ios - block 中的 NSMutableArray addObject 给出空数组

Question

我有一组列表 ALAsset URL。我想将该 URL 逐一转换为 ALAsset 并将其添加到新数组中。

这是我的代码:

-(void)retrieveAssetsWithArray:(NSArray *)assetsArray
{

    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        // Background work
        __block NSMutableArray *retrievedAssetsArray = [[NSMutableArray alloc] init];
        for (int i = 0; i < [assetsArray count]; i++)
        {
            ALAssetsLibrary *library = [[ALAssetsLibrary alloc] init];
            [library assetForURL:[NSURL URLWithString:[assetsArray objectAtIndex:i]]
                     resultBlock:^(ALAsset *asset)
             {
                 if (asset)
                 {
                     NSLog(@"assetss: %@", asset);
                     [retrievedAssetsArray addObject:asset];
                     NSLog(@"assets arayyyy: %@", retrievedAssetsArray);
                 }
             }
             failureBlock:^(NSError *error)
             {
                 NSLog(@"Error: Cannot load asset - %@", [error localizedDescription]);
             }
             ];
        }

        dispatch_async(dispatch_get_main_queue(), ^{
            // Update UI
            if ([self.delegate respondsToSelector:@selector(getRetrievedAssetsFromPhotoLibrary:)])
            {
                NSLog(@"retrievedAssetsArray :%@", retrievedAssetsArray);
                [self.delegate getRetrievedAssetsFromPhotoLibrary:retrievedAssetsArray];
            }
        });
    });
}

将 URL 转换为 ALAsset 的部分工作正常。但是retrievedAssetsArray当我尝试将其记录在 dispatch_async(dispatch_get_main_queue() 中时，返回如下:

retrievedAssetsArray :(
    "ALAsset - Type:Unknown, URLs:(null)",
    "ALAsset - Type:Unknown, URLs:(null)",
    "ALAsset - Type:Unknown, URLs:(null)",
    "ALAsset - Type:Unknown, URLs:(null)"
)

为什么会发生这种情况？谁能告诉我如何解决这个问题？干杯。

Answer 1

我找到了问题的答案:

-(void)retrieveAssetsWithArray:(NSArray *)assetsArray
{
    __block NSMutableArray *retrievedAssetsArray = [[NSMutableArray alloc] init];
    dispatch_semaphore_t sema = dispatch_semaphore_create(0);
    dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT,0);
    ALAssetsLibrary *library = [[ALAssetsLibrary alloc] init];

    for (NSString *string in assetsArray) {
        dispatch_async(queue, ^{
            [library assetForURL:[NSURL URLWithString:string] resultBlock:^(ALAsset *asset) {
                [retrievedAssetsArray addObject:asset];
                dispatch_semaphore_signal(sema);
            } failureBlock:^(NSError *error) {
                dispatch_semaphore_signal(sema);
            }];
        });

        dispatch_semaphore_wait(sema, DISPATCH_TIME_FOREVER);
    }

    dispatch_release(sema);

    /* Check out ALAssets and return in delegate methods*/    
    dispatch_async(dispatch_get_main_queue(), ^{
        // Update UI
        if ([self.delegate respondsToSelector:@selector(getRetrievedAssetsFromPhotoLibrary:)])
        {
            NSLog(@"retrieve %@", retrievedAssetsArray);
            [self.delegate getRetrievedAssetsFromPhotoLibrary:retrievedAssetsArray];
        }
    });

}