ios - swift 5 : Non-terrible solutions for sharing of JSON Key-values for Codable Structs?

Question

作为值类型和纯函数的粉丝，我让我所有的模型都是结构体。但是我的一些模型共享我需要进行 JSON 编码的属性。因为我不能使用继承(主要是好的，但在这种情况下不好，但我仍然想坚持使用结构)我需要为每个结构声明这些属性。但是 Swift 的 Codable 不允许

使用 Swift 5.1，是否有任何优雅的解决方案(如避免代码重复和反驳 NSJSONSerialization 和字典黑客)来共享结构的 JSON 键值？尤其是在编码 struct 时。

简单的例子

protocol SerializableModel: Codable {
    static var version: Int { get }
    static var serializerName: String { get }
}

extension SerializableModel {
    // Default value
    static var version: Int { 100 }

    // Static -> Instance convenience
    var version: Int { Self.version }
    var serializerName: String { Self.serializerName }
}

enum SharedCodingKeyValues {}
extension SharedCodingKeyValues {
    static let version = "version"
    static let serializer = "serializer"
}

struct Person {
    let name: String
}

extension Person: SerializableModel {
    static var serializerName: String = "com.mycompany.person"
}

// MARK: Encodable
extension Person {
    func encode(to encoder: Encoder) throws {
        var container = encoder.container(keyedBy: CodingKeys.self)
        try container.encode(name, forKey: .name)

        // Does not even compile: `Cannot invoke 'encode' with an argument list of type '(Int, forKey: String)'`
//        try container.encode(version, forKey: SharedCodingKeyValues.version)

        // Runtime crash: => `Unexpectedly found nil while unwrapping an Optional value`
//        try container.encode(version, forKey: CodingKeys.init(stringValue: SharedCodingKeyValues.version)!)
//        try container.encode(serializerName, forKey: CodingKeys.init(stringValue: SharedCodingKeyValues.serializer)!)
    }
}

呸!好的，这没有用，目前，我已经屈服于这个我不满意的解决方案:

struct Person {
    let name: String
}

extension Person: SerializableModel {
    static var serializerName: String = "com.mycompany.person"
}

// MARK: CodingKeys
extension Person {

    // Uh, terrible! I REALLY do not wanna do this, because for large models with many stored properties I have to declare ALL my
    // JSON keys, even if they are unchanged (waiting for Swift to improve specifically this point)
    // Also since just be declaring this, auto-synthesizing of `init(from decoder: Decoder)` stopped working, uhh! This is terrible.
    enum CodingKeys: String, CodingKey {
        case name

        // Does not even compile: `Raw value for enum case must be a literal`
//        case serializer = SharedCodingKeyValues.serializer

        case serializer // uh, terrible, the name of this enum has to match ALL my other models' CodingKeys.serializer value, and if the JSON key changes server side I need to find and update them all, since I cannot share this value
        case version
    }
}

// MARK: Encodable
extension Person {

    func encode(to encoder: Encoder) throws {
        var container = encoder.container(keyedBy: CodingKeys.self)
        try container.encode(name, forKey: .name)

        try container.encode(version, forKey: .version)
        try container.encode(serializerName, forKey: .serializer)
    }
}

// MARK: Decodable
extension Person {
    init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self)
        self.name = try container.decode(String.self, forKey: .name)
        // Actually for now we don't care about any incoming values for the keys `version` and `serializerName` has already been used elsewhere
    }
}

这个解决方案有很多不好的地方，请阅读代码内嵌的注释。

我们怎样才能做得更好？

Answer 1

您可以做的是定义一个自定义 CodingKey它基本上结合了你的模型的编码键和你想要的任何其他编码键，就像这样(警告 - 这里有很多代码，但我做了它所以它足够通用，可以在任何项目中的任何模型中重复使用):

protocol CompoundableCodingKey: CodingKey {
    associatedtype OtherCodingKeys1
    associatedtype OtherCodingKeys2

    init(otherCodingKeys1: OtherCodingKeys1)
    init(otherCodingKeys2: OtherCodingKeys2)
    init(intValue: Int)
    init(stringValue: String)
}

