当超类也是可编码的时，如何在子类中使用CodingKeys？

Question

I'm working on JSON encode and decode, but some problems are very annoying and I don't know how to use CodingKeys in inherit classes.

I have two classes ResponseBean and ResponseWithObjectBean<T> .

Here is the response class definition:

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
}

and I will get a user from the response:

public class User: Codable
{
    private enum CodingKeys: String, CodingKey
    {
        case name
        case age
    }

    public var name: String? = nil
    public var age: Int32? = nil
}

Here is the test json:

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

The following is how I run:

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{

}

I don't know how to define CodingKeys in ResponseWithObjectBean class, and even I did, it dosen't work at all. But if I implement init(from decoder: Decoder) throws construction and manully pass coding keys I defined in ResponseWithObjectBean , I can get all properties.

Answer 1

This is pretty simple, you just have to do the coding and decoding by hand, in the child class:

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)
    }

}

This way you can decode and encode the way you want:

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 : In order to prevent you from writing manually all this boilerplate, you can use generation tools like Sourcery: https://github.com/krzysztofzablocki/Sourcery