在CollectionView中拖放:PBItemCollectionService连接已断开

我正在努力实现 Drag&Drop .以下是我正在使用的方法:

func collectionView(_ collectionView: UICollectionView, itemsForBeginning session: UIDragSession, at indexPath: IndexPath) -> [UIDragItem] {
        // This just gets the view model for the cell at that indexPath
        let rackModel = adapter.object(atSection: indexPath.section) as! RoomListData.RackModel

        // My RackModel class conforms to NSItemProviderWriting and Reading
        let itemProvider = NSItemProvider(object: rackModel)
        let dragItem = UIDragItem(itemProvider: itemProvider)
        return [dragItem]
}

func collectionView(_ collectionView: UICollectionView, itemsForAddingTo session: UIDragSession, at indexPath: IndexPath, point: CGPoint) -> [UIDragItem] {
        let rackModel = adapter.object(atSection: indexPath.section) as! RoomListData.RackModel
        let itemProvider = NSItemProvider(object: rackModel)
        let dragItem = UIDragItem(itemProvider: itemProvider)
        return [dragItem]
}

func collectionView(_ collectionView: UICollectionView, performDropWith coordinator: UICollectionViewDropCoordinator) {
    // This isn't being called  
    print("okay")
}


func collectionView(_ collectionView: UICollectionView, dragPreviewParametersForItemAt indexPath: IndexPath) -> UIDragPreviewParameters? {
        let preview = UIDragPreviewParameters()
        preview.backgroundColor = .clear
        return preview
}

我还设置了 dropDelegate 和 dragDelegate 对于 UICollectionView . 拖动是可行的,但当我放下对象时,我会在控制台(和 performDrop 方法(未被调用):

PBItemCollectionService连接已断开

这是什么?它发生在模拟器上,也发生在我的iPad上。我找不到问题。

谢谢你的帮助!如果你需要更多的信息,请告诉我。我不知道调试这个需要什么。