GradientView.swift

import UIKit

//@IBDesignable // allows you to render inside the story board
class GradientView: UIView { // it is inhertating from UIView since that is what we are going to put it in

    // these IBInspectablea allow you to assign different colors in the right hand panel
    @IBInspectable var topColor: UIColor = UIColor.blue{
        didSet{
            self.setNeedsLayout()
        }
    }
    
    @IBInspectable var bottomColor: UIColor = UIColor.green {
        didSet{
            self.setNeedsLayout()
        }
    }
    
    override func layoutSubviews() {
        let gradientLayer = CAGradientLayer()
        // taking the colors from what you set above and giving them to the gradients layer
        // you could have also hardcored these values in here as well
        gradientLayer.colors = [topColor.cgColor, bottomColor.cgColor]
        // specifying where the first and second color is going to be coming from
        gradientLayer.startPoint = CGPoint(x:0,y:0)
        gradientLayer.endPoint = CGPoint(x:1,y:1)
        // giving it the bounds
        gradientLayer.frame = self.bounds
        // setting the sub layer
        self.layer.insertSublayer(gradientLayer, at: 0)
    }

}