create inner shadow - forget original source

InnerShadow.swift

// use after view has been layout

extension UIView
{
    // different inner shadow styles
    public enum innerShadowSide
    {
        case all, left, right, top, bottom, topAndLeft, topAndRight, bottomAndLeft, bottomAndRight, exceptLeft, exceptRight, exceptTop, exceptBottom
    }
    
    // define function to add inner shadow
    public func addInnerShadow(onSide: innerShadowSide, shadowColor: UIColor, shadowSize: CGFloat, cornerRadius: CGFloat = 0.0, shadowOpacity: Float)
    {
        // define and set a shaow layer
        let shadowLayer = CAShapeLayer()
        shadowLayer.frame = bounds
        shadowLayer.shadowColor = shadowColor.cgColor
        shadowLayer.shadowOffset = CGSize(width: 0.0, height: 0.0)
        shadowLayer.shadowOpacity = shadowOpacity
        shadowLayer.shadowRadius = shadowSize
        shadowLayer.fillRule = .evenOdd
        
        // define shadow path
        let shadowPath = CGMutablePath()
        
        // define outer rectangle to restrict drawing area
        let insetRect = bounds.insetBy(dx: -shadowSize * 2.0, dy: -shadowSize * 2.0)
        
        // define inner rectangle for mask
        let innerFrame: CGRect = { () -> CGRect in
            switch onSide
            {
                case .all:
                    return CGRect(x: 0.0, y: 0.0, width: frame.size.width, height: frame.size.height)
                case .left:
                    return CGRect(x: 0.0, y: -shadowSize * 2.0, width: frame.size.width + shadowSize * 2.0, height: frame.size.height + shadowSize * 4.0)
                case .right:
                    return CGRect(x: -shadowSize * 2.0, y: -shadowSize * 2.0, width: frame.size.width + shadowSize * 2.0, height: frame.size.height + shadowSize * 4.0)
                case .top:
                    return CGRect(x: -shadowSize * 2.0, y: 0.0, width: frame.size.width + shadowSize * 4.0, height: frame.size.height + shadowSize * 2.0)
                case.bottom:
                    return CGRect(x: -shadowSize * 2.0, y: -shadowSize * 2.0, width: frame.size.width + shadowSize * 4.0, height: frame.size.height + shadowSize * 2.0)
                case .topAndLeft:
                    return CGRect(x: 0.0, y: 0.0, width: frame.size.width + shadowSize * 2.0, height: frame.size.height + shadowSize * 2.0)
                case .topAndRight:
                    return CGRect(x: -shadowSize * 2.0, y: 0.0, width: frame.size.width + shadowSize * 2.0, height: frame.size.height + shadowSize * 2.0)
                case .bottomAndLeft:
                    return CGRect(x: 0.0, y: -shadowSize * 2.0, width: frame.size.width + shadowSize * 2.0, height: frame.size.height + shadowSize * 2.0)
                case .bottomAndRight:
                    return CGRect(x: -shadowSize * 2.0, y: -shadowSize * 2.0, width: frame.size.width + shadowSize * 2.0, height: frame.size.height + shadowSize * 2.0)
                case .exceptLeft:
                    return CGRect(x: -shadowSize * 2.0, y: 0.0, width: frame.size.width + shadowSize * 2.0, height: frame.size.height)
                case .exceptRight:
                    return CGRect(x: 0.0, y: 0.0, width: frame.size.width + shadowSize * 2.0, height: frame.size.height)
                case .exceptTop:
                    return CGRect(x: 0.0, y: -shadowSize * 2.0, width: frame.size.width, height: frame.size.height + shadowSize * 2.0)
                case .exceptBottom:
                    return CGRect(x: 0.0, y: 0.0, width: frame.size.width, height: frame.size.height + shadowSize * 2.0)
            }
        }()
        
        // add outer and inner rectangle to shadow path
        shadowPath.addRect(insetRect)
        shadowPath.addRect(innerFrame)
        
        // set shadow path as show layer's
        shadowLayer.path = shadowPath
        
        // add shadow layer as a sublayer
        layer.addSublayer(shadowLayer)
        
        // hide outside drawing area
        clipsToBounds = true
    }
}