ios - UICollectionViewDropDelegate 和 DiffableDataSource

Question

我正在尝试实现 UICollectionViewDropDelegate使用 DiffableDataSource(在 iOS 13 中引入)。

我已经实现 UICollectionViewDragDelegate ，这很好用。

我设置了collectionView.dropDelegate = self

我实现了 func collectionView(_ collectionView: UICollectionView, performDropWith coordinator: UICollectionViewDropCoordinator) (你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，但是一旦您开始拖动单元格，该应用程序立即崩溃并显示以下消息:

Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: 'UICollectionView must be updated via the UICollectionViewDiffableDataSource APIs when acting as the UICollectionView's dataSource: please do not call mutation APIs directly on UICollectionView.

所以看起来这个方法试图直接修改 UICollectionView (也许在你移动项目时移动单元格？)我不知道如何绕过这种行为。

想法？

Answer 1

这是一个相当容易的解决方案。从 UICollectionViewDropDelegate 函数 performDropWith 开始。请注意，我只需要一个 .move 而不需要 .copy ，但您只需添加一个类似于下面的 reorderItems() 方法的 copyItems() 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
   }
}

然后我们有函数 reorderItems() 我们需要处理 collectionView 中的实际更改。需要明确的一件事是快照 (NSDiffableDataSourceSnapshot<>) 和数据源 (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)
   }
}