assetsLibrary是一个简单的bug吗？

Question

我真的迷路了。为什么我为每个UIImage获取两次NSLog？

 //------ get the images from the camera roll ----------
    assets=[[NSMutableArray alloc]init];
    NSMutableArray *cameraRollPictures=[[NSMutableArray alloc]init];
    ALAssetsLibrary *assetsLibrary = [[ALAssetsLibrary alloc] init];
    [assetsLibrary enumerateGroupsWithTypes:ALAssetsGroupSavedPhotos usingBlock:^(ALAssetsGroup *group, BOOL *stop)
    {

        NSInteger numberOfAssets = [group numberOfAssets];
        NSLog(@"NUM OF IMAGES:%d",numberOfAssets);
        if (numberOfAssets > 0)
        {


            for (int i = 0; i <= numberOfAssets-1; i++)
            {

                [group enumerateAssetsAtIndexes:[NSIndexSet indexSetWithIndex:i] options:0 usingBlock:^(ALAsset *result, NSUInteger index, BOOL *stop)
                 {
                    UIImage *thumbnail = [UIImage imageWithCGImage:[result thumbnail]];
                    [assets addObject:thumbnail];
                     NSLog(@"theObject!!!! -- (%d) %@",i,thumbnail);

            //******* for each i its here twice !!   ********

                }];
            }
        }

Answer 1

由于某些原因，enumerateAssetsAtIndexes (和enumerateAssetsUsingBlock)在枚举的末尾使用result == nil和index == NSNotFound执行额外的块调用。如果将NSLog()更改为

NSLog(@"i=%d, index=%ld, result=%@", i, (unsigned long)index, result);

然后，您将获得输出

NUM OF IMAGES:2
i=0, index=0, result=ALAsset - Type:Photo, URLs:assets-library://asset/asset.PNG?id=...
i=0, index=2147483647, result=(null)
i=1, index=1, result=ALAsset - Type:Photo, URLs:assets-library://asset/asset.PNG?id=...
i=1, index=2147483647, result=(null)

因此，您必须检查result的值并忽略nil值：

for (int i = 0; i <= numberOfAssets-1; i++) {
     [group enumerateAssetsAtIndexes:[NSIndexSet indexSetWithIndex:i] options:0 usingBlock:^(ALAsset *result, NSUInteger index, BOOL *stop)
      {
          if (result != nil) {
              UIImage *thumbnail = [UIImage imageWithCGImage:[result thumbnail]];
              [assets addObject:thumbnail];
          }
      }];
}

请注意，您可以将枚举简化为

[group enumerateAssetsUsingBlock:^(ALAsset *result, NSUInteger index, BOOL *stop) {
     if (result != nil) {
         UIImage *thumbnail = [UIImage imageWithCGImage:[result thumbnail]];
         [assets addObject:thumbnail];
     }
}];