NSCollectionView不会接受掉落到物品上的物品

Question

我正在尝试创建一个集合视图，该视图将接受来自Finder的文件。我的委托人的validateDrop方法始终返回NSDragOperationGeneric。如果建议的放置操作为NSCollectionViewDropBefore，则一切正常。但是，如果在我释放鼠标按钮时建议的放置操作为NSCollectionViewDropOn，则永远不会调用acceptDrop委托方法。我可能会缺少什么？

收藏夹视图项仅包含图像。我按原样使用NSCollectionViewItem，而不是子类。

初始化：

    [self.collectionView registerForDraggedTypes: @[NSPasteboardTypeFileURL]];
    NSNib* itemNib = [[NSNib alloc] initWithNibNamed: @"IconItemView" bundle: nil];
    [self.collectionView registerNib: itemNib forItemWithIdentifier: @"app"];

我的委托方法：

- (NSDragOperation)collectionView:(NSCollectionView *)collectionView
    validateDrop:(id<NSDraggingInfo>)draggingInfo
    proposedIndexPath:(NSIndexPath * _Nonnull *)proposedDropIndexPath
    dropOperation:(NSCollectionViewDropOperation *)proposedDropOperation
{
    NSLog(@"Validate drop op %d", (int)*proposedDropOperation );
    return NSDragOperationGeneric;
}

- (BOOL)collectionView:(NSCollectionView *)collectionView
        acceptDrop:(id<NSDraggingInfo>)draggingInfo
        indexPath:(NSIndexPath *)indexPath
        dropOperation:(NSCollectionViewDropOperation)dropOperation
{
    NSLog(@"Accept drop op %d", (int)dropOperation );

    [draggingInfo enumerateDraggingItemsWithOptions: 0
        forView: self.collectionView
        classes: @[ [NSURL class] ]
        searchOptions: @{}
        usingBlock:
            ^(NSDraggingItem * _Nonnull draggingItem, NSInteger idx, BOOL * _Nonnull stop)
            {
                NSURL* itemURL = draggingItem.item;
                NSImage* theImage = [[NSWorkspace sharedWorkspace]
                    iconForFile: itemURL.path];
                NSDictionary* itemRec = @{
                    @"icon": theImage,
                    @"url": itemURL
                };
                NSMutableArray* secArray = [self arrayForSection: indexPath.section];
                [secArray insertObject: itemRec atIndex: indexPath.item];
            }];

    [self.collectionView reloadData];

    return YES;
}