prior swift 3 i was adding shadow in my UIView like this :
//toolbar is an UIToolbar (UIView)
toolbar.layer.masksToBounds = false
toolbar.layer.shadowOffset
Please try this, it's working for me.
extension UIView {
func dropShadow() {
layer.shadowColor = UIColor.black.cgColor
layer.shadowOffset = CGSize(width: 2, height: 3)
layer.masksToBounds = false
layer.shadowOpacity = 0.3
layer.shadowRadius = 3
//layer.shadowPath = UIBezierPath(rect: bounds).cgPath
layer.rasterizationScale = UIScreen.main.scale
layer.shouldRasterize = true
}}