Mask a view in Objective-C

In ActionScript 3 we can apply a mask of to a visual object like this:

SomeVisualObject.mask = maskShapeObject; 

How can I achieve similar result in Objective-C? Assume I have two UIImageView objects, I want something like this:

imageView1.mask = imageView2;

How can I use one UIImageView to mask, or clip the shape of, another?