidateLayout()
self.collectionView?.layoutIfNeeded()

根本不会出现故障。 它与动画有关。 你有什么想法？

谢谢

PS：这是代码（还有更多IGListKit东西，但这并不重要）：

class MyCustomLayout: UICollectionViewFlowLayout {

    fileprivate var cellPadding: CGFloat = 6

    fileprivate var cache = [UICollectionViewLayoutAttributes]()

    fileprivate var contentHeight: CGFloat = 300
    fileprivate var contentWidth: CGFloat = 0

    var gap: IndexPath? = nil
    var gapPosition: CGPoint? = nil

    override var collectionViewContentSize: CGSize {
        return CGSize(width: contentWidth, height: contentHeight)
    }

    override func prepare() {
        // cache contains the cached layout attributes
        guard cache.isEmpty, let collectionView = collectionView else {
            return
        }

        // I'm using IGListKit, so every cell is in its own section in my case
        for section in 0..<collectionView.numberOfSections {
            let indexPath = IndexPath(item: 0, section: section)
            // If a gap has been set, just make the current offset (contentWidth) bigger to
            // simulate a "missing" item, which creates a gap
            if let gapPosition = self.gapPosition {
                if gapPosition.x >= (contentWidth - 100) && gapPosition.x < (contentWidth + 100) {
                    contentWidth += 100
                }
            }

            // contentWidth is used as x origin
            let frame = CGRect(x: contentWidth, y: 10, width: 100, height: contentHeight)
            contentWidth += frame.width + cellPadding

            let attributes = UICollectionViewLayoutAttributes(forCellWith: indexPath)
            attributes.frame = frame
            cache.append(attributes)
        }
    }

    public func makeAGap(at indexPath: IndexPath, position: CGPoint) {
        gap = indexPath
        self.cache = []
        self.contentWidth = 0
        self.gapPosition = position


        collectionView?.performBatchUpdates({
            self.invalidateLayout()
            self.collectionView?.layoutIfNeeded()
        }, completion: nil)

        //invalidateLayout() // Using this, the glitch does NOT appear
    }

    override func layoutAttributesForElements(in rect: CGRect) -> [UICollectionViewLayoutAttributes]? {

        var visibleLayoutAttributes = [UICollectionViewLayoutAttributes]()

        // Loop through the cache and look for items in the rect
        for attributes in cache {
            if attributes.frame.intersects(rect) {
                visibleLayoutAttributes.append(attributes)
            }
        }
        return visibleLayoutAttributes
    }

    override func layoutAttributesForItem(at indexPath: IndexPath) -> UICollectionViewLayoutAttributes? {
        return cache[indexPath.item]
    }
}

class ViewController: UIViewController {

    /// IGListKit stuff: Data for self.collectionView ("its cells", which represent the rows)
    public var data: [String] {
        return (0...100).compactMap { "\($0)" }
    }

    /// This collectionView will consist of cells, that each have their own collectionView.
    private lazy var collectionView: UICollectionView = {
        let layout = MyCustomLayout()
        layout.scrollDirection = .horizontal
        let collectionView = UICollectionView(frame: .zero, collectionViewLayout: layout)
        collectionView.backgroundColor = UIColor(hex: 0xeeeeee)

        adapter.collectionView = collectionView
        adapter.dataSource = self
        view.addSubview(collectionView)
        collectionView.translatesAutoresizingMaskIntoConstraints = false
        collectionView.frame = CGRect(x: 0, y: 50, width: 1000, height: 300)

        return collectionView
    }()

    /// IGListKit stuff. Data manager for the collectionView
    private lazy var adapter: ListAdapter = {
        let adapter = ListAdapter(updater: ListAdapterUpdater(), viewController: self, workingRangeSize: 0)
        return adapter
    }()

    override func viewDidLoad() {
        super.viewDidLoad()
        _ = collectionView

        let pan = UIPanGestureRecognizer(target: self, action: #selector(handlePan(pan:)))
        pan.maximumNumberOfTouches = 1
        pan.delegate = self
        view.addGestureRecognizer(pan)
    }

    @objc private func handlePan(pan: UIPanGestureRecognizer) {
        guard let indexPath = collectionView.indexPathForItem(at: pan.location(in: collectionView)) else {
            return
        }

        (collectionView.collectionViewLayout as? MyCustomLayout)?.makeAGap(at: indexPath, position: pan.location(in: collectionView))
    }
}

extension ViewController: ListAdapterDataSource, UIGestureRecognizerDelegate {
    func objects(for listAdapter: ListAdapter) -> [ListDiffable] {
        return data as [ListDiffable]
    }

    func listAdapter(_ listAdapter: ListAdapter, sectionControllerFor object: Any) -> ListSectionController {
        return RowListSection()
    }

    func emptyView(for listAdapter: ListAdapter) -> UIView? {
        return nil
    }

    func gestureRecognizer(_ gestureRecognizer: UIGestureRecognizer, shouldRecognizeSimultaneouslyWith otherGestureRecognizer: UIGestureRecognizer) -> Bool {
        return true
    }
}

Answer 1

好的，这一直在发生。 我花了几个小时试图解决问题，放弃了在这里问一个问题，几分钟后，我找到了一个解决方案。

因此，我无法完全解释为什么会发生这种故障，但似乎与屏幕绘制有关。 我添加了一个CADisplayLink来使布局无效与屏幕的刷新率同步，现在故障已消失（下面的代码段供感兴趣的用户使用）。

但是，我很想知道那里到底发生了什么，以及为什么同步失效可以解决图形故障。 我也会去研究它，但是我没有那么丰富的经验，所以如果有人知道这样的事情， 我将非常感谢对此问题提出一个新的（更详细的）答案：）

override func viewDidLoad() {
        super.viewDidLoad()
        _ = collectionView

        let pan = UIPanGestureRecognizer(target: self, action: #selector(handlePan(pan:)))
        pan.maximumNumberOfTouches = 1
        pan.delegate = self
        view.addGestureRecognizer(pan)

        let displayLink = CADisplayLink(target: self, selector: #selector(update))
        displayLink.add(to: .current, forMode: .common)
    }

    var indexPath: IndexPath? = nil
    var position: CGPoint? = nil

    @objc private func update() {
        if let indexPath = self.indexPath, let position = self.position {
            (collectionView.collectionViewLayout as? MyCustomLayout)?.makeAGap(at: indexPath, position: position)
        }
    }

    @objc private func handlePan(pan: UIPanGestureRecognizer) {
        guard let indexPath = collectionView.indexPathForItem(at: pan.location(in: collectionView)) else {
            return
        }

        // buffer the values
        self.indexPath = indexPath
        position = pan.location(in: collectionView)
}