ios - UICollectionView BatchUpdate 边缘案例失败

Question

我在 UICollectionView 的 batchUpdate 操作中确定了一个简单的边缘情况，它应该可以工作但失败了

attempt to perform an insert and a move to the same index path ( {length = 2, path = 0 - 2})

我的操作是从[A, B] --> [C, B', A]。这是通过更新完成的:

• 从索引 0 移动到 2
• 重新加载索引 1
• 在索引 0 处插入

很明显错误不对，insert索引和move TO索引不一样。

我设置了演示以确保这是一个 UICollectionView 问题，如果您想查看它的实际效果，这是我的代码:

@implementation ViewController {
  UICollectionView *_collection;
  NSArray *_values;
}

- (instancetype)init
{
  self = [super init];
  if (self) {
    UICollectionViewFlowLayout *layout = [[UICollectionViewFlowLayout alloc] init];
    _collection = [[UICollectionView alloc] initWithFrame:CGRectZero collectionViewLayout:layout];
    _collection.dataSource = self;
    _values = @[@1, @2];
    [_collection registerClass:[UICollectionViewCell class]
      forCellWithReuseIdentifier:@"reuse"];
    [self.view addSubview:_collection];
  }
  return self;
}

- (void)viewDidLoad {
  [super viewDidLoad];
  _collection.frame = self.view.bounds;

  double delayInSeconds = 2.0;
  dispatch_time_t popTime = dispatch_time(DISPATCH_TIME_NOW, (int64_t)(delayInSeconds * NSEC_PER_SEC));
  dispatch_after(popTime, dispatch_get_main_queue(), ^(void){
    [self triggerBatchUpdate];
  });
}

- (void)triggerBatchUpdate {
  [_collection performBatchUpdates:^{
    _values = @[@4, @5, @1];
    [_collection insertItemsAtIndexPaths:@[[self index:0]]];
    [_collection moveItemAtIndexPath:[self index:0] toIndexPath:[self index:2]];

    // Works with this line
//    [_collection moveItemAtIndexPath:[self index:1] toIndexPath:[self index:1]];

    // Fails with this line
    [_collection reloadItemsAtIndexPaths:@[[self index:1]]];
  } completion:nil];
}

- (NSIndexPath *)index:(NSUInteger)ind {
  return [NSIndexPath indexPathForRow:ind inSection:0];
}

- (NSInteger)collectionView:(UICollectionView *)collectionView
     numberOfItemsInSection:(NSInteger)section {
  return _values.count;
}

- (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView
                  cellForItemAtIndexPath:(NSIndexPath *)indexPath {
  UICollectionViewCell *cell = [_collection dequeueReusableCellWithReuseIdentifier:@"reuse"
                                                                      forIndexPath:indexPath];
  cell.backgroundColor = [UIColor grayColor];
  return cell;
}


@end

事实上如果我使用代码就可以工作

[_collection moveItemAtIndexPath:[self index:1] toIndexPath:[self index:1]];

代替

[_collection reloadItemsAtIndexPaths:@[[self index:1]]];

让我怀疑。这两个操作应该是等价的。

知道这里发生了什么吗？我认为这是 UICollectionView 错误吗？

编辑:另一个与此有关的崩溃:

attempt to perform a delete and a move from the same index path ( {length = 2, path = 0 - 5})

Answer 1

这似乎是一个错误，虽然 performBatchUpdates:completion 的文档解释了插入和删除操作中索引的上下文，并提到允许重新加载操作，但它没有详细说明重新加载操作中索引的上下文。

经过一些实验，似乎“在幕后”重新加载是作为删除和插入实现的。这似乎会导致一些其他操作与重新加载索引之间存在重叠的问题。

不过，我确实发现用显式删除和插入替换重新加载似乎可行，因此您可以使用:

- (void)triggerBatchUpdate {
    [_collection performBatchUpdates:^{
        _values = @[[UIColor blueColor], [UIColor yellowColor], [UIColor redColor]];
        [_collection moveItemAtIndexPath:[self index:0] toIndexPath:[self index:2]];
        [_collection insertItemsAtIndexPaths:@[[self index:0]]];

        [_collection deleteItemsAtIndexPaths:@[[self index:1]]];
        [_collection insertItemsAtIndexPaths:@[[self index:1]]];
    } completion:nil];
}