ios - block 的执行总是延迟

我对 IOS 很陌生。我想在 viewDidLoad 中获取设备上的所有图片。但是该 block 总是在我在代码中调用 NSLog(@"%d", photos.count) 之后执行，如下所示。这种情况如何处理？

__block NSMutableArray* photos = [[[NSMutableArray alloc] init] retain];

ALAssetsLibrary *library = [[ALAssetsLibrary alloc] init];

void (^assertsEnumerator)(ALAsset *, NSUInteger , BOOL *) =  ^(ALAsset *result, NSUInteger index, BOOL *stop)
{
        if(result)
        {   
            NSLog(@"Assert name : %@", [result valueForProperty:ALAssetPropertyURLs]);         
            [photos addObject:[UIImage imageWithCGImage:[result aspectRatioThumbnail]]];   
        }
};

void (^groupEnumerator)(ALAssetsGroup*, BOOL*) = ^(ALAssetsGroup *group, BOOL *stop) {
    if(group != nil) {
        NSLog(@"Adding group %@", [group valueForProperty:ALAssetsGroupPropertyName]);
        [group enumerateAssetsUsingBlock: assertsEnumerator];
    }
};

[library enumerateGroupsWithTypes: ALAssetsGroupSavedPhotos
                       usingBlock:groupEnumerator
                     failureBlock:^(NSError * err) {NSLog(@"Erorr: %@", [err localizedDescription]);}];
[library release];
NSLog(@"%d", photos.count);
[photos release];

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Best Answer-推荐答案

作为 ALAssetsLibrary documentation状态，enumerateGroupsWithTypes:usingBlock:failureBlock: 是异步的，所以不是在原地调用，而是计划在下一个运行循环周期内调用。文档清楚地说明了这是什么原因以及您应该如何进行:

This method is asynchronous. When groups are enumerated, the user may be asked to confirm the application's access to the data; the method, though, returns immediately. You should perform whatever work you want with the assets in enumerationBlock.

关于ios - block 的执行总是延迟，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/9478456/