ios - 将文档目录中的图像异步加载到 UICollectionView

Question

我正在尝试将文档目录中的图像加载到 UICollectionView 中。它的工作原理:流畅，快速....(笑)。但我遇到一个问题:当我调用 [collectionView reloadData] 时，图像闪烁(一次)。这是我的代码:

- (UICollectionViewCell*)collectionView:(UICollectionView *)collectionView_ cellForItemAtIndexPath:(NSIndexPath *)indexPath {
    NSString *cellIdentifier = NSStringFromClass([AFMyRecentCollectionViewCell class]);
    AFMyRecentCollectionViewCell *cell = (AFMyRecentCollectionViewCell*)[collectionView_ dequeueReusableCellWithReuseIdentifier:cellIdentifier forIndexPath:indexPath];
    AFMyRecentObject *recent = [[AFRecentManager sharedInstance].arrayRecent objectAtIndex:indexPath.row];

    dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0ul);
    dispatch_async(queue, ^{
        UIImage *image;
        NSFileManager *fileManager = [NSFileManager defaultManager];
        NSString *filePath = [self databasePathWithPathComponent:[NSString stringWithFormat:@"%@.jpg", recent.id_film]];
        if ([fileManager fileExistsAtPath:filePath] == YES) {
            image = [UIImage imageWithContentsOfFile:filePath];
        }

        dispatch_async(dispatch_get_main_queue(), ^{
            if (image) {
                cell.imageView.image = image;
            } else {
                cell.imageView.image = [UIImage imageNamed:@"loadding.png"];
            }
        });
    });

    cell.layer.shouldRasterize = YES;
    cell.layer.rasterizationScale = [UIScreen mainScreen].scale;

    return cell;
}

- (NSString *)databasePathWithPathComponent:(NSString*)pathComponent {
    NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
    NSString *documentsDirectory = [paths objectAtIndex: 0];
    NSString *path = [documentsDirectory stringByAppendingPathComponent:pathComponent];
    return path;
}

Answer 1

您必须放置默认/占位符图像以防止出现这种情况..

    // this prevents the flicker/blinks
    cell.imageView.image = nil; // if you dont have any image as placeholder

    //since you have 
    cell.imageView.image = [UIImage imageNamed:@"loadding.png"]; // this i assume your placeholder image?

    dispatch_async(queue, ^{
        //your codes..
        dispatch_async(dispatch_get_main_queue(), ^{
            // your codes..
        });
    });