用UICollectionViewDiffableDataSource和NSFetchedResultsController重排细胞

Question

我使用一个UICollectionViewDiffableDataSource和一个NSFetchedResultsController在NSFetchedResultsController中填充我的UICollectionView。

为了增加重新排序单元的能力，我添加了一个UILongPressGestureRecognizer和子类UICollectionViewDiffableDataSource，以便使用它的canMoveItemAt:和moveItemAt:方法。

当重新排序单元格时，会发生以下情况：

1. 调用moveItemAt:，我更新对象位置属性并保存MOC。
2. 调用controllerDidChangeContent: of NSFetchedResultsControllerDelegate，我从当前的fetchedObjects创建一个新的快照并应用它。

当我应用dataSource?.apply(snapshot, animatingDifferences: true)时，单元格会立即切换回位置。如果我设置了animatingDifferences: false，它就能工作，但是所有的单元格都可以明显地重新加载。

这里有什么最佳实践吗?如何在UICollectionViewDiffableDataSource和NSFetchedResultsController上实现单元格重新排序？

以下是我提到的方法：

// ViewController 
func createSnapshot(animated: Bool = true) {
    var snapshot = NSDiffableDataSourceSnapshot<Int, Favorite>()
    snapshot.appendSections([0])
    snapshot.appendItems(provider.fetchedResultsController.fetchedObjects ?? [])
    dataSource?.apply(snapshot, animatingDifferences: animated)
}

// NSFetchedResultsControllerDelegate
func controllerDidChangeContent(_ controller: NSFetchedResultsController<NSFetchRequestResult>) {
    createSnapshot(animated: false)
}

// Subclassed UICollectionViewDiffableDataSource
override func collectionView(_ collectionView: UICollectionView, moveItemAt sourceIndexPath: IndexPath, to destinationIndexPath: IndexPath) {
    provider.moveFavorite(from: sourceIndexPath.row, to: destinationIndexPath.row)
}

// Actual cell moving in a provider class
public func moveFavorite(from source: Int, to destination: Int) {
    guard let favorites = fetchedResultsController.fetchedObjects else { return }
    if source < destination {
        let partialObjects = favorites.filter({ $0.position <= destination && $0.position >= source })

        for object in partialObjects {
            object.position -= 1
        }

        let movedFavorite = partialObjects.first
        movedFavorite?.position = Int64(destination)
    }
    else {
        let partialObjects = favorites.filter({ $0.position >= destination && $0.position <= source })

        for object in partialObjects {
            object.position += 1
        }

        let movedFavorite = partialObjects.last
        movedFavorite?.position = Int64(destination)
    }
    do {
        try coreDataHandler.mainContext.save()
    } catch let error as NSError {
        print(error.localizedDescription)
    }
}

Answer 1

相同问题的解决方案是对UICollectionViewDiffableDataSource进行子类化，并在子类中实现canMoveItemAt方法以回答true。如果longPressAction的.ended案例做了三件事，动画对我来说似乎很好：

1. 更新模型
2. 呼叫dateSource.collectionView(..moveItemAt:..)
3. 运行您的dataSource.apply

用于拖动行为的其他常用方法也必须实现，看起来就像您所做的那样。其他方法-这些方法在UICollectionView的“交互式重新排序”一节中有很好的文档。https://developer.apple.com/documentation/uikit/uicollectionview

class PGLDiffableDataSource: UICollectionViewDiffableDataSource<Int, Int> {
    override func collectionView(_ collectionView: UICollectionView, canMoveItemAt indexPath: IndexPath) -> Bool {
        return true
    }

}

Answer 2

不需要子类。从iOS 14.0开始，UICollectionViewDiffableDataSource支持您可以实现的重新排序处理程序。

let data_source = UICollectionViewDiffableDataSource<MySection, MyModelObject>( collectionView: collection_view, cellProvider:
{
    [weak self] (collection_view, index_path, video) -> UICollectionViewCell? in
        
    let cell = collection_view.dequeueReusableCell( withReuseIdentifier: "cell", for: index_path ) as! MyCollectionViewCell

    if let self = self
    {
        //setModel() is my own method to update the view in MyCollectionViewCell
        cell.setModel( self.my_model_objects[index_path.item] )
    }

    return cell
})

// Allow every item to be reordered as long as there's 2 or more
diffable_data_source.reorderingHandlers.canReorderItem = 
{ 
    item in 
    my_model_objects.count >= 2 return true 
}

//Update your model objects before the reorder occurs. 
//You can also use didReorder, but it might be useful to have your
//model objects in the correct order before dequeueReusableCell() is 
//called so you can update the cell's view with the correct model object.
diffable_data_source.reorderingHandlers.willReorder = 
{ 
    [weak self] transaction in
    
    guard let self = self else { return }
    
    self.my_model_objects = transaction.finalSnapshot.itemIdentifiers
}