swift - 初始化器要求 'init(json:)' 只能由非最终类 `required` 定义中的 'UIColor' 初始化器满足

Question

我正在尝试编写一个扩展来满足扩展中的协议(protocol):

extension UIColor: JSONRepresentable {
    convenience init?(json: Any) {
        guard let colourArray = json as? [CGFloat] else {
            print("json was not an array of CGFloats")
            return nil
        }
    
        self.init(
            red: colourArray[0],
            green: colourArray[1],
            blue: colourArray[2],
            alpha: colourArray[3]
        )
    }
}

我收到这个错误:

Initializer requirement 'init(json:)' can only be satisfied by a required initializer in the definition of non-final class 'UIColor'.

如果我添加一个 required 关键字，我会得到这个错误

'required' initializer must be declared directly in class 'UIColor' (not in an extension).

是否有原因或有任何解决方法？

编辑:为了清楚起见，这里是协议(protocol)

protocol JSONRepresentable {
    init?(json: Any)
}

Answer 1

struct Color: Codable {
    let red, green, blue, alpha: CGFloat
}

extension Color {
    var uiColor: UIColor { return UIColor(color: self) }
    var cgColor: CGColor { return uiColor.cgColor }
    var ciColor: CIColor { return CIColor(color: uiColor) }
    var data: Data { return try! JSONEncoder().encode(self) }
}

extension UIColor {
    convenience init(color: Color) {
        self.init(red: color.red, green: color.green, blue: color.blue, alpha: color.alpha)
    }
    var color: Color {
        let color = CIColor(color: self)
        return Color(red: color.red, green: color.green, blue: color.blue, alpha: color.alpha)
    }
}
extension Data {
    var string: String {
        return String(data: self, encoding: .utf8) ?? ""
    }
}

---

Playground 测试

let json = """
{"red": 0.5, "green": 0.0, "blue": 0.0, "alpha": 1.0}
"""

if let color = try? JSONDecoder().decode(Color.self, from: Data(json.utf8)) {
    print(color)                  // "Color(red: 0.5, green: 0.0, blue: 0.0, alpha: 1.0)\n"
    print(color.uiColor)          // "UIExtendedSRGBColorSpace 0.5 0 0 1\n
    print(color.data)         // "40 bytes\n"
    print(color.data.string)  // "{"red":0.5,"alpha":1,"blue":0,"green":0}\n"
}

let redColor = UIColor.red.color
let jsonData = redColor.data.string  // "{"red":1,"alpha":1,"blue":0,"green":0}"

---

---

如果您需要使用 CGFloats 数组，您可以覆盖 JSON 编码器和解码器初始化器:

extension Color {
    public init(from decoder: Decoder) throws {
        var container = try decoder.unkeyedContainer()
        red   = try container.decode(CGFloat.self)
        green = try container.decode(CGFloat.self)
        blue  = try container.decode(CGFloat.self)
        alpha = try container.decode(CGFloat.self)
    }
    public func encode(to encoder: Encoder) throws {
        var container = encoder.unkeyedContainer()
        try container.encode(red)
        try container.encode(green)
        try container.encode(blue)
        try container.encode(alpha)
    }
}

---

测试

let values: [CGFloat] = [0.5,0.0,0.0,1.0]
let jsonData = try JSONSerialization.data(withJSONObject: values) // 11 bytes
let json = jsonData.string   // "[0.5,0,0,1]"

do {
    let color = try JSONDecoder().decode(Color.self, from: jsonData)
    print(color)                  // "Color(red: 0.5, green: 0.0, blue: 0.0, alpha: 1.0)\n"
    print(color.uiColor)          // "UIExtendedSRGBColorSpace 0.5 0 0 1\n
    print(color.data)                                  // "11 bytes\n"
    print(color.data.string)                           // "[0.5,0,0,1]\n"
    let encodedData = try JSONEncoder().encode(color)  // 11 bytes
    print(encodedData == jsonData)                     // true
} catch {
    print(error)
}