title: How to trigger auto layout offscreen without adding the view to view hierarchy
body: <p>My scenario is that I need to generate PNG Image data from an UIView whose layout I will setup myself. I create an UIView viewA in memory, which I will use to create PNG data. This happens in a framework I'm working on, meaning that my viewA can only exist in memory and will never get attached to view hierarchy/go on screen.</p>
  <p>However, based on my current testing, autolayout can only be effective when the view is attached to view hierarchy so that the system can trigger layout pass and related events such as layoutSubview and updateConstraint. Without being on screen, calls like setNeedsLayout and layoutSubview are ineffective because no layout pass will be run at all.</p>
  <p>I can still manually setup frame of ViewA, but just wondering is there a way to trigger autolayout even when the view stays offscreen.</p>