struct CompoundCodingKeys<OtherCodingKeys1: CodingKey, OtherCodingKeys2: CodingKey>: CompoundableCodingKey {
    private let otherCodingKeys1: OtherCodingKeys1?
    private let otherCodingKeys2: OtherCodingKeys2?
    private let internalIntValue: Int?
    private let internalStringValue: String

    var intValue: Int? {
        if let otherCodingKeys1 = otherCodingKeys1 {
            return otherCodingKeys1.intValue
        } else if let otherCodingKeys2 = otherCodingKeys2 {
            return otherCodingKeys2.intValue
        }

        return internalIntValue
    }

    var stringValue: String {
        if let otherCodingKeys1 = otherCodingKeys1 {
            return otherCodingKeys1.stringValue
        } else if let otherCodingKeys2 = otherCodingKeys2 {
            return otherCodingKeys2.stringValue
        }

        return internalStringValue
    }

    init(intValue: Int) {
        if let otherCodingKeys1 = OtherCodingKeys1(intValue: intValue) {
            self.otherCodingKeys1 = otherCodingKeys1
            otherCodingKeys2 = nil
            internalIntValue = nil
            internalStringValue = otherCodingKeys1.stringValue
        } else if let otherCodingKeys2 = OtherCodingKeys2(intValue: intValue) {
            otherCodingKeys1 = nil
            self.otherCodingKeys2 = otherCodingKeys2
            internalIntValue = nil
            internalStringValue = otherCodingKeys2.stringValue
        } else {
            otherCodingKeys1 = nil
            otherCodingKeys2 = nil
            internalIntValue = intValue
            internalStringValue = intValue.description
        }
    }

    init(stringValue: String) {
        if let otherCodingKeys1 = OtherCodingKeys1(stringValue: stringValue) {
            self.otherCodingKeys1 = otherCodingKeys1
            otherCodingKeys2 = nil
            internalIntValue = nil
            internalStringValue = otherCodingKeys1.stringValue
        } else if let otherCodingKeys2 = OtherCodingKeys2(stringValue: stringValue) {
            otherCodingKeys1 = nil
            self.otherCodingKeys2 = otherCodingKeys2
            internalIntValue = nil
            internalStringValue = otherCodingKeys2.stringValue
        } else {
            otherCodingKeys1 = nil
            otherCodingKeys2 = nil
            internalIntValue = nil
            internalStringValue = stringValue
        }
    }

    init(otherCodingKeys1: OtherCodingKeys1) {
        self.init(stringValue: otherCodingKeys1.stringValue)
    }

    init(otherCodingKeys2: OtherCodingKeys2) {
        self.init(stringValue: otherCodingKeys2.stringValue)
    }
}

extension KeyedEncodingContainerProtocol where Key: CompoundableCodingKey {
    mutating func encode<T>(_ value: T, forKey key: Key.OtherCodingKeys1) throws where T: Encodable {
        try encode(value, forKey: Key(otherCodingKeys1: key))
    }

    mutating func encode<T>(_ value: T, forKey key: Key.OtherCodingKeys2) throws where T: Encodable {
        try encode(value, forKey: Key(otherCodingKeys2: key))
    }

    mutating func encodeIfPresent<T>(_ value: T?, forKey key: Key.OtherCodingKeys1) throws where T: Encodable {
        try encodeIfPresent(value, forKey: Key(otherCodingKeys1: key))
    }

    mutating func encodeIfPresent<T>(_ value: T?, forKey key: Key.OtherCodingKeys2) throws where T: Encodable {
        try encodeIfPresent(value, forKey: Key(otherCodingKeys2: key))
    }

    mutating func encodeConditional<T>(_ object: T, forKey key: Key.OtherCodingKeys1) throws where T: AnyObject, T: Encodable {
        try encodeConditional(object, forKey: Key(otherCodingKeys1: key))
    }

    mutating func encodeConditional<T>(_ object: T, forKey key: Key.OtherCodingKeys2) throws where T: AnyObject, T: Encodable {
        try encodeConditional(object, forKey: Key(otherCodingKeys2: key))
    }
}

extension KeyedEncodingContainerProtocol where Key: CompoundableCodingKey, Key.OtherCodingKeys1: CompoundableCodingKey {
    mutating func encode<T>(_ value: T, forKey key: Key.OtherCodingKeys1.OtherCodingKeys1) throws where T: Encodable {
        try encode(value, forKey: Key.OtherCodingKeys1(otherCodingKeys1: key))
    }

