title: rounded imageView 's border is not smooth after setting image property
body: <pre><code>class PPAvatarCollectionCell: UICollectionViewCell {
      var imageView:UIImageView!
  
  override init(frame: CGRect) {
      super.init(frame: frame)
  
      imageView = UIImageView(frame: CGRect(origin: CGPointMake(0, 0), size: CGSizeMake(frame.size.width, frame.size.height)))
  
      self.addSubview(imageView)
  
      imageView.contentMode = .ScaleAspectFill
      imageView.backgroundColor = UIColor.whiteColor()
  
      imageView.layer.borderColor = UIColor.greenColor().CGColor
      imageView.layer.borderWidth = 10
      imageView.image = UIImage(named: "demo")
      imageView.layer.cornerRadius = frame.size.width*0.5
      imageView.clipsToBounds = true
  
  }
  </code></pre>
  
  <p>the border is smooth and great with above code 
  but after  add    <code>imageView.image = UIImage(named: "demo")</code></p>
  
  <p>the imageView's border is no longer smooth.
  Why did this happen ?</p>
  
  <p><a href="https://i.stack.imgur.com/MRN2E.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/MRN2E.png" alt="smooth image"></a></p>
  
  <p><a href="https://i.stack.imgur.com/7U3OI.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/7U3OI.png" alt="not smooth image"></a></p>
  
  <hr>
  
  <p>UPDATE:</p>
  
  <p>Seems something wrong with layer.radius ,the border is smooth even with image property set on imageView after remove    <code>imageView.layer.cornerRadius = frame.size.width*0.5</code></p>
  
  <p><a href="https://i.stack.imgur.com/Vfn6C.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/Vfn6C.png" alt="enter image description here"></a></p>
  
  <hr>
  
  <p>UPDATE 2:
  turn out to be something wrong with UICollectionViewCell , imageView is a part of UICollectionViewCell</p>
  
  <hr>