无法拖动到NSCollectionViewItem中的图像上

class MyCollectionViewDelegate: NSObject, NSCollectionViewDelegate, NSCollectionViewDelegateFlowLayout, NSDraggingSource, NSDraggingDestination {
    // 实现拖放代理方法
    // ...
}

extension MyCollectionViewDelegate {
    func collectionView(_ collectionView: NSCollectionView, writeItemsAt indexPaths: Set<IndexPath>, to pasteboard: NSPasteboard) -> Bool {
        // 将拖动的数据写入粘贴板
        // ...
        return true
    }
    
    func collectionView(_ collectionView: NSCollectionView, validateDrop draggingInfo: NSDraggingInfo, proposedIndexPath proposedDropIndexPath: AutoreleasingUnsafeMutablePointer<NSIndexPath>, dropOperation proposedDropOperation: UnsafeMutablePointer<NSCollectionView.DropOperation>) -> NSDragOperation {
        // 验证拖放操作
        // ...
        return .move
    }
    
    func collectionView(_ collectionView: NSCollectionView, acceptDrop draggingInfo: NSDraggingInfo, indexPath: IndexPath, dropOperation: NSCollectionView.DropOperation) -> Bool {
        // 接受拖放操作
        // ...
        return true
    }
}

let collectionView = NSCollectionView()
let delegate = MyCollectionViewDelegate()
collectionView.delegate = delegate
collectionView.dataSource = delegate
collectionView.register(MyCollectionViewItem.self, forItemWithIdentifier: NSUserInterfaceItemIdentifier(rawValue: "MyCollectionViewItem"))