objective-c - 使用 ALAssetsLibrary 枚举 block 强制执行顺序

Question

我正在使用 Apple 的 ALAssetsLibrary 框架进行一些迭代。我需要在开始播放视频之前完成此迭代。

这里是简化的示例代码，显示了我遇到的问题。

@property (nonatomic) NSMutableArray *assetsToPlay;

- (void)viewDidLoad
{
    ALAssetsLibrary *library = [[ALAssetsLibrary alloc] init];

    [library enumerateGroupsWithTypes:ALAssetsGroupAll usingBlock:^(ALAssetsGroup *group, BOOL *stop) {
        [group enumerateAssetsUsingBlock:^(ALAsset *result, NSUInteger index, BOOL *stop) {
            AVAsset *asset = <#Get Asset#>
            [self.assetsToPlay addObject:asset];
        }];
    } failureBlock:nil];

    [self playem]; // BUG: This code is invoked immediately. I would like it to run only after the above block iteration finished.
}

- (void)playem
{
    for(AVAsset *asset in self.assetsToPlay) {
        NSLog(@"%@", asset);
    }
}

Answer 1

就 Elad 而言，枚举方法异步运行，完成后 group 将为 nil。

因此，与大多数异步情况一样，您应该将调用移至完成 block 中的 playem 内部，而不是在其之后。您想在 group 为 nil 时调用 playem(即您已完成枚举):

[library enumerateGroupsWithTypes:ALAssetsGroupAll usingBlock:^(ALAssetsGroup *group, BOOL *stop) {
    if (group) {
        // if `group` is non-`nil`, then still enumerating the groups

        [group enumerateAssetsUsingBlock:^(ALAsset *result, NSUInteger index, BOOL *stop) {
            if (result) { 
                // note, only do this if `result` is not `nil` (`nil` signifies end of asset enumeration)

                AVAsset *asset = <#Get Asset#>
                [self.assetsToPlay addObject:asset];
            }
        }];
    } else {
        // if `group` is `nil`, then done with enumeration, so call method

        [self playem]; // this will be called when the enumeration is done.
    }
} failureBlock:^(NSError *error) {
    // handle the user's rejection of permission here
}];