Sketch drop shadow effect to iOS Swift Code.

DropShadowEffect.swift

import Foundation
import UIKit

struct DropShadowStyle {
    var color: UIColor = .black
    var alpha: Float = 0.1
    let x: CGFloat
    let y: CGFloat
    let blur: CGFloat
    let spread: CGFloat
    let cornerRadius: CGFloat
    
    func applyToLayer(layer: CALayer, shouldRasterize: Bool = false) {
        layer.masksToBounds = false
        layer.cornerRadius = cornerRadius
        layer.shadowColor = color.cgColor
        layer.shadowOpacity = alpha
        layer.shadowOffset = CGSize(width: x, height: y)
        layer.shadowRadius = blur / 2.0
        if spread == 0 {
            // Code below can custom corner position.
            // self.layer.shadowPath = UIBezierPath(roundedRect: self.bounds, byRoundingCorners: .allCorners, cornerRadii: cornerRadii).cgPath
            layer.shadowPath = UIBezierPath(roundedRect: layer.bounds, cornerRadius: cornerRadius).cgPath
        } else {
            let offset = -(spread)
            let rect = layer.bounds.insetBy(dx: offset, dy: offset)
            layer.shadowPath = UIBezierPath(roundedRect: rect, cornerRadius: cornerRadius).cgPath
        }
        
        if shouldRasterize {
            layer.shouldRasterize = true
            layer.rasterizationScale = UIScreen.main.scale
        }
    }
    
    // TODO: this is not a elegant way to configure shadow
    // TestCase: weekHeaderShadowStyle.applyToLayer(layer: weekInfiniteSlideView.layer) not working.
    func attach(_ mainView: UIView, below targetView: UIView, shadowView: UIView) {
        shadowView.bounds = targetView.bounds
        shadowView.frame.origin = .zero
        self.applyToLayer(layer: shadowView.layer, shouldRasterize: true)
        if !shadowView.isDescendant(of: mainView) {
            mainView.insertSubview(shadowView, belowSubview: targetView)
        }
    }
}