ALAssetsGroup enumerateAssetsUsingBlock: not returning assets

Question

I have implemented a custom gallery with two UIViewController subclasses: AlbumController and PhotosController . The first one shows all gallery albums, and the second one, all photos from the selected album.

Everything works fine except when I am downloading images from a remote repository (Flickr at the moment) to the Camera Roll album. In that case, whenever I go to the AlbumController I see all the albums, and the album cover of the Camera Roll is refreshed with the last downloaded image, but when I go to the PhotosController for the Camera Roll album, I don't see anything. If I enter the Camera Roll album after all images have been downloaded, then I see them all.

PhotosController.h:

ALAssetsGroup *album;

PhotosController.m:

-(void) viewDidAppear:(BOOL)animated{
    if( albumHasChanged ){
        [self performSelectorInBackground:@selector(prepareAlbumPhotos) 
                               withObject:nil];
        [self performSelector:@selector(reloadPhotosTableView) 
                   withObject:nil 
                   afterDelay:.5];            
        albumHasChanged = NO;
    }
}

#pragma mark - Private methods

-(void)prepareAlbumPhotos {
    [albumAssets removeAllObjects];

    @autoreleasepool {
        [self.album enumerateAssetsUsingBlock:^(ALAsset *result, NSUInteger index, BOOL *stop) 
         {
             // No assets are returned while images are being downloaded!!

             // ... albumAssets gets filled
         }];

        [self reloadPhotosTableView];
    }
}

-(void) reloadPhotosTableView{
    [photosTableView reloadData];
}

Inside another class I download the images and add them to the gallery like so:

UIImage *img = [UIImage imageWithData:data];
UIImageWriteToSavedPhotosAlbum(img, nil, nil, nil);    

Can anybody explain why enumerateAssetsUsingBlock: does not return any assets while writing images to its ALAssetsGroup ?

Is there a workaround to read assets from the Camera Roll album in this case?

Answer 1

The problem was that every time you save a new image in the phone Gallery, the reference to the Camera Roll ALAssetsGroup changes and is no longer valid (not nil though).

In my case I was downloading and writing many images so each time I tried to view them in my gallery, the reference to the album had changed, and therefore, enumerateAssetsUsingBlock: wasn't returning anything.

To avoid this problem, first I download all images to a folder, and then, I copy the images to the gallery blocking the application with an UIAlertView message, with no buttons, notifying the process. With this implementation I solved the problem.

If you want to save some images at the same time, you could find another problem. Have a look at this question and answer as well: https://stackoverflow.com/a/8177014