How to draw a rainbow with Bezier Paths

RainbowPlayground.swift

//: Playground - noun: a place where people can play

import UIKit
import Foundation
import PlaygroundSupport

extension UIColor {
    
    class var indigo: UIColor {
        return UIColor(colorLiteralRed: 75.0/255.0, green: 0, blue: 130.0/255.0, alpha: 1)
    }
    
    class var violet: UIColor {
        return UIColor(colorLiteralRed: 139.0/255.0, green: 0, blue: 255.0/255.0, alpha: 1)
    }
    
    static let rainbow = [
        UIColor.red,
        UIColor.orange,
        UIColor.yellow,
        UIColor.green,
        UIColor.blue,
        UIColor.indigo,
        UIColor.violet
    ]
}

class RainbowView: UIView {
    
    override func draw(_ rect: CGRect) {
        let paths = rainbowBezierPaths(rect: rect)
        for (index, path) in paths.enumerated() {
            draw(bezierPath: path, color: UIColor.rainbow[index])
        }
    }
    
    private func draw(bezierPath: UIBezierPath, color: UIColor) {
        color.setStroke()
        color.setFill()
        bezierPath.stroke()
    }
    
    private func rainbowBezierPaths(rect: CGRect) -> [UIBezierPath] {
        let gapFactor: CGFloat = 0.90
        let diameter = max(rect.maxX, rect.midY)
        let maxRadius: CGFloat = (diameter / 2.0) * gapFactor
        let arcCenter = CGPoint(x: rect.midX, y: rect.midY + (maxRadius / 2.0))
        let lineWidth: CGFloat = maxRadius / CGFloat(UIColor.rainbow.count) / 2.0
        var paths = [UIBezierPath]()
        
        for index in 0..<(UIColor.rainbow.count) {
            let radius = maxRadius - (CGFloat(index) * lineWidth)
            let path = UIBezierPath(arcCenter: arcCenter, radius: radius, startAngle: 0, endAngle: CGFloat.pi, clockwise: false)
            path.lineWidth = lineWidth
            paths.append(path)
        }
        
        return paths
    }
}

let rainbowView = RainbowView(frame: CGRect(x: 0, y: 0, width: 800, height: 800))
rainbowView.backgroundColor = .white
PlaygroundPage.current.liveView = rainbowView