如何使用Photos框架平滑加载照片并保持质量？

Question

我试图从图书馆获取照片并在collectionView中显示。我的collectionView启用了分页功能。有我的代码，它的工作原理：

- (void)fetchImage:(PHAsset *)asset completion:(void(^)(UIImage *image))completion {
    //Fetch image from asset
    CGFloat scale = [UIScreen mainScreen].scale;
    CGFloat scaleWidth = [UIScreen mainScreen].bounds.size.width * scale;
    CGFloat scaleHeight = [UIScreen mainScreen].bounds.size.height * scale;
    CGSize retinaSquare = CGSizeMake(scaleWidth, scaleHeight);

    representedAssetIdentifier = [NSString stringWithFormat:@"%@", asset.localIdentifier];

    PHImageRequestOptions *option = [PHImageRequestOptions new];
    option.resizeMode = PHImageRequestOptionsResizeModeFast;
    option.synchronous = YES;

    [[PHImageManager defaultManager] requestImageForAsset:asset
                                               targetSize:retinaSquare
                                              contentMode:PHImageContentModeAspectFill
                                                  options:option
                                            resultHandler:^(UIImage *result, NSDictionary *info) {
                                                // Set the cell's thumbnail image if it's still showing the same asset.
                                                if ([representedAssetIdentifier isEqualToString:asset.localIdentifier]) {
                                                    completion(result);
                                                }
                                            }];
}

在cellForItemAtIndexPath：

- (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath {
    PhotosCell *cell = (PhotosCell *)[collectionView dequeueReusableCellWithReuseIdentifier:[self cellIdentifier] forIndexPath:indexPath];

    PhotoItemModel *item = _controller.assetModels[indexPath.row];

    [_controller fetchImage:item.asset completion:^(UIImage *image) {
        dispatch_async(dispatch_get_main_queue(), ^{
            cell.imageItem.image = image;
            cell.imageItem.clipsToBounds = YES;
            cell.imageItem.contentMode = UIViewContentModeScaleAspectFill;
        });
    }];

    return cell;
}

但我有一个问题：当我尝试滚动到下一张照片时，collectionView并不粗糙。它会延迟一段时间。 我知道问题来自代码option.synchronous = YES;。当我删除此代码时，collectionView很顺利。但我得到另一个问题：有时，照片的质量不好。像那样： 真实的形象：

＆＃34;有时＆＃34;：

我的问题是：如何平滑加载照片并保持照片质量？