How to get multiply blend mode on a plain UIView (not UIImage)

Question

I have an image in my iPad app and I basically want to place a color filter on top of it. For this I have a colored UIView that is masked to a certain shape that I put over the

Answer 1

Take a look at the docs for CALayer's compositingFilter: https://developer.apple.com/documentation/quartzcore/calayer/1410748-compositingfilter

On your color overlay view you can set view.layer.compositingFilter to a CICategoryCompositeOperation to achieve blending with the content behind it. Here is some sample playground code with a multiply blend mode applied to test it out (replace the UIImage(named: "") with your own image for testing)

import UIKit
import PlaygroundSupport

class MyViewController : UIViewController {
    override func loadView() {

        let mainView = UIView()
        self.view = mainView

        let image = UIImageView()
        image.translatesAutoresizingMaskIntoConstraints = false;
        image.image = UIImage(named: "maxresdefault.jpg")
        mainView.addSubview(image)

        let overlay = UIView()
        overlay.translatesAutoresizingMaskIntoConstraints = false;
        overlay.backgroundColor = .red
        overlay.layer.compositingFilter = "multiplyBlendMode"
        mainView.addSubview(overlay)

        mainView.addConstraints(NSLayoutConstraint.constraints(withVisualFormat: "H:|-0-[subview]-0-|", options: .directionLeadingToTrailing, metrics: nil, views: ["subview": view]))
        mainView.addConstraints(NSLayoutConstraint.constraints(withVisualFormat: "V:|-0-[subview]-0-|", options: .directionLeadingToTrailing, metrics: nil, views: ["subview": view]))

        mainView.addConstraints(NSLayoutConstraint.constraints(withVisualFormat: "H:|-0-[subview]-0-|", options: .directionLeadingToTrailing, metrics: nil, views: ["subview": overlay]))
        mainView.addConstraints(NSLayoutConstraint.constraints(withVisualFormat: "V:|-0-[subview]-0-|", options: .directionLeadingToTrailing, metrics: nil, views: ["subview": overlay]))
    }
}

PlaygroundPage.current.liveView = MyViewController()