UICollectionViewDropDelegate和DiffableDataSource

Question

我正在尝试用UICollectionViewDropDelegate (在iOS 13中引入)来实现iOS。

• I已经实现了UICollectionViewDragDelegate，它工作得很好。
• ，我设置了

实现func collectionView(_ collectionView: UICollectionView, performDropWith coordinator: UICollectionViewDropCoordinator) (您甚至可以在方法为空的情况下尝试)。

该应用程序将构建并运行，但一旦您开始拖动一个单元格，应用程序就会立即崩溃，并收到以下消息：

终止应用程序由于非常规异常'NSInternalInconsistencyException'，原因：'UICollectionView必须通过UICollectionViewDiffableDataSource API更新时充当UICollectionView的dataSource:请不要在UICollectionView上直接调用突变API。

因此，这个方法似乎是试图直接修改UICollectionView (可能是在移动项目时移动单元格？)我想不出该怎么绕过这种行为。

有什么想法？

Answer 1

这是一个相当容易解决的问题。从UICollectionViewDropDelegate函数performDropWith开始。请注意，我只需要一个.move，而不需要.copy，但是您只需要添加一个copyItems()方法，类似于下面的reorderItems()方法：

func collectionView(_ collectionView: UICollectionView, performDropWith coordinator: UICollectionViewDropCoordinator) {
   var destinationIndexPath: IndexPath
   if let indexPath = coordinator.destinationIndexPath {
      destinationIndexPath = indexPath
   } else {
      let section = collectionView.numberOfSections - 1
      let row = collectionView.numberOfItems(inSection: section)
      destinationIndexPath = IndexPath(row: row, section: section)
   }
   switch coordinator.proposal.operation {
   case .move:
      self.reorderItems(coordinator: coordinator, destinationIndexPath: destinationIndexPath, collectionView: collectionView)
      break
   case .copy:
      // Not copying between collections so this block not needed.
      return
   default:
      return
   }
}

然后我们有了函数reorderItems()，我们需要处理collectionView中的实际更改。需要明确的一点是，快照(NSDiffableDataSourceSnapshot<>)和dataSource (UICollectionViewDiffableDataSource<>)都是类变量。

/// reorderItems method
/// This method moves a cell from the sourceIndexPath to the destinationIndexPath within the same UICollectionView
///
/// - Parameters:
///   - coordinator: UICollectionViewDropCoordinator obtained in performDropWith
///   - destinationIndexPath: IndexPath where user dropped the element.
///   - collectionView: UICollectionView object where reordering is done.
private func reorderItems(coordinator: UICollectionViewDropCoordinator, destinationIndexPath: IndexPath, collectionView: UICollectionView) {
   let items = coordinator.items
   if items.count == 1, let item = items.first, let sourceIndexPath = item.sourceIndexPath {
      var destIndexPath = destinationIndexPath
      if destIndexPath.row >= collectionView.numberOfItems(inSection: destIndexPath.section) {
         destIndexPath.row = collectionView.numberOfItems(inSection: destIndexPath.section) - 1
      }

      /// Since my collectionView data is attributed to a Firebase.storage set of data, this is where I write my changes back to the store.

      snapshot.moveItem(dataSource.itemIdentifier(for: sourceIndexPath)!, beforeItem: dataSource.itemIdentifier(for: destinationIndexPath)!)
      dataSource.apply(snapshot, animatingDifference: true)
      coordinator.drop(items.first!.dragItem, toItemAt: destIndexPath)
   }
}

Answer 2

如果您正在使用collectionView.performBatchUpdates()进行更改以更新您的collectionView，这将导致崩溃，似乎您需要对数据源(UICollectionViewDiffableDataSource)上的快照进行操作，您可以在错误上看到："UICollectionViewDiffableDataSource API时充当UICollectionView的dataSource:请不要直接调用UICollectionView上的突变API“

试试这个：

// MARK: - Properties
  var dataSource: UICollectionViewDiffableDataSource<Int, UIImage>?

  var entry: Entry? {
    didSet {
      guard let entry = entry else { return }
      let dateFormatter = DateFormatter()
      dateFormatter.setLocalizedDateFormatFromTemplate("MMM dd yyyy, hh:mm")
      title = dateFormatter.string(from: entry.dateCreated)
    }
  }

其中，您的条目是一个结构：

struct Entry {
  let id = UUID().uuidString
  let dateCreated = Date()
  var log: String?
  var images: [UIImage] = []
  var isFavorite: Bool = false
}

extension Entry: Hashable {
  func hash(into hasher: inout Hasher) {
    hasher.combine(dateCreated)
    hasher.combine(log)
  }

  static func == (lhs: Entry, rhs: Entry) -> Bool {
    return lhs.dateCreated == rhs.dateCreated &&
      lhs.log ?? "" == rhs.log ?? "" &&
      lhs.images == rhs.images &&
      lhs.isFavorite == rhs.isFavorite
  }
}

reloadSnapshot是：

private func reloadSnapshot(animated: Bool) {
    var snapshot = NSDiffableDataSourceSnapshot<Int, UIImage>()
    snapshot.appendSections([0])
    snapshot.appendItems(entry?.images ?? [])
    dataSource?.apply(snapshot, animatingDifferences: animated)
  }

最后在您的UICollectionViewDropDelegate上：

func collectionView( _ collectionView: UICollectionView, performDropWith coordinator: UICollectionViewDropCoordinator) {

let destinationIndex = coordinator.destinationIndexPath?.item ?? 0

for item in coordinator.items {
  if coordinator.session.localDragSession != nil,
    let sourceIndex = item.sourceIndexPath?.item {

    self.entry?.images.remove(at: sourceIndex)
  }

  item.dragItem.itemProvider.loadObject(ofClass: UIImage.self) {
    (object, error) in
    guard let image = object as? UIImage, error == nil else {
      print(error ?? "Error: object is not UIImage")
      return
    }
    DispatchQueue.main.async {
      self.entry?.images.insert(image, at: destinationIndex)
      self.reloadSnapshot(animated: true)
    }
  }
}
}

我所有这些都是基于阅读和工作从raywenderlich.com的“催化剂由教程”

Answer 3

还有一件事：

如果您使用UICollectionViewDropDelegate方法返回一个UICollectionViewDropProposal，collectionView(_:dropSessionDidUpdate:withDestinationIndexPath destinationIndexPath:)，则该方法在动画化拖放时将调用遮罩下的不可扩散方法。

我猜苹果从来没有用不同的数据源对此进行过测试。您将需要删除此方法，并以不同的方式实现它的动画。