swift - 用幻像类型解码泛型

Question

我正在尝试定义一个 Currency 类型，以防止混淆数字和字母货币代码:

public protocol ISO4217Type {}

public enum ISO4217Alpha: ISO4217Type {}
public enum ISO4217Num: ISO4217Type {}

public struct Currency<T: ISO4217Type> {

    public let value: String
}

extension Currency where T == ISO4217Alpha {

    public init?(value: String) {
        let isLetter = CharacterSet.letters.contains
        guard value.unicodeScalars.all(isLetter) else { return nil }
        self.value = value
    }
}

extension Currency where T == ISO4217Num {

    public init?(value: String) {
        let isDigit = CharacterSet.decimalDigits.contains
        guard value.unicodeScalars.all(isDigit) else { return nil }
        self.value = value
    }
}

这很好用。现在，是否可以添加 Codable 一致性，当尝试使用错误的有效负载解码货币代码时会抛出解码错误？ (例如，将 USD 解码为数字货币代码。)

Answer 1

关键的启示是可以在幻像类型上使用静态函数来自定义行为:

public protocol ISO4217Type {

    static func isValidCode(_ code: String) -> Bool
}

public enum ISO4217Alpha: ISO4217Type {

    public static func isValidCode(_ code: String) -> Bool {
        let isLetter = CharacterSet.letters.contains
        return code.unicodeScalars.all(isLetter)
    }
}

public enum ISO4217Num: ISO4217Type {

    public static func isValidCode(_ code: String) -> Bool {
        let isDigit = CharacterSet.decimalDigits.contains
        return code.unicodeScalars.all(isDigit)
    }
}

public struct Currency<T: ISO4217Type> {

    public let value: String

    private init(uncheckedValue value: String) {
        self.value = value
    }

    public init?(value: String) {
        guard T.isValidCode(value) else { return nil }
        self.value = value
    }
}

extension Currency: Codable {

    public func encode(to encoder: Encoder) throws {
        var c = encoder.singleValueContainer()
        try c.encode(value)
    }

    public init(from decoder: Decoder) throws {
        let c = try decoder.singleValueContainer()
        let value = try c.decode(String.self)
        guard T.isValidCode(value) else {
            throw DecodingError.dataCorruptedError(in: c,
                debugDescription: "Invalid \(type(of: T.self)) code")
        }
        self.init(uncheckedValue: value)
    }
}