Set gradient color on custom UIView boarder

I have a UIView that contains a 2 UILabels with a button inside and I would like to add a gradient color to its boarder. I have managed to add it and button has ended up moving outside the custom UIView with the custom UIView also shrinking all the way outside on smaller devices. How can I fix the UIView to remain the same size when I add a gradient color

Here is the initial UIView with two UILabels and a button inside with a normal border colour before

And here how it looks after applying a gradient color to it

Here is my code on how I apply the gradient.

@IBOutlet weak var customView: UIView!

let gradient = CAGradientLayer()
        gradient.frame.size = self.customView.frame.size
        gradient.colors = [UIColor.green.cgColor, UIColor.yellow.cgColor, UIColor.red.cgColor]
        gradient.startPoint = CGPoint(x: 0.1, y: 0.78)
        gradient.endPoint = CGPoint(x: 1.0, y: 0.78)
        let shapeLayer = CAShapeLayer()
        shapeLayer.path = UIBezierPath(rect:   self.customView.bounds).cgPath
        shapeLayer.fillColor = UIColor.clear.cgColor
        shapeLayer.strokeColor = UIColor.black.cgColor
        shapeLayer.lineWidth = 4
        gradient.mask = shapeLayer
        self.customView.layer.addSublayer(gradient)

Layers do not resize when the view resizes, so you want to create a custom view and override layoutSubviews().

Here's an example:

@IBDesignable
class GradBorderView: UIView {

    var gradient = CAGradientLayer()

    override init(frame: CGRect) {
        super.init(frame: frame)
        commonInit()
    }
    
    required init?(coder aDecoder: NSCoder) {
        super.init(coder: aDecoder)
        commonInit()
    }
    
    func commonInit() -> Void {
        layer.addSublayer(gradient)
    }
    
    override func layoutSubviews() {
        
        gradient.frame = bounds
        gradient.colors = [UIColor.green.cgColor, UIColor.yellow.cgColor, UIColor.red.cgColor]
        gradient.startPoint = CGPoint(x: 0.1, y: 0.78)
        gradient.endPoint = CGPoint(x: 1.0, y: 0.78)
        let shapeLayer = CAShapeLayer()
        shapeLayer.path = UIBezierPath(rect: bounds).cgPath
        shapeLayer.fillColor = UIColor.clear.cgColor
        shapeLayer.strokeColor = UIColor.black.cgColor
        shapeLayer.lineWidth = 4
        gradient.mask = shapeLayer
        
    }
    
}

Now, when your view changes size based on constraints and auto-layout, your gradient border will "auto-resize" correctly.

Also, by using @IBDesignable, you can see the results when laying out your views in Storyboard / Interface Builder.

Here's how it looks with the Grad Border View width set to 240:

and with the Grad Border View width set to 320:

Edit

If we want to use rounded corners, we can set the shape layer path to a rounded rect bezier path, and then also set the corner radius of the view's layer.

For example:

override func layoutSubviews() {

    let cRadius: CGFloat = 8.0
    let bWidth: CGFloat = 4.0
    
    // layer border is centered on layer edge
    let half: CGFloat = bWidth * 0.5
    // make gradient frame size of view + half the border width
    gradient.frame = bounds.insetBy(dx: -half, dy: -half)
    gradient.colors = [UIColor.green.cgColor, UIColor.yellow.cgColor, UIColor.red.cgColor]
    gradient.startPoint = CGPoint(x: 0.1, y: 0.78)
    gradient.endPoint = CGPoint(x: 1.0, y: 0.78)
    
    let shapeLayer = CAShapeLayer()
    // make shapeLayer path the size of view OFFSET by half the border width
    //  with rounded corners
    shapeLayer.path = UIBezierPath(roundedRect: bounds.offsetBy(dx: half, dy: half), cornerRadius: cRadius).cgPath
    shapeLayer.fillColor = UIColor.clear.cgColor
    shapeLayer.strokeColor = UIColor.black.cgColor
    shapeLayer.lineWidth = bWidth
    gradient.mask = shapeLayer
    
    // same corner radius as shapeLayer path
    layer.cornerRadius = cRadius
}