当超级类也可以编码时，如何在子类中使用编码键

Question

我正在研究JSON Encode和Decode，但是有些问题很烦人，我不知道如何在继承类中使用CodingKeys。

我有两个类ResponseBean和ResponseWithObjectBean<T>。

这是响应类定义：

public class ResponseBean: Codable
{
    //This is only sample, I define `CodingKeys` because the property in json is in different name.
    private enum CodingKeys: String, CodingKey
    {
        case intA
        case intB
    }
    public var intA: Int32 = 0
    public var intB: Int32 = 0
}
public class ResponseWithObjectBean<T: Codable> : ResponseBean
{
    /*
    Here I don't know how to define an enum to confirm protocl CondingKey. 
    I defined an enum named CodingKeys or whatever, they just don't work and 
    the testMessage and obj are still nil.
    But if I implement the init(from decoder: Decoder) construction and manually
    pass the coding keys which I defined to the decode function, all works fine.
    */
    public var testMessage: String? = nil
    public var obj: T? = nil
}

我将从响应中获得用户：

public class User: Codable
{
    private enum CodingKeys: String, CodingKey
    {
        case name
        case age
    }
    public var name: String? = nil
    public var age: Int32? = nil
}

这是测试JSON：

var testJson = """
{
    "intA": 10,
    "intB": 20,
    "testMessage": "This is a test json",
    "obj":{
        "name": "LiHong",
        "age": 11
    }
}
"""

以下是我运行的方式：

do{
    var responseData = testJson.data(using: .utf8)
    var decoder = JSONDecoder()
    var response: ResponseWithObjectBean<User> = try decoder.decode(ResponseWithObjectBean<User>.self, from: responseData)
}catch let e{
}

我不知道如何在ResponseWithObjectBean类中定义CodingKeys，即使我这样做，也根本无法使用。但是，如果我在ResponseWithObjectBean中定义了init(from decoder: Decoder) throws构造和Manully通过编码键，则可以获取所有属性。

Answer 1

这很简单，您只需要手工进行编码和解码，在子类中：

public class ResponseWithObjectBean<T: Codable> : ResponseBean {
    public var testMessage: String? = nil
    public var obj: T? = nil
    // Create another CodingKey compliant enum with another name for the new keys
    private enum CustomCodingKeys: String, CodingKey {
        case testMessage
        case obj
    }
    // Override the decoder
    required init(from decoder: Decoder) throws {
        try super.init(from: decoder)
        let container = try decoder.container(keyedBy: CustomCodingKeys.self)
        testMessage = try container.decode(String?.self, forKey: .testMessage)
        obj = try container.decode(T?.self, forKey: .obj)
    }
    // And the coder
    public override func encode(to encoder: Encoder) throws {
        try super.encode(to: encoder)
        var container = encoder.container(keyedBy: CustomCodingKeys.self)
        try container.encode(testMessage, forKey: .testMessage)
        try container.encode(obj, forKey: .obj)
    }
}

这样，您可以以所需的方式解码和编码：

let decoder = JSONDecoder()
let response = try decoder.decode(ResponseWithObjectBean<User>.self, from: responseData)
let data = try JSONEncoder().encode(response)
print(String(data: data, encoding: .utf8))

edit ：为了防止您手动编写所有这些样板，您可以使用诸如sourcery的生成工具：https：//github.com/krzysztofzablocki/sourcery/sourcery