title: How to check if a view is inViewHierarchy or not?
body: <p>For example, if a view of a viewController is in viewHierarchy and I just finished downloading stuff, I want to update stuff quickly. If self.view is not in viewHierarchy, I want to postpone updating.</p>
  
  <p>I suppose, I can just add a boolean variable to indicate that and put it on viewWillAppear and viewWillDisappear.</p>
  
  <p>I can also scan windows and see if UIView is in the viewHierarchy or not.</p>
  
  <p>Is there a more straightforward way?</p>