ios UICollectionView slow scrolling and memory warning - ios

Question

I am currently working on a project that involves a UICollectionView populated by AVAssets imported from a UIImagepickerController, after 10 or so item are in the collection, Scrolling becomes laggy and slow, and occasionally I receive memory warnings. I believe the problem to be in the thumbnail generation which happens in realtime, here is the code i use:
- (void) setAsset:(AVAsset *)asset
{
_asset = asset;
AVAssetImageGenerator *generate = [[AVAssetImageGenerator alloc] initWithAsset:_asset];
NSError *err = NULL;
CMTime time = CMTimeMake(1, 60);
generate.appliesPreferredTrackTransform = YES;
CGImageRef imgRef = [generate copyCGImageAtTime:time actualTime:NULL error:&err];
self.VideoImageView.image = [UIImage imageWithCGImage:(imgRef)];
}
Is there another less "expensive way" to achieve this without delay?
Any help on the matter would be greatly appreciated.

Answer 1

You can do the real time thumbnail generation on a background thread, and then jump back to the main thread when the operation is done to set the actual thumbnail.
Right now you're doing everything on the main thread, which blocks the UI, and makes the scrolling jerky.

Answer 2

You can set a placeholder image in your cell and generate thumbnail in background queue.
Then set it to your image view.
AsynImageView might be of some use to you.

Answer 3

Open another thread to deal with the image thing. When your image done switch back to main thread to update a cell. You can do this with GCD

Answer 4

Here are the code that asynchronous ganerate thumbnail image for video.
-(void)generateImage
{
NSURL *url = [NSURL fileURLWithPath:_videoPath];
AVURLAsset *asset=[[AVURLAsset alloc] initWithURL:url options:nil];
AVAssetImageGenerator *generator = [[AVAssetImageGenerator alloc] initWithAsset:asset];
generator.appliesPreferredTrackTransform=TRUE;
CMTime thumbTime = CMTimeMakeWithSeconds(30,30);
AVAssetImageGeneratorCompletionHandler handler = ^(CMTime requestedTime, CGImageRef im, CMTime actualTime, AVAssetImageGeneratorResult result, NSError *error){
if (result != AVAssetImageGeneratorSucceeded) {
NSLog(#"couldn't generate thumbnail, error:%#", error);
}
// TODO Do something with the image
};
CGSize maxSize = CGSizeMake(128, 128);
generator.maximumSize = maxSize;
[generator generateCGImagesAsynchronouslyForTimes:[NSArray arrayWithObject:[NSValue valueWithCMTime:thumbTime]] completionHandler:handler];
}
Hope this help you.