title: UIView.animate() does not animate the thing
body: <p>I'm trying to animate sprite resizing. Size changes, but without any animation.</p>
  
  <p>Here's the problematic code:</p>
  
  <pre><code>func layout_scene() {
      backgroundColor = UIColor(red: 14/255, green: 145/255, blue: 137/255, alpha: 1.0);
  
      var char_trail: SKSpriteNode!
      char_trail = SKSpriteNode(imageNamed: "trail")
      char_trail.size = CGSize(width: frame.width / 8, height: frame.width / 12)
      char_trail.position = CGPoint(x: frame.minX + frame.width / 10,
                                    y: frame.minY + frame.height / 4.4)
      char_trail.zPosition = -0.1
  
      timer = Timer.scheduledTimer(timeInterval: 1, target: self, selector: #selector(fire), userInfo: nil, repeats: true)
  }
  
  
  @objc func    fire()
  {
      UIView.animate(withDuration: 1, animations: {
          self.char_trail.size.width += 10
      })
  }
  </code></pre>
  
  <p>I expect char_trail size to change smoothly, but it just resizes in a moment. Tell me what I'm doing wrong, please</p>