Question

我有一个PHAsset列表，我需要获取相关的URL并对每个URL执行一些操作。处理完所有资产后，我需要执行另一项任务。我尝试使用__block来计算处理的资产，但由于竞争条件，它不可靠。有没有更好的方法来了解何时处理所有资产？

    PHFetchResult* photosAsset = [PHAsset fetchAssetsInAssetCollection:collection options:fetchOptions2];
    __block int count = 0;

    for (int i = 0; i < photosAsset.count; ++i) {
        [[PHImageManager defaultManager] requestAVAssetForVideo:[photosAsset objectAtIndex:i] options:nil resultHandler:
         ^(AVAsset *avAsset, AVAudioMix *audioMix, NSDictionary *info) {
             NSURL *url = [(AVURLAsset *)avAsset URL];
             // then do something with the url here...

             ++count;
             NSLog(@"%d", count);
             if (count == photosAsset.count) {
                 NSLog(@"FINISHED!");
             }
        }];
    }

Answer 1

所以这是基于@ Larme建议的解决方案：

    dispatch_group_t group = dispatch_group_create();

    for (int i = 0; i < photosAsset.count; ++i) {
        dispatch_group_enter(group);
        [[PHImageManager defaultManager] requestAVAssetForVideo:[photosAsset objectAtIndex:i] options:nil resultHandler:
         ^(AVAsset *avAsset, AVAudioMix *audioMix, NSDictionary *info) {
             dispatch_group_leave(group);
         }];
    }

    dispatch_group_notify(group,dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^ {
        NSLog(@"FINISHED!");
    });

    dispatch_release(group);

使用__block进行竞争条件

1 个答案: