从 Document 目录加载多个图像时 UICollectionView 卡住

Question

我正在加载超过 30-50 张图像，这些图像存储在文档目录中。他们所有的图像都立即加载到collectionView，但是当滚动collectionView时它会卡住。

- (__kindof UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath
{
    static NSString *cellIdentifier = @"ImageCell";
    ImageCell *cell = [collectionView dequeueReusableCellWithReuseIdentifier:cellIdentifier forIndexPath:indexPath];

    //Load image from document  directory
    NSString *imageName = [arrCollection objectAtIndex:indexPath.row];
    NSString *imagePath = [NSString stringWithFormat:@"%@/%@",(AppObj).imagefolderPath, imageName];
    UIImage *localImg = [[UIImage alloc]initWithContentsOfFile:imagePath];

    cell.imgSketch.image = localImg;

    cell.txtName.text = [NSString stringWithFormat:@"%@",[imageName stringByReplacingOccurrencesOfString:@".png" withString:@""]];

    cell.layer.masksToBounds = YES;
    return cell;
}

我也尝试了以下调度块

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0), ^{
                UIImage *localImg = [[UIImage alloc]initWithContentsOfFile:[NSString stringWithFormat:@"%@/%@",(AppObj).imagefolderPath,[arrCollection objectAtIndex:indexPath.row]]];
                dispatch_async(dispatch_get_main_queue(), ^{
                    cell.imgSketch.image = localImg;
                });
            });

请建议我解决此问题的任何解决方案。

提前致谢

Answer 1

您可以使用SDWebImage 也可以显示文档目录中的图像。

[cell.imgSketch sd_setImageWithURL:[NSURL fileURLWithPath:[NSString stringWithFormat:@"%@/%@",(AppObj).imagefolderPath,[arrCollection objectAtIndex:indexPath.row]]]];

注意：在创建NSURL 的对象时使用fileURLWithPath，因为它是文件的路径。

Answer 2

我的建议是将您的图像加载操作移至后台线程。

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, NULL), ^{
   UIImage* localImage = [[UIImage alloc] initWithContentsOfFile:[NSString stringWithFormat:@"%@/%@",(AppObj).imagefolderPath,[arrCollection objectAtIndex:indexPath.row]]];    
   dispatch_async(dispatch_get_main_queue(), ^{
      cell.imgSketch.image = localImage;
   });
});

Answer 3

我建议在cellForRowAt 中只加载一次所有图像

NSMutableArray*images; // every instance is of type UIImage not imageName

而不是这个

UIImage *localImg = [[UIImage alloc]initWithContentsOfFile:[NSString stringWithFormat:@"%@/%@",(AppObj).imagefolderPath,[arrCollection objectAtIndex:indexPath.row]]];

注意：这个方法initWithContentsOfFile在mainQueue中运行所以是卡住的主要原因

Answer 4

如果我们使用大图像，collectionview/tableview 单元格会闪烁。尝试使用小尺寸图片或在后台队列中的cellForItemAtIndexPath 创建一张。

您可以通过在数组中缓存图像来提高性能。 注意：如果容量增加限制，请尝试清理它。

cell.imgSketch.image = nil;
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{

     UIImage *localImg = [[UIImage alloc] initWithContentsOfFile:[NSString stringWithFormat:@"%@/%@", (AppObj).imagefolderPath, [arrCollection objectAtIndex:indexPath.row]]];

     NSData *imgData = UIImageJPEGRepresentation(localImg, 0.5);
     localImg = [UIImage imageWithData:imgData];

     dispatch_async(dispatch_get_main_queue(), ^{
         ImageCell *imageCell = [collectionView cellForItemAtIndexPath:indexPath];
         if (imageCell) {
             cell.imgSketch.image = localImg;
         }
    });
});