Swift-便利构造hexColor

import UIKit

public extension UIColor {
    
    /// 便利构造Hex颜色
    ///
    /// - Parameters:
    ///   - string: hex值
    ///   - alpha: alpha值，默认1.0
    convenience init(hex string: String, alpha: CGFloat = 1.0) {
        
        var hex = string.hasPrefix("#") ? String(string.dropFirst()) : string
        guard hex.count == 3 || hex.count == 6  else {
            self.init(white: 1.0, alpha: 0.0)
            return
        }
        
        if hex.count == 3 {
            for (indec, char) in hex.enumerated() {
                hex.insert(char, at: hex.index(hex.startIndex, offsetBy: indec * 2))
            }
        }
        
        self.init(
            red: CGFloat((Int(hex, radix: 16)! >> 16) & 0xFF) / 255.0,
            green: CGFloat((Int(hex, radix: 16)! >> 8) & 0xFF) / 255.0,
            blue: CGFloat((Int(hex, radix: 16)!) & 0xFF) / 255.0,
            alpha: alpha
        )
    }
}