Question

我觉得下面的代码中存在一个小错误，它使得集合视图单元格第一次出现，因为它们很快就会移动，然后到达它们的位置，这是不好的。

- (UICollectionViewCell *)collectionView:(UICollectionView *)cv cellForItemAtIndexPath:(NSIndexPath *)indexPath;
{
    BookCell *cell = [cv dequeueReusableCellWithReuseIdentifier:@"BookCell" forIndexPath:indexPath];
    NSString *coverName = [[_dataSource objectAtIndex:indexPath.row] objectForKey:@"cover"];
    NSString *cover = [[NSBundle mainBundle] pathForResource:coverName ofType:@"jpg" inDirectory:@"covers"];
    cell.coverImage.image = self.placeHolderImage;

    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        UIImage *coverImage = [[UIImage alloc] initWithContentsOfFile:cover];

        CGSize imageSize = coverImage.size;
        UIGraphicsBeginImageContext(imageSize);
        [coverImage drawInRect:CGRectMake(0, 0, imageSize.width, imageSize.height)];
        coverImage = UIGraphicsGetImageFromCurrentImageContext();
        UIGraphicsEndImageContext();

        dispatch_async(dispatch_get_main_queue(), ^{

                cell.coverImage.image = coverImage;
        });
    });
    ...    
    return cell;
}

更新：如果我将日志放在dispatch_async(dispatch_get_main_queue()内，我看到每个单元格都在更新两次，这会导致重排效果。但每次例如单元格0具有不同的图像。不确定是什么导致了这一点。

index: 0, cover: <UIImage: 0x156988d0>
index: 1, cover: <UIImage: 0x155b48f0>
index: 3, cover: <UIImage: 0x156b0660>
index: 2, cover: <UIImage: 0x1569d5a0>
index: 4, cover: <UIImage: 0x155eca20>
index: 5, cover: <UIImage: 0x155dece0>
index: 7, cover: <UIImage: 0x155c7d90>
index: 6, cover: <UIImage: 0x155d5920>
index: 8, cover: <UIImage: 0x155c7dc0>
index: 9, cover: <UIImage: 0x155b6d80>
index: 0, cover: <UIImage: 0x155cf410>
index: 1, cover: <UIImage: 0x1569c9a0>
index: 2, cover: <UIImage: 0x15692ca0>
index: 3, cover: <UIImage: 0x1555b7c0>
index: 4, cover: <UIImage: 0x155c9c90>
index: 5, cover: <UIImage: 0x156baee0>
index: 9, cover: <UIImage: 0x155cdf50>
index: 7, cover: <UIImage: 0x155b4630>
index: 8, cover: <UIImage: 0x15684af0>
index: 6, cover: <UIImage: 0x155c7950>

Answer 1

    dispatch_async(dispatch_get_main_queue(), ^{

        BookCell *updateCell = (BookCell*)[self.collectionView cellForItemAtIndexPath:indexPath];
        if (updateCell)
        {
            cell.coverImage.image = coverImage;
        }
    });

此处不需要递归调用。该块可以捕获BookCell的值，因此您只需要：

    dispatch_async(dispatch_get_main_queue(), ^{
            cell.coverImage.image = coverImage;
    });

设置生成的图像。这可能是您问题的根源。

作为一种风格问题，你在collectionView:cellForItemAtIndexPath:中做得太多了。在您的自定义BookCell课程中完成所有这些操作，以便更好地分离问题。

如何防止对UICollectionView的洗牌效果？

1 个答案: