具有自定義流布局的UICollectionView-滾動時具有活動的平移手勢時，單元會出現毛刺

Question

所以我要建立自己的拖放系統。 為此，我需要一種方法在用戶將鼠標懸停在其中的collectionViews單元格之間創建“間隙”。

我現在正在嘗試一些東西，並使用可移動單元格的自定義流程布局進行了基本演示。

我所做的演示包括一個簡單的collectionView （使用IGListKit但這對這個問題無關緊要，我很確定）和一個UIPanGestureRecognizer ，它允許您在collectionView上平移以在手指下創建一個間隙。

我通過在每次pan gesture reconizer更改時使布局無效來實現此目的。 它以這種方式工作，但是當我在平移collectionView同時滾動時，單元格似乎會出現一點故障。 看起來像這樣（看起來單元格的渲染跟不上）：

我很確定問題出在包含此調用的makeAGap函數內：

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

如果我不為無效設置動畫，像這樣

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

根本不會出現故障。 它與動畫有關。 你有什么想法？

謝謝

PS：這是代碼（還有更多IGListKit東西，但這並不重要）：

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，這一直在發生。 我花了幾個小時試圖解決問題，放棄了在這里問一個問題，幾分鍾后，我找到了一個解決方案。

因此，我無法完全解釋為什么會發生這種故障，但似乎與屏幕繪制有關。 我添加了一個CADisplayLink來使布局無效與屏幕的刷新率同步，現在故障已消失（下面的代碼段供感興趣的用戶使用）。

但是，我很想知道那里到底發生了什么，以及為什么同步失效可以解決圖形故障。 我也會去研究它，但是我沒有那么豐富的經驗，所以如果有人知道這樣的事情， 我將非常感謝對此問題提出一個新的（更詳細的）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)
}