将集合视图中的单元格移动到另一部分的最后一个单元格之外不起作用

Question

为了能够在集合视图中移动/平移单元格，应该实现 UIGestureRecognizer。这在单节集合视图中工作正常。但是，在多节集合视图中，当单元格被平移/移动到另一个节的最后一个单元格之外时，会发生错误。 尽管“cellForItemAtIndexPath”在平移期间被正确触发以显示“新”单元格，但不应在平移超出最后一个可用单元格后触发（即在这种情况下 indexPath.row == numberOfItemsInSection 并导致错误（参见下面的代码）） .

在表格视图中，（默认）移动/平移实现不会出现此问题。

这可能是 Swift 中的一个错误吗？是否有人建议在平移超出部分中的最后一个单元格时避免触发“cellForItemAtIndexPath”？

下面是 Swift 2.3 中的一个工作代码示例来说明问题。

class CustomViewController: UIViewController, UICollectionViewDataSource, UICollectionViewDelegate, UICollectionViewDelegateFlowLayout {

    private var list = ["one", "two", "three","four","five","six","seven","eight"]
    private var itemsList: [[String]]!

    private var collectionView : UICollectionView!

    override func viewDidLoad() {
        super.viewDidLoad()

        itemsList = [list, list, list] // Create three sections

        collectionView = UICollectionView(frame: CGRectZero, collectionViewLayout: UICollectionViewFlowLayout())
        self.collectionView.dataSource = self
        self.collectionView.delegate = self
        self.collectionView.translatesAutoresizingMaskIntoConstraints = false
        self.collectionView.registerClass(CustomCell.self, forCellWithReuseIdentifier: "cell")

        let panGesture = UIPanGestureRecognizer(target: self, action: #selector(CustomViewController.handleGesture(_:)))
        self.collectionView.addGestureRecognizer(panGesture)
        self.view.addSubview(self.collectionView)
    }

    override func viewWillLayoutSubviews() {
        let margins = view.layoutMarginsGuide
        self.collectionView.leadingAnchor.constraintEqualToAnchor(margins.leadingAnchor, constant: 0).active = true
        self.collectionView.trailingAnchor.constraintEqualToAnchor(margins.trailingAnchor, constant: 0).active = true
        self.collectionView.bottomAnchor.constraintEqualToAnchor(view.bottomAnchor, constant: 0).active = true
        self.collectionView.topAnchor.constraintEqualToAnchor(view.topAnchor, constant: 0).active = true
    }

    override func viewWillAppear(animated: Bool) {
        super.viewWillAppear(animated)
    }

    func numberOfSectionsInCollectionView(collectionView: UICollectionView) -> Int   {
        return itemsList.count
    }

    func collectionView(collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
         return itemsList[section].count
    }

    func collectionView(collectionView: UICollectionView, cellForItemAtIndexPath indexPath: NSIndexPath) -> UICollectionViewCell {
        let cell = collectionView.dequeueReusableCellWithReuseIdentifier("cell", forIndexPath: indexPath) as! CustomCell

        let items = itemsList[indexPath.section]

        cell.label.text = items[indexPath.row] // The error occurs here, after panning beyond the last cell of another section stating: "fatal error: Index out of range"
        return cell
    }

    func collectionView(collectionView: UICollectionView, canMoveItemAtIndexPath indexPath: NSIndexPath) -> Bool {
        return true
    }

    func collectionView(collectionView: UICollectionView, moveItemAtIndexPath sourceIndexPath: NSIndexPath, toIndexPath destinationIndexPath: NSIndexPath) {
        let itemToMove = itemsList[sourceIndexPath.section][sourceIndexPath.row]
        itemsList[sourceIndexPath.section].removeAtIndex(sourceIndexPath.row)
        itemsList[destinationIndexPath.section].insert(itemToMove, atIndex: destinationIndexPath.row)
    }


    func collectionView(collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, referenceSizeForHeaderInSection section: Int) -> CGSize {
                  return CGSizeMake(collectionView.bounds.size.width, 50)
    }

    func collectionView(collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAtIndexPath indexPath: NSIndexPath) -> CGSize {
        return CGSizeMake(collectionView.bounds.size.width, 40)
    }

    func handleGesture(gesture: UIPanGestureRecognizer) {
        switch(gesture.state) {
        case UIGestureRecognizerState.Began:

            guard let selectedIndexPath = self.collectionView.indexPathForItemAtPoint(gesture.locationInView(self.collectionView)) else {
                break }
            collectionView.beginInteractiveMovementForItemAtIndexPath(selectedIndexPath)
        case UIGestureRecognizerState.Changed:
            collectionView.updateInteractiveMovementTargetPosition(gesture.locationInView(gesture.view!))
        case UIGestureRecognizerState.Ended:
            collectionView.endInteractiveMovement()
        default:
            collectionView.cancelInteractiveMovement()
        }
    }
}


class CustomCell: UICollectionViewCell {
    var label: UILabel!
    required init(coder aDecoder: NSCoder) {
        super.init(coder: aDecoder)!
    }

    override init(frame: CGRect) {
        super.init(frame: frame)
        self.backgroundColor = UIColor.whiteColor()
        self.label = UILabel()
        self.label.translatesAutoresizingMaskIntoConstraints = false
        self.contentView.addSubview(self.label)
    }

    override func layoutSubviews() {
        super.layoutSubviews()
        let margins = self.contentView.layoutMarginsGuide
        self.label.leadingAnchor.constraintEqualToAnchor(margins.leadingAnchor).active = true
        self.label.trailingAnchor.constraintEqualToAnchor(margins.trailingAnchor).active = true
        self.label.centerYAnchor.constraintEqualToAnchor(self.contentView.centerYAnchor).active = true
    }
}

Answer 1

根据 Apple 的说法，我的问题中描述的问题是 iOS 中的一个错误，并且似乎在当前版本的 iOS 10 beta (xcode 8 GM) 中得到了解决。

不幸的是，集合视图的其他问题尚未修复，例如将项目移动到空白部分的基本支持。