title: How to replace one UI element by another with custom animation?
body: <p>I want to implement nested commentaries(like stickers) in my own document viewer.
  At first, it should be <code>UITextView</code>, but when <code>resignFirstResponder</code> executes, it should become just a small button. 
  The main question is: how to animate this?
  I've read Quartz 2d programming guide from Apple, but it didn't gave me any ideas.
  I don't asking for exact or ready solution: keywords, links to articles or documentation are enough. 
  Thanks for future responses.</p>