How to mask and add shadow to a UIView

Thanks @WilsonXJ I changed mask to addSubLayer.

This is the answer that worked for me:

    let p = UIBezierPath()
    p.moveToPoint(CGPointMake(20, 20))
    p.addLineToPoint(CGPointMake(100, 20))
    p.addLineToPoint(CGPointMake(100, 50))
    p.addLineToPoint(CGPointMake(110, 55))
    p.addLineToPoint(CGPointMake(100, 60))
    p.addLineToPoint(CGPointMake(100, 100))
    p.addLineToPoint(CGPointMake(20, 100))
    p.closePath()

    let s = CAShapeLayer()
    s.fillColor = UIColor.whiteColor().CGColor
    s.frame = layer.bounds
    s.path = p.CGPath

    layer.backgroundColor = UIColor.clearColor().CGColor
    layer.addSublayer(s)

    layer.masksToBounds = true
    layer.shadowColor = UIColor.yellowColor().CGColor
    layer.shadowOffset = CGSizeZero
    layer.shadowOpacity = 0.9
    layer.shadowPath = p.CGPath
    layer.shadowRadius = 10

I don't think that current answer is the right one, because there is no layer.mask usage anymore.

In case when you need to use layer.mask and to drop shadow of masked layer - the obvious solution is to add another layer below masked layer that will have same shape as layer.mask and drop it's shadow

example:

let view = UIView(frame: CGRect(origin: .zero, size: CGSize(width: 500, height: 500)))
view.backgroundColor = .white
PlaygroundPage.current.liveView = view

let path: CGPath = ...

let maskedView = UIView(frame: path.boundingBox)
maskedView.center = view.center
maskedView.backgroundColor = .green
view.addSubview(maskedView)

let maskLayer = CAShapeLayer()
maskLayer.frame = maskedView.bounds
maskLayer.path = path
maskedView.layer.mask = maskLayer

let shadowLayer = CAShapeLayer()
shadowLayer.path = path
shadowLayer.frame = maskedView.frame

shadowLayer.shadowOpacity = 0.4
shadowLayer.shadowRadius = 2
shadowLayer.shadowColor = UIColor.black.cgColor
shadowLayer.shadowOffset = CGSize(width: 4, height: 4)

maskedView.superview!.layer.insertSublayer(shadowLayer, below: maskedView.layer)