How to ScaleAspectFit in CALayer content

Question

I want to add some Layers on my imageView to make an animation. I am facing a problem that don't know how to scale fit layer.content from images. The following are my code:

CALayer *topLayer = [[CALayer alloc] init];
topLayer.masksToBounds = YES;
topLayer.backgroundColor = [[UIColor whiteColor] CGColor];  
[imageView.layer addSublayer:topLayer] ;
topLayer.contents = (id)[[UIImage imageNamed:@"test.jpg"] CGImage];

Thank you.