title: How to use sizeThatFits in Swift?
body: <p>I have got a text view and a view like this</p>
  
  <pre><code>let lb = UITextView()
  let view = UIView()
  background_img_view.addSubview(about_txt)
  </code></pre>
  
  <p><code>lb</code> doesn't have fixed height,it can be 30 or 300px,how can i use <code>sizeThatFits</code> to make <code>background_img_view</code>'s height depend on <code>lb</code>'s?</p>