    mutating func encode<T>(_ value: T, forKey key: Key.OtherCodingKeys1.OtherCodingKeys2) throws where T: Encodable {
        try encode(value, forKey: Key.OtherCodingKeys1(otherCodingKeys2: key))
    }

    mutating func encodeIfPresent<T>(_ value: T?, forKey key: Key.OtherCodingKeys1.OtherCodingKeys1) throws where T: Encodable {
        try encodeIfPresent(value, forKey: Key.OtherCodingKeys1(otherCodingKeys1: key))
    }

    mutating func encodeIfPresent<T>(_ value: T?, forKey key: Key.OtherCodingKeys1.OtherCodingKeys2) throws where T: Encodable {
        try encodeIfPresent(value, forKey: Key.OtherCodingKeys1(otherCodingKeys2: key))
    }

    mutating func encodeConditional<T>(_ object: T, forKey key: Key.OtherCodingKeys1.OtherCodingKeys1) throws where T: AnyObject, T: Encodable {
        try encodeConditional(object, forKey: Key.OtherCodingKeys1(otherCodingKeys1: key))
    }

    mutating func encodeConditional<T>(_ object: T, forKey key: Key.OtherCodingKeys1.OtherCodingKeys2) throws where T: AnyObject, T: Encodable {
        try encodeConditional(object, forKey: Key.OtherCodingKeys1(otherCodingKeys2: key))
    }
}

extension KeyedEncodingContainerProtocol where Key: CompoundableCodingKey, Key.OtherCodingKeys2: CompoundableCodingKey {
    mutating func encode<T>(_ value: T, forKey key: Key.OtherCodingKeys2.OtherCodingKeys1) throws where T: Encodable {
        try encode(value, forKey: Key.OtherCodingKeys2(otherCodingKeys1: key))
    }

    mutating func encode<T>(_ value: T, forKey key: Key.OtherCodingKeys2.OtherCodingKeys2) throws where T: Encodable {
        try encode(value, forKey: Key.OtherCodingKeys2(otherCodingKeys2: key))
    }

    mutating func encodeIfPresent<T>(_ value: T?, forKey key: Key.OtherCodingKeys2.OtherCodingKeys1) throws where T: Encodable {
        try encodeIfPresent(value, forKey: Key.OtherCodingKeys2(otherCodingKeys1: key))
    }

    mutating func encodeIfPresent<T>(_ value: T?, forKey key: Key.OtherCodingKeys2.OtherCodingKeys2) throws where T: Encodable {
        try encodeIfPresent(value, forKey: Key.OtherCodingKeys2(otherCodingKeys2: key))
    }

    mutating func encodeConditional<T>(_ object: T, forKey key: Key.OtherCodingKeys2.OtherCodingKeys1) throws where T: AnyObject, T: Encodable {
        try encodeConditional(object, forKey: Key.OtherCodingKeys2(otherCodingKeys1: key))
    }

    mutating func encodeConditional<T>(_ object: T, forKey key: Key.OtherCodingKeys2.OtherCodingKeys2) throws where T: AnyObject, T: Encodable {
        try encodeConditional(object, forKey: Key.OtherCodingKeys2(otherCodingKeys2: key))
    }
}

extension KeyedDecodingContainerProtocol where Key: CompoundableCodingKey {
    func decode<T>(_ type: T.Type, forKey key: Key.OtherCodingKeys1) throws -> T where T: Decodable {
        try decode(type, forKey: Key(otherCodingKeys1: key))
    }

    func decode<T>(_ type: T.Type, forKey key: Key.OtherCodingKeys2) throws -> T where T: Decodable {
        try decode(type, forKey: Key(otherCodingKeys2: key))
    }

    func decodeIfPresent<T>(_ type: T.Type, forKey key: Key.OtherCodingKeys1) throws -> T? where T: Decodable {
        try decodeIfPresent(type, forKey: Key(otherCodingKeys1: key))
    }

    func decodeIfPresent<T>(_ type: T.Type, forKey key: Key.OtherCodingKeys2) throws -> T? where T: Decodable {
        try decodeIfPresent(type, forKey: Key(otherCodingKeys2: key))
    }
}

