How to set corner radius of imageView?

Layer draws out of clip region, you need to set it to mask to bounds:

self.mainImageView.layer.masksToBounds = true

From the docs:

By default, the corner radius does not apply to the image in the layer’s contents property; it applies only to the background color and border of the layer. However, setting the masksToBounds property to true causes the content to be clipped to the rounded corners

There is one tiny difference in Swift 3.0 and Xcode8

Whenever you want to apply corner radius to UIView, make sure you call yourUIView.layoutIfNeeded() before calling cornerRadius.

Otherwise, it will return the default value for UIView's height and width (1000.0) which will probably make your View disappear.

Always make sure that all effects that changes the size of UIView (Interface builder constraints etc) are applied before setting any layer properties.

Example of UIView class implementation

class BadgeView: UIView {

  override func awakeFromNib() {

    self.layoutIfNeeded()
    layer.cornerRadius = self.frame.height / 2.0
    layer.masksToBounds = true

   }
 }

You can define border radius of any view providing an "User defined Runtime Attributes", providing key path "layer.cornerRadius" of type string and then the value of radius you need ;) See attached images below:

try this

self.mainImageView.layer.cornerRadius = CGRectGetWidth(self.mainImageView.frame)/4.0
self.mainImageView.clipsToBounds = true