基于另一个值的具有多个对象类型的可编码解码属性 [英] Codable decode property with multiple object types BASED on another value

问题描述

这与解码多种类型(int，字符串)的属性值无关.

It's not about decoding a property value with multiple types (int, string)..

我有一个名为 data 的对象，它可以返回多种类型，这时可以做的事情可能看起来像这样:

I have an object called data it can return multiple types, What could be done at this point may look something like this :

enum MyData: Codable {
    case ObjOne(groupObject)
    case ObjTwo(imageObject)

    init(from decoder: Decoder) throws {
        let value = try decoder.singleValueContainer()

        if let v = try? value.decode(groupObject.self) {
            self = .ObjOne(v)
            return
        } else if let v = try? value.decode(imageObject.self) {
            self = .ObjTwo(v)
            return
        }

        throw Rating.ParseError.notRecognizedType(value)
    }

    enum ParseError: Error {
        case notRecognizedType(Any)
    }
}

这里的问题是我试图使 MyData 根据先前解码过程中使用的另一个值来解码对象，简而言之，我想将一个值传递给MyData ，因此可以确定要解码的内容

The issue here is that i try to make MyData decode the object based on another value that was used in the previous decoding process, in short words, i want to pass a value to MyData so it can determine which to decode

我有这个

enum ContentType: String, Codable {
    case linear
    case grid
    case slider
}

我想让 MyData 知道此 ContentType 值，以便 MyData 可以确定流程的进行方式，

And i want MyData to know about this ContentType value so MyData can determine how the flow will go,

那么ContentType是从哪里来的呢?它位于上一个主对象的相同属性列表中，来自类似这样的东西

So where did ContentType come from ? it's in the same list of properties in the previous main object, coming from something that looks like this

struct Catalog: Codable {
    var dataType: ContentType?
    var data: MyData? 
}

我想用更简单的语言实现什么?

What i want to achieve in more simple words ?

struct Catalog: Codable {
    var dataType: ContentType?
    var data: MyData<dataType>? <--// i know this is not possible,
    // -- but i want MyData to know about the dataType value that will be decoded
}

---------我要解析的JSON

--------- JSON i want to parse

[{
  "data_type": "group",
  "data": {
    "group_id": 127 // this refers to object : groupObject
  }
},
{
  "data_type": "image",
  "data": {
    "image": "http://google.com/favicon" // this is referring : imageObject
  }
}
]

您看到了上面的观点，即数据"可以基于data_type的值返回不同的对象

You see the point above, is that "data" can return different objects, based on the value of data_type

推荐答案

我没有使用泛型，而是创建了一个符合 Decodable 的空协议，并将其用作 data的类型.然后，内容结构需要符合此协议.

Rather than using generics I created an empty protocol that conforms to Decodable and used that as the type for data. Then the content structs needs to conform to this protocol.

protocol MyData: Decodable {}

struct Group: MyData {
    let groupId: Int
}

struct Image: MyData {
    let image: String
}

struct Catalog: Decodable {
    var dataType: String
    var data: MyData

    enum CodingKeys: String, CodingKey  {
        case dataType, data
    }

    enum ParseError: Error {
        case notRecognizedType(Any)
    }

    init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self)
        dataType = try container.decode(String.self, forKey: .dataType)
        switch dataType {
        case "group":
            data = try container.decode(Group.self, forKey: .data)
        case "image":
            data = try container.decode(Image.self, forKey: .data)
        default:
            throw ParseError.notRecognizedType(dataType)
        }
    }
}

请注意，我没有在 init 中使用枚举 ContentType ，因为它与示例json数据不匹配，但