assetsLibrary a simple bug? - objective-c

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];
}
}];