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- (void)setupDragCollectVieCell
{
    // 开启拖放手势，设置代理。
    self.collectionView.dragInteractionEnabled = YES;
    self.collectionView.dragDelegate = self;
    self.collectionView.dropDelegate = self;
}

- (NSArray<UIDragItem *> *)collectionView:(UICollectionView *)collectionView itemsForBeginningDragSession:(id<UIDragSession>)session atIndexPath:(NSIndexPath *)indexPath
{
    GIRPhotoWallData *data = [self.photos objectAtIndex:indexPath.item];
    NSItemProvider *itemProvider = [[NSItemProvider alloc] initWithObject:data.imageURL];
    UIDragItem *dragItem = [[UIDragItem alloc] initWithItemProvider:itemProvider];
    dragItem.localObject = data;
    
    if (data.media == nil) {
        return @[];
    }
    return @[dragItem];
}

// 是否接收拖动的item。
- (BOOL)collectionView:(UICollectionView *)collectionView canHandleDropSession:(id<UIDropSession>)session
{
    return [session canLoadObjectsOfClass:[NSString class]];
}

// 拖动过程中不断反馈item位置。
- (UICollectionViewDropProposal *)collectionView:(UICollectionView *)collectionView dropSessionDidUpdate:(id<UIDropSession>)session withDestinationIndexPath:(NSIndexPath *)destinationIndexPath
{
    UICollectionViewDropProposal *dropProposal;
    if (session.localDragSession) {
        // 拖动手势源自同一app。
        dropProposal = [[UICollectionViewDropProposal alloc] initWithDropOperation:UIDropOperationMove intent:UICollectionViewDropIntentInsertAtDestinationIndexPath];
    } else {
        // 拖动手势源自其它app。
        dropProposal = [[UICollectionViewDropProposal alloc] initWithDropOperation:UIDropOperationCopy intent:UICollectionViewDropIntentInsertAtDestinationIndexPath];
    }
    return dropProposal;
}

- (void)collectionView:(UICollectionView *)collectionView performDropWithCoordinator:(id<UICollectionViewDropCoordinator>)coordinator
{
    // 如果coordinator.destinationIndexPath存在，直接返回；如果不存在，则返回（0，0)位置。
    NSIndexPath *destinationIndexPath = coordinator.destinationIndexPath ? coordinator.destinationIndexPath : [NSIndexPath indexPathForItem:0 inSection:0];

    GIRPhotoWallData *data = [self.photos objectAtIndex:destinationIndexPath.row];
    if (data.media == nil) {
        return;
    }
    
    // 在collectionView内，重新排序时只能拖动一个cell。
    if (coordinator.items.count == 1 && coordinator.items.firstObject.sourceIndexPath) {
        NSIndexPath *sourceIndexPath = coordinator.items.firstObject.sourceIndexPath;

        // 将多个操作合并为一个动画。
        [collectionView performBatchUpdates:^{
            // 将拖动内容从数据源删除，插入到新的位置。
            GIRPhotoWallData *imageName = coordinator.items.firstObject.dragItem.localObject;
            
            [self.photos removeObjectAtIndex:sourceIndexPath.item];
            
            [self.photos insertObject:imageName atIndex:destinationIndexPath.item];

            // 更新collectionView。
            [collectionView moveItemAtIndexPath:sourceIndexPath toIndexPath:destinationIndexPath];
        } completion:nil];
        
        // 以动画形式移动cell。
        [coordinator dropItem:coordinator.items.firstObject.dragItem toItemAtIndexPath:destinationIndexPath];
    }
}