title: layoutSubviews getting called repeatedly when the app enters background state
body: <p>I want labels to have a font size that is proportional to the size of the screen. I've subclassed the <code>UILabel</code> class to accomplish this:</p>
  
  <pre><code>@IBDesignable class MyCustomLabel: UILabel {
      override func layoutSubviews() {
          super.layoutSubviews()
          self.font = UIFont(name: "myFontName", size: (self.font?.pointSize)!)
          self.adjustsFontSizeToFitWidth = true
      }
  }
  </code></pre>
  
  <p>The labels (which have a proportional width to superview constraint attached to them) resize correctly when the app is first launched, but when it enters background state <code>layoutSubviews</code> gets repeatedly called. The app no longer responses to users' input and keeps assigning a font size to the font. </p>
  
  <p>Why is this happening?</p>