assetsLibrary a simple bug?

Question

I am really lost . why i get NSLog twice for each UIImage ?

 //------ 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

For some reason, enumerateAssetsAtIndexes (and enumerateAssetsUsingBlock) do an additional invocation of the block with result == nil and index == NSNotFound at the end of the enumeration. This becomes obvious if you change the NSLog() to

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

Then you will get the output

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)

Therefore you have to check the value of result and ignore a nil value:

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];
          }
      }];
}

Note that you can simplify the enumeration to

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