IOS: change UICollectionView Cell size according to the image size

Question

I want to generate an image gallery, without changing the aspect ratio of the images. Images are in different sizes. I'm using UICollectionView and positioning an UIImageView inside the cell. I use below code to get the image size. but it always return size 0.

- (CGSize)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout*)collectionViewLayout sizeForItemAtIndexPath:(NSIndexPath *)indexPath{

NSString* url = [selectedImages objectAtIndex:indexPath.row];

UIImageView *thumb;
ALAssetsLibraryAccessFailureBlock failureBlock  = ^(NSError *myError) {
    NSLog(@"failed loading asset");
};
ALAssetsLibraryAssetForURLResultBlock resultBlock = ^(ALAsset *myAsset) {
    [thumb setImage:[UIImage imageWithCGImage:[myAsset aspectRatioThumbnail]]];
};
ALAssetsLibrary *lib = [[ALAssetsLibrary alloc] init];

[lib assetForURL:[NSURL URLWithString:url] resultBlock:resultBlock failureBlock:failureBlock];
NSLog(@"%f,%f",thumb.image.size.height,thumb.image.size.width);
return thumb.image.size;   
}

Answer 1

You always got image size return size 0 because your UIImageview () get image in ^{} block and it is not executed on mainthread , so, you have to calculate image size before sizeForItemAtIndexPath this method or

you can try like this, but i guess it is not proper way,

ALAssetsLibraryAssetForURLResultBlock resultBlock = ^(ALAsset *myAsset) {
    [thumb setImage:[UIImage imageWithCGImage:[myAsset aspectRatioThumbnail]]];
return thumb.image.size;   

};