Swift：无法长按拖动单元格到 UICollectionViewController 和 UITableView 中的空白部分

Question

以前，我尝试用 UITableView 中的长按拖动来替换标准的苹果重新排序控件（从右侧手柄拖动单元格）。然而，由于某种原因，通过长按拖动，我无法将单元格移动到其中已经没有单元格的部分。现在我正在尝试实现一个功能，用户可以在 UICollectionViewController 而不是 UITableView 中的两个部分之间拖动单元格。我实现了长按拖动功能，但由于某种原因我遇到了同样的问题。我如何向这些部分添加一个虚拟单元，以便它们永远不为空，或者是否有更好的方法来解决这个问题？还有一种无需长按即可拖动单元格的方法吗？

这些是我添加到 UICollectionViewController 类中以启用长按拖动的函数：

override func viewDidLoad() {
    super.viewDidLoad()

    let longPressGesture = UILongPressGestureRecognizer(target: self, action: "handleLongGesture:")
    self.collectionView!.addGestureRecognizer(longPressGesture)
}

func handleLongGesture(gesture: UILongPressGestureRecognizer) {

    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()
    }
}

override func collectionView(collectionView: UICollectionView, moveItemAtIndexPath sourceIndexPath: NSIndexPath, toIndexPath destinationIndexPath: NSIndexPath) {

    let fromRow = sourceIndexPath.row
    let toRow = destinationIndexPath.row
    let fromSection = sourceIndexPath.section
    let toSection = destinationIndexPath.section

    var item: Item
    if fromSection == 0 {
        item = section1Items[fromRow]
        section1Items.removeAtIndex(fromRow)
    } else {
        item = section2Items[sourceIndexPath.row]
        section2Items.removeAtIndex(fromRow)
    }

    if toSection == 0 {
        section1Items.insert(score, atIndex: toRow)
    } else {
        section2Items.insert(score, atIndex: toRow)
    }
}

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

谢谢

Answer 1

要首先回答问题的第二部分，请使用 UIPanGestureRecogniser 而不是 UILongPressRecogniser。

对于空部分，您可以将不可见的虚拟单元添加到空部分。在故事板中创建一个没有子视图的原型单元，并确保它具有与集合视图相同的背景颜色。

如果该部分为空，您需要安排显示此单元格。但当在只有 1 个单元格的部分中开始移动时，您还需要添加一个虚拟单元格，否则该部分将在移动过程中折叠，并且用户无法将单元格移回其开始的部分。 在手势处理程序中，开始移动时添加临时单元。如果尚未删除单元格，则在拖动完成时也将其删除（如果单元格实际上并未移动，则不会调用委托 moveItemAtIndexPath 方法）：

var temporaryDummyCellPath:NSIndexPath?

func handlePanGesture(gesture: UIPanGestureRecognizer) {

    switch(gesture.state) {
    case UIGestureRecognizerState.Began:
        guard let selectedIndexPath = self.collectionView.indexPathForItemAtPoint(gesture.locationInView(self.collectionView)) else {
            break
        }
        if model.numberOfPagesInSection(selectedIndexPath.section) == 1 {
            // temporarily add a dummy cell to this section
            temporaryDummyCellPath = NSIndexPath(forRow: 1, inSection: selectedIndexPath.section)
            collectionView.insertItemsAtIndexPaths([temporaryDummyCellPath!])
        }
        collectionView.beginInteractiveMovementForItemAtIndexPath(selectedIndexPath)
    case UIGestureRecognizerState.Changed:
        collectionView.updateInteractiveMovementTargetPosition(gesture.locationInView(gesture.view!))
    case UIGestureRecognizerState.Ended:
        collectionView.endInteractiveMovement()

        // remove dummy path if not already removed
        if let dummyPath = self.temporaryDummyCellPath {
            temporaryDummyCellPath = nil
            collectionView.deleteItemsAtIndexPaths([dummyPath])
        }

    default:
        collectionView.cancelInteractiveMovement()

        // remove dummy path if not already removed
        if let dummyPath = temporaryDummyCellPath {
            temporaryDummyCellPath = nil
            collectionView.deleteItemsAtIndexPaths([dummyPath])
        }
    }
}

在 

collectionView(collectionView: UICollectionView, numberOfItemsInSection section: Int)

中，始终返回比模型中项目数多 1 的值，或者如果添加了临时虚拟单元，则返回一个附加单元。

func collectionView(collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
    // special case: about to move a cell out of a section with only 1 item
    // make sure to leave a dummy cell 
    if section == temporaryDummyCellPath?.section {
        return 2
    }

    // always keep one item in each section for the dummy cell
    return max(model.numberOfPagesInSection(section), 1)
}

在 

collectionView(collectionView: UICollectionView, cellForItemAtIndexPath indexPath: NSIndexPath)

中，如果该行等于该部分模型中的项目数，则分配虚拟单元。

通过为虚拟单元返回 false 并为其他单元返回 true 来禁用选择

collectionView(collectionView: UICollectionView, shouldSelectItemAtIndexPath indexPath: NSIndexPath)

中的单元。

同样禁用

collectionView(collectionView: UICollectionView, canMoveItemAtIndexPath indexPath: NSIndexPath)

 中的单元格移动

确保目标移动路径仅限于模型中的行：

func collectionView(collectionView: UICollectionView, targetIndexPathForMoveFromItemAtIndexPath originalIndexPath: NSIndexPath, toProposedIndexPath proposedIndexPath: NSIndexPath) -> NSIndexPath
{
    let proposedSection = proposedIndexPath.section
    if model.numberOfPagesInSection(proposedSection) == 0 {
        return NSIndexPath(forRow: 0, inSection: proposedSection)
    } else {
        return proposedIndexPath
    }

}

现在你需要处理最后一步。更新您的模型，然后根据需要添加或删除虚拟单元：

func collectionView(collectionView: UICollectionView, moveItemAtIndexPath sourceIndexPath: NSIndexPath, toIndexPath destinationIndexPath: NSIndexPath)
{
    // move the page in the model
    model.movePage(sourceIndexPath.section, fromPage: sourceIndexPath.row, toSection: destinationIndexPath.section, toPage: destinationIndexPath.row)

    collectionView.performBatchUpdates({

        // if original section is left with no pages, add a dummy cell or keep already added dummy cell
        if self.model.numberOfPagesInSection(sourceIndexPath.section) == 0 {
            if self.temporaryDummyCellPath == nil {
                let dummyPath = NSIndexPath(forRow: 0, inSection: sourceIndexPath.section)
                collectionView.insertItemsAtIndexPaths([dummyPath])
            } else {
                // just keep the temporary dummy we already created
                self.temporaryDummyCellPath = nil
            }
        }

        // if new section previously had no pages remove the dummy cell
        if self.model.numberOfPagesInSection(destinationIndexPath.section) == 1 {
            let dummyPath = NSIndexPath(forRow: 0, inSection: destinationIndexPath.section)
            collectionView.deleteItemsAtIndexPaths([dummyPath])
        }
    }, completion: nil)

}

最后确保虚拟单元没有辅助功能项目，以便在打开语音时跳过它。

Answer 2

Xcode 10.2 (10E125)、Swift 5、iOS 11

想法

我想建议处理

func collectionView(_ collectionView: UICollectionView, dragSessionWillBegin session: UIDragSession) {...}
func collectionView(_ collectionView: UICollectionView, dragSessionDidEnd session: UIDragSession) {...}

开始拖动时，您可以将 

临时 单元格添加到空（或所有）部分的末尾。而且你的空部分不会是空的）。然后你可以使用标准 UIKit api 删除你的单元格。在拖动结束之前，您可以删除 临时 单元格。

为什么？

我不建议使用手势处理程序，因为：

“制造自行车”的大好机会

1. 该解决方案可能非常耗时且维护起来困难/昂贵。
2. 用于使用经过更深入测试的表/集合的标准 API
样品

拖放的完整示例

更多信息

支持集合视图中的拖放
UICollectionView
• 拖动委托
UICollectionView
• Drop委托

Answer 3

这是我的解决方案：

如果您有节标题（SuplementaryViews），您可以通过查找最接近的 SupplementaryView 及其到当前拖动位置的 IndexPath 来计算正确的 IndexPath：

func collectionView(_ collectionView: UICollectionView, performDropWith coordinator: UICollectionViewDropCoordinator) {
    guard let destinationIndexPath = coordinator.destinationIndexPath ?? closestIndexPath(coordinator.session, collectionView) else { return }

    // Do your reordering work here
}

private func closestIndexPath(_ session: UIDropSession, _ collectionView: UICollectionView) -> IndexPath? {
    let location = session.location(in: collectionView)

    let indexPaths = collectionView.indexPathsForVisibleSupplementaryElements(ofKind: "SectionHeaderSupplementaryViewIdentifier")
    let supplementaryViews = collectionView.visibleSupplementaryViews(ofKind: "SectionHeaderSupplementaryViewIdentifier")

    var correctDestination: IndexPath?
    collectionView.performUsingPresentationValues {
        correctDestination = closestIndexPath(to: location, using: supplementaryViews, with: indexPaths)
    }
    return correctDestination
}

private func closestIndexPath(to point: CGPoint, using views: [UIView], with indexPaths: [IndexPath]) -> IndexPath? {
    guard views.count == indexPaths.count else {
        print("Views and indexPaths arrays must have the same count.")
        return nil
    }

    var minDistance: CGFloat = .greatestFiniteMagnitude
    var closestIndexPath: IndexPath?

    for (index, view) in views.enumerated() {
        let distance = abs(point.y - view.frame.maxY)
        if distance < minDistance {
            minDistance = distance
            closestIndexPath = indexPaths[index]
        }
    }

    return closestIndexPath
}

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