Convert RGB(A) hex string to UIColor

UIColor+Hex.swift

import UIKit

fileprivate extension Int {
    var rgbaPercentage: CGFloat {
        return CGFloat(self) / 255.0
    }
}

enum UIColorInputError: Error {
	case invalidInputColor
}

extension UIColor {
    convenience init?(hex: String) {
        let chars = hex.characters.map { String($0) }
        let start = Int((chars.index(of: "#") ?? -1) + 1)
        let channels: [CGFloat] = stride(from: start, to: chars.count - 1, by: 2)
            .flatMap { Int(chars[$0] + chars[$0 + 1], radix: 16)?.rgbaPercentage }
        self.init(channels: channels)
    }
    
    private convenience init?(channels: [CGFloat]) {
        switch channels.count {
        case 3, 4: self.init(
            red: channels[0],
            green: channels[1],
            blue: channels[2],
            alpha: channels.count == 4 ? channels[3] : 1.0)
        default: return nil
        }
    }

	func hexString(withAlpha hasAlpha: Bool = false) throws -> String {
		var r: CGFloat = 0
		var g: CGFloat = 0
		var b: CGFloat = 0
		var a: CGFloat = 0
		getRed(&r, green: &g, blue: &b, alpha: &a)
		
		guard r >= 0 && r <= 1 && g >= 0 && g <= 1 && b >= 0 && b <= 1 else {
			throw UIColorInputError.invalidInputColor
		}
		
		func makeChannel(with percentage: CGFloat) -> Int {
			return Int(percentage * 255)
		}
		
		guard hasAlpha else {
			return String(format: "#%02X%02X%02X",
			              makeChannel(with: r),
			              makeChannel(with: g),
			              makeChannel(with: b))
		}
		return String(format: "#%02X%02X%02X%02X",
		              makeChannel(with: r),
		              makeChannel(with: g),
		              makeChannel(with: b),
		              makeChannel(with: a))
	}
}

let rgbColorWithHash = UIColor(hex: "#0FF1CE")!
let rgbColorWithoutHash = UIColor(hex: "BADA55")!
let rgbaColorWithHash = UIColor(hex: "#CAFEBABE")!
let rgbaColorWithoutHash = UIColor(hex: "DEADBEEF")!

rgbColorWithHash.hexString() // 0FF1CE
rgbaColorWithoutHash.hexString(withAlpha: true) // DEADBEEF