Cocoa IBDesignable gradient view

GradientView.swift

//  GradientView.swift
//
//  Created by Robert Ryan on 12/29/16.
//  Copyright © 2016 Robert Ryan. All rights reserved.
//

import Cocoa

@IBDesignable
public class GradientView: NSView {
    
    @IBInspectable public var margin:     CGFloat = 5  { didSet { needsLayout = true } }
    @IBInspectable public var startColor: NSColor = #colorLiteral(red: 0, green: 0.4772562545, blue: 1, alpha: 1) { didSet { updateColors() } }
    @IBInspectable public var endColor:   NSColor = #colorLiteral(red: 0.9254902005, green: 0.2352941185, blue: 0.1019607857, alpha: 1) { didSet { updateColors() } }
    
    private var gradientLayer: CAGradientLayer!
    
    convenience init() {
        self.init(frame: .zero)
    }
    
    override init(frame frameRect: NSRect) {
        super.init(frame: frameRect)
        
        configure()
    }
    
    required public init?(coder: NSCoder) {
        super.init(coder: coder)
        
        configure()
    }

    private func configure() {
        wantsLayer = true
        
        gradientLayer = CAGradientLayer()
        gradientLayer.startPoint = CGPoint(x: 0, y: 0.5)
        gradientLayer.endPoint = CGPoint(x: 1, y: 0.5)
        gradientLayer.name = "gradientLayer"

        updateColors()
    }
    
    override public func layout() {
        super.layout()

        if gradientLayer.superlayer == nil {
            layer!.addSublayer(gradientLayer)
            gradientLayer.contentsScale = layer!.contentsScale
        }
        
        gradientLayer.frame = bounds.insetBy(dx: margin, dy: margin)
    }
    
    private func updateColors() {
        gradientLayer.colors = [startColor, endColor].map { $0.cgColor }
    }
}

Discussed in http://stackoverflow.com/q/41386423/1271826

By the way, I made this @IBDesignable (so put it in a separate framework target and you can then use this view in Interface Builder), but you don't have to do that if you don't want to. Also, I made the margin and the colors @IBInspectable, that way you can customize this right in Interface Builder. Again, not necessary.