PHAsset fetch returns 50% of the collection as nil

Question

When I take a PHAssetCollection and give it to PHAsset.fetchAssetsInAssetCollection:options I am given back a collection of PHAssets. For every UIImage given back to me, there is a nil value given back as well.

The following is my cellForItemAtIndexPath:indexPath method.

public override func collectionView(collectionView: UICollectionView, cellForItemAtIndexPath indexPath: NSIndexPath) -> UICollectionViewCell {
    let cell = collectionView.dequeueReusableCellWithReuseIdentifier("AssetCellIdentifier", forIndexPath: indexPath) as? AssetCollectionViewCell;

    let asset = self.assetFetchResult!.objectAtIndex(indexPath.row);
    self.imageManager.requestImageForAsset(
        asset as! PHAsset,
        targetSize: CGSize.init(width: asset.pixelWidth, height: asset.pixelHeight),
        contentMode: PHImageContentMode.AspectFill,
        options: nil) { (image, info) -> Void in
            print (image);
            if (image == nil) {
                return;
            }


            cell?.assetThumbnail.contentMode = UIViewContentMode.ScaleAspectFit;
            cell?.assetThumbnail.image = image;
    }

    return cell!;
}

If I don't put the nil check

if (image == nil) {
    return;
}

Then the UICollectionView just renders nothing. As soon as I add the nil check, the collection view starts rendering the images from the photo library's asset collection.

Why are there always the same number of nils given back to me as there are UIImage objects? Is there something in my request that I am messing up?

This is the output of my print invocation above.

Optional(<UIImage: 0x12f8263d0>, {40, 30})
Optional(<UIImage: 0x12e5265a0>, {40, 30})
Optional(<UIImage: 0x12e664c90>, {40, 30})
Optional(<UIImage: 0x12e74c860>, {40, 30})
Optional(<UIImage: 0x12e58c580>, {40, 30})
Optional(<UIImage: 0x12e60b6f0>, {40, 30})
Optional(<UIImage: 0x12e7657d0>, {40, 30})
Optional(<UIImage: 0x12e6655e0>, {40, 30})
Optional(<UIImage: 0x12e743ca0>, {40, 30})
Optional(<UIImage: 0x12e5fb6e0>, {40, 30})
Optional(<UIImage: 0x12e5fb8e0>, {40, 30})
Optional(<UIImage: 0x12f85f3b0>, {40, 30})
nil
nil
nil
nil
nil
nil
nil
nil
nil
nil
nil
nil

Update:

It seems that this happens across every PHAssetCollection that I am given back.

Update 2:

This is the bits from the WWDC example I am trying to replicate. The one thing I did notice that it does, that I am not, is tagging the cells in order to prevent overriding it if it's being reused.

- (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath
{
    AAPLGridViewCell *cell = [collectionView dequeueReusableCellWithReuseIdentifier:CellReuseIdentifier forIndexPath:indexPath];

    // Increment the cell's tag
    NSInteger currentTag = cell.tag + 1;
    cell.tag = currentTag;

    PHAsset *asset = self.assetsFetchResults[indexPath.item];
    [self.imageManager requestImageForAsset:asset
                                 targetSize:AssetGridThumbnailSize
                                contentMode:PHImageContentModeAspectFill
                                    options:nil
                              resultHandler:^(UIImage *result, NSDictionary *info) {

        // Only update the thumbnail if the cell tag hasn't changed. Otherwise, the cell has been re-used.
        if (cell.tag == currentTag) {
            cell.thumbnailImage = result;
        }

    }];

    return cell;
}

Answer 1

You cannot call self.imageManager.requestImageForAsset in the way you are calling it in the middle of cellForItemAtIndexPath. This method, cellForItemAtIndexPath, returns immediately. Meanwhile, requestImageForAsset is asynchronous - meaning that your completion handler is called later. By the time it is called, we've moved on to a different cell; and remember, cells are reused. So your entire approach is nonsensical.

Take a look at Apple's example code to see how to populate a collection view's cells using the caching version of the image manager.

Also, I notice that you are ignoring the PHImageRequestOptions. It's very important. You should be thinking carefully about what you want here. If you set no value, this means that you run the risk that your completion handler will be called more than once, which again makes no sense when we are no longer in the process of populating this cell.