Question

我希望在处理每张图像后加载太多图像但当我上下滚动，懒惰加载..
这是我的代码。

- (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath
{
UICollectionViewCell *cell;
cell = [collectionView dequeueReusableCellWithReuseIdentifier:@"FaceImageCell" forIndexPath:indexPath];
UIImageView *fImageView = (UIImageView*)[cell viewWithTag:1];
[faceImageView setImage:[UIImage imageNamed:@"ic_loading"]];
NSLog(@"Loaded Image row : %d", indexPath.row);

ALAsset *asset;
    asset = [_imageList objectAtIndex:indexPath.row];
    dispatch_async(all_f_d_queue, ^{
        ALAssetRepresentation *representation = [asset defaultRepresentation];
        NSString *filename = [representation filename];
        NSLog(@"%@", filename);
        UIImage *image, *fullImage;
        if ((fullImage = [_fullImageCache objectForKey:filename]) == nil) {
            image = [UIImage imageWithCGImage:[asset thumbnail]
                                             scale:[representation scale]
                                       orientation:(UIImageOrientation)[representation orientation]];
            vector<cv::Rect> f = [ImageUtils findFeature:image minsize:MIN_FACE_SIZE
                                                 withCascade:face_cascade];

            Mat imageMat = [ImageUtils cvMatFromUIImage:image];
            fullImage = [ImageUtils UIImageFromCVMat:imageMat];
            [_fullImageCache setObject:fullImage forKey:filename];
        }
        dispatch_async(dispatch_get_main_queue(), ^{
            [fImageView setImage:fullImage];
            [cell setNeedsDisplay];
        });
    });
return cell;
}

由于队列很多，它发生了延迟加载当加载单元格不可见时，我想停止或取消调度队列我该如何检测单元格是不可见的以及停止调度队列？
请告诉我。
我可以在android中使用asynctask。但我不知道如何在iOS中实现它。

Answer 1

您可以使用NSOperation和NSOperationQueue。它与dispatchlib几乎相同，但是面向对象。您可以将工作包装在NSOperation上，然后将其添加到NSOperationQueue。然后您可以随时取消NSOperation。可能必须阅读Apple文档以更好地解释这两个类。

Answer 2

看看this tutorial。它显示了如何使用NSOperationQueue异步下载数据，并在用户滚动并且此单元格不再可见时取消此队列。

使用dispatch_sync在uicollectionview上延迟加载

2 个答案: