Add a gradient on UIImageView

You can use extension for swift 3, swift 4 and swift 5

Create a new file for your extension of UIImageView like UIImageView_extension.swift and set into this code:

UIImageView is extends UIView so if you change UIImageView to UIView then it become more dynamic i.e can used by all components who extents UIView. So I used UIView instead of UIImageView.

import UIKit

extension UIView{
   // For insert layer in Foreground
   func addBlackGradientLayerInForeground(frame: CGRect, colors:[UIColor]){
    let gradient = CAGradientLayer()
    gradient.frame = frame
    gradient.colors = colors.map{$0.cgColor}
    self.layer.addSublayer(gradient)
   }
   // For insert layer in background
   func addBlackGradientLayerInBackground(frame: CGRect, colors:[UIColor]){
    let gradient = CAGradientLayer()
    gradient.frame = frame
    gradient.colors = colors.map{$0.cgColor}
    self.layer.insertSublayer(gradient, at: 0)
   }
}

and in your ViewController.swift you can use it:

class myViewController: UIViewController{
    @IBOutlet weak var imageView: UIImageView!

    override func viewDidLoad() {
    super.viewDidLoad()

        imageView.addBlackGradientLayerInBackground(frame: view.bounds, colors:[.clear, .black])
        //Alternative
        //imageView.addBlackGradientLayerInBackground(frame: imageView.frame, colors: [.clear, .black])
    }
}

This function needs a frame, so you only need a frame from view or yourself imageView. Always think like generic function, then you can change the gradient colors without troubles in the future in other views.

I would suggest putting a UIView with the gradient on top of the UIImageView:

@IBOutlet weak var shanghaiImage: UIImageView!

let view = UIView(frame: profileImageView.frame)

let gradient = CAGradientLayer()

gradient.frame = view.frame

gradient.colors = [UIColor.clear.cgColor, UIColor.black.cgColor]

gradient.locations = [0.0, 1.0]

view.layer.insertSublayer(gradient, at: 0)

shanghaiImage.addSubview(view)

shanghaiImage.bringSubview(toFront: view)

Objective-C:

UIView *view = [[UIView alloc] initWithFrame: profileImageView.frame];

CAGradientLayer *gradient = [[CAGradientLayer alloc] init];

gradient.frame = view.frame;

gradient.colors = @[ (id)[[UIColor clearColor] CGColor], (id)[[UIColor blackColor] CGColor] ];

gradient.locations = @[@0.0, @1.0];

[view.layer insertSublayer: gradient atIndex: 0];

[shanghaiImage addSubview: view];

[shanghaiImage bringSubviewToFront: view];

In Swift 5.1 I have create a custom class that inherits from UIImageView and supports multiple gradient configurations.

import UIKit
import SnapKit

class GradientImageView: UIImageView {

    //MARK: - View model
    enum GradientDirection {
        case upDown
        case downUp
        case leftRight
        case rightLeft
        case topLeftBottomRight
        case topRightBottomLeft
        case bottomLeftTopRight
        case bottomRightTopLeft
    }
    
    //MARK: - Properties
    var colors: [UIColor] = [] {
        didSet {
            updateGradient()
        }
    }
    private var cgColors: [CGColor] {
        return colors.map({ $0.cgColor })
    }
    var gradientDirection: GradientDirection = .downUp {
        didSet {
            updateGradient()
        }
    }
    private lazy var gradientLayer: CAGradientLayer = {
        let layer = CAGradientLayer()
    
        layer.shouldRasterize = true
        
        return layer
    }()
    
    
    //MARK: UI
    private lazy var overlayView: UIView = { return UIView() }()
    
    
    //MARK: - Constructor
    init(colors: [UIColor], gradientDirection: GradientDirection) {
        super.init(frame: .zero)
        
        self.colors = colors
        self.gradientDirection = gradientDirection
    }
    
    required init?(coder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }
    
}


//MARK: - Lifecycle methods methods
extension GradientImageView {
    
    override func didMoveToSuperview() {
        super.didMoveToSuperview()
        
        if superview != nil {
            setupUI()
            updateGradient()
        }
        
    }
    
    override func layoutSubviews() {
        super.layoutSubviews()
        
        gradientLayer.frame = overlayView.frame
    
    }
    
}


//MARK: - Private methods
private extension GradientImageView {
    
    func setupUI() {
        addSubview(overlayView)
        //With Snapkit
        overlayView.snp.makeConstraints { (maker) in
            maker.edges.equalToSuperview()
        }

        //Without Snapkit
        //overlayView.translatesAutoresizingMaskIntoConstraints = false
        //overlayView.topAnchor.constraint(equalTo: overlayView.superview!.topAnchor).isActive = true
        //overlayView.leftAnchor.constraint(equalTo: overlayView.superview!.leftAnchor).isActive = true
        //overlayView.bottomAnchor.constraint(equalTo: overlayView.superview!.bottomAnchor).isActive = true
        //overlayView.rightAnchor.constraint(equalTo: overlayView.superview!.rightAnchor).isActive = true

        overlayView.layer.addSublayer(gradientLayer)
    }
    
    func updateGradient() {
        
        gradientLayer.colors = cgColors
        
        switch gradientDirection {
        case .upDown:
            gradientLayer.startPoint = CGPoint(x: 0.5, y: 0)
            gradientLayer.endPoint = CGPoint(x: 0.5, y: 1)
        case .downUp:
            gradientLayer.startPoint = CGPoint(x: 0.5, y: 1)
            gradientLayer.endPoint = CGPoint(x: 0.5, y: 0)
        case .leftRight:
            gradientLayer.startPoint = CGPoint(x: 0, y: 0.5)
            gradientLayer.endPoint = CGPoint(x: 1, y: 0.5)
        case .rightLeft:
            gradientLayer.startPoint = CGPoint(x: 1, y: 0.5)
            gradientLayer.endPoint = CGPoint(x: 0, y: 0.5)
        case .topLeftBottomRight:
            gradientLayer.startPoint = CGPoint(x: 0, y: 0)
            gradientLayer.endPoint = CGPoint(x: 1, y: 1)
        case .topRightBottomLeft:
            gradientLayer.startPoint = CGPoint(x: 1, y: 0)
            gradientLayer.endPoint = CGPoint(x: 0, y: 1)
        case .bottomLeftTopRight:
            gradientLayer.startPoint = CGPoint(x: 0, y: 1)
            gradientLayer.endPoint = CGPoint(x: 1, y: 0)
        case .bottomRightTopLeft:
            gradientLayer.startPoint = CGPoint(x: 1, y: 1)
            gradientLayer.endPoint = CGPoint(x: 0, y: 0)
        }
        
    }
    
}

USAGE

let gradientImageView = GradientImageView(colors: [YOUR COLORS], gradientDirection: .upDown)
gradientImageView.image = //YOUR Image