extension KeyedDecodingContainerProtocol where Key: CompoundableCodingKey, Key.OtherCodingKeys1: CompoundableCodingKey {
    func decode<T>(_ type: T.Type, forKey key: Key.OtherCodingKeys1.OtherCodingKeys1) throws -> T where T: Decodable {
        try decode(type, forKey: Key.OtherCodingKeys1(otherCodingKeys1: key))
    }

    func decode<T>(_ type: T.Type, forKey key: Key.OtherCodingKeys1.OtherCodingKeys2) throws -> T where T: Decodable {
        try decode(type, forKey: Key.OtherCodingKeys1(otherCodingKeys2: key))
    }

    func decodeIfPresent<T>(_ type: T.Type, forKey key: Key.OtherCodingKeys1.OtherCodingKeys1) throws -> T? where T: Decodable {
        try decodeIfPresent(type, forKey: Key.OtherCodingKeys1(otherCodingKeys1: key))
    }

    func decodeIfPresent<T>(_ type: T.Type, forKey key: Key.OtherCodingKeys1.OtherCodingKeys2) throws -> T? where T: Decodable {
        try decodeIfPresent(type, forKey: Key.OtherCodingKeys1(otherCodingKeys2: key))
    }
}

extension KeyedDecodingContainerProtocol where Key: CompoundableCodingKey, Key.OtherCodingKeys2: CompoundableCodingKey {
    func decode<T>(_ type: T.Type, forKey key: Key.OtherCodingKeys2.OtherCodingKeys1) throws -> T where T: Decodable {
        try decode(type, forKey: Key.OtherCodingKeys2(otherCodingKeys1: key))
    }

    func decode<T>(_ type: T.Type, forKey key: Key.OtherCodingKeys2.OtherCodingKeys2) throws -> T where T: Decodable {
        try decode(type, forKey: Key.OtherCodingKeys2(otherCodingKeys2: key))
    }

    func decodeIfPresent<T>(_ type: T.Type, forKey key: Key.OtherCodingKeys2.OtherCodingKeys1) throws -> T? where T: Decodable {
        try decodeIfPresent(type, forKey: Key.OtherCodingKeys2(otherCodingKeys1: key))
    }

    func decodeIfPresent<T>(_ type: T.Type, forKey key: Key.OtherCodingKeys2.OtherCodingKeys2) throws -> T? where T: Decodable {
        try decodeIfPresent(type, forKey: Key.OtherCodingKeys2(otherCodingKeys2: key))
    }
}

然后，如果我们改变SharedCodingKeyValues进入 CodingKey枚举，像这样:

enum SharedCodingKeys: String, CodingKey {
    case version
    case serializer
}

...然后我们可以得到一个以 CompoundCodingKeys<CodingKeys, SharedCodingKeys> 类型为键的容器，它允许您指定 任一个 Person.CodingKeys (不需要手动定义)，或 SharedCodingKeys ，像这样:

// MARK: Encodable
extension Person {
    func encode(to encoder: Encoder) throws {
        var container = encoder.container(keyedBy: CompoundCodingKeys<CodingKeys, SharedCodingKeys>.self)

        // From `Person.CodingKeys`
        try container.encode(name, forKey: .name)

        // From `SharedCodingKeys`
        try container.encode(version, forKey: .version)
        try container.encode(serializerName, forKey: .serializer)
    }
}

自 CompoundCodingKeys本身符合 CodingKey ，您甚至可以更进一步，将任意多的编码键组合成一个，例如 CompoundCodingKeys<CodingKeys, CompoundCodingKeys<SomeOtherCodingKeys, SharedCodingKeys>> :

enum SomeOtherCodingKeys: String, CodingKey {
    case someOtherKey
}

// MARK: Encodable
extension Person {
    func encode(to encoder: Encoder) throws {
        var container = encoder.container(keyedBy: CompoundCodingKeys<CodingKeys, CompoundCodingKeys<SomeOtherCodingKeys, SharedCodingKeys>>.self)

        // From `Person.CodingKeys`
        try container.encode(name, forKey: .name)

        // From `SharedCodingKeys`
        try container.encode(version, forKey: .version)
        try container.encode(serializerName, forKey: .serializer)

        // From `SomeOtherCodingKeys`
        try container.encode(serializerName, forKey: .someOtherKey)
    }
}