shodow-with-layer.swift

// use in a custom view

override func layoutSubviews() {
    super.layoutSubviews()
    setupShadow()
}

private var shadowLayer: CAShapeLayer! // layer for shadow back
private func setupShadow() {
    guard columnStackView.bounds.size != .zero else { return }

    if shadowLayer == nil {
        shadowLayer = CAShapeLayer()
        shadowLayer.path = UIBezierPath(roundedRect: <target view>.frame, cornerRadius: <shadow corner radius>).cgPath
        shadowLayer.fillColor = <shadow color>.cgColor

        shadowLayer.shadowColor = <shadow color>.cgColor
        shadowLayer.shadowPath = shadowLayer.path
        
        // width for x offset according to <target view>'s frame
        // height for y offset according to <target view>'s frame
        shadowLayer.shadowOffset = CGSize(width: 0, height: 15)
        
        shadowLayer.shadowOpacity = 1
        shadowLayer.shadowRadius = 8
    }

    layer.insertSublayer(shadowLayer, at: 0)
}