如何实现上下文可控的 Swift Codable Encode 功能？

Question

我正在尝试为 Codable 结构实现编码器，我可以通过传递一个上下文来控制它，该上下文确定我将添加到输出 JSON 中的字段。但我想不出一个好的方法。

例如说我有以下内容：

import Foundation

struct MyStruct: Encodable {
    var a:Int = 0
    var b:Int = 0

    enum CodingKeys: String, CodingKey {
        case a
        case b
    }

    enum Context {
        case summary, full
    }

    // I can write something like this
    func encode(to encoder: Encoder) throws {
        var container = encoder.container(keyedBy: CodingKeys.self)
        try container.encode(a, forKey: .a)
        try container.encode(b, forKey: .b)
    }

    // ...but I want to write is something like this
    func encode(to encoder: Encoder, withContextcontext: Context ) throws {
        var container = encoder.container(keyedBy: CodingKeys.self)
        try container.encode(a, forKey: .a)

        if context == .full
        {
            try container.encode(b, forKey: .b)
        }
    }

}

var myObject = MyStruct()
myObject.a = 10
myObject.b = 5

let jsonEncoder = JSONEncoder()
do {
    let jsonData = try jsonEncoder.encode(myObject)
    // let jsonData = try jsonEncode.encode(myObject, withContext:.full)
    let jsonString = String(data: jsonData, encoding: .utf8)!
    print(jsonString)
} catch {
    print(error)
}

有没有什么好的解决方案可以产生与我想象中的 encode(:WithContext) 功能相同的效果？

我有一个我不太喜欢的解决方案是将上下文变量添加到结构中并在调用 encode() 之前对其进行设置：

struct MyStruct: Encodable {
    var a:Int = 0
    var b:Int = 0
    var context = .full

    enum CodingKeys: String, CodingKey {
        case a
        case b
    }

    enum Context {
        case summary, full
    }



    // But I want to write is something like this
    func encode(to encoder) throws {
        var container = encoder.container(keyedBy: CodingKeys.self)
        try container.encode(a, forKey: .a)

        if context == .full
        {
            try container.encode(b, forKey: .b)
        }
    }

}

var myObject = MyStruct()
myObject.a = 10
myObject.b = 5
myObject.context = .summary

let jsonEncoder = JSONEncoder()
do {
    let jsonData = try jsonEncoder.encode(myObject)
    let jsonString = String(data: jsonData, encoding: .utf8)!
    print(jsonString)
} catch {
    print(error)
}

不确定我是否要将此 Context 变量添加到我拥有的每个结构中。还有更优雅的方式吗？

Answer 1

如果您需要将一些上下文传递给Encoder，您可以随时使用Encoder.userInfo：

用户为编码设置的任何上下文信息。

例子：

struct MyStruct: Encodable {
    var a:Int = 0
    var b:Int = 0

    enum CodingKeys: String, CodingKey {
        case a
        case b
    }

    enum Context {
        case summary, full

        static let encodingKey = CodingUserInfoKey(rawValue: "my_struct_context")!
    }

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

        if (encoder.userInfo[Context.encodingKey] as? Context) == .full {
            try container.encode(b, forKey: .b)
        }
    }
}

var myObject = MyStruct()
myObject.a = 10
myObject.b = 5

let jsonEncoder = JSONEncoder()
jsonEncoder.userInfo = [
    MyStruct.Context.encodingKey: MyStruct.Context.summary
]

do {
    let jsonData = try jsonEncoder.encode(myObject)
    let jsonString = String(data: jsonData, encoding: .utf8)!
    print(jsonString)
} catch {
    print(error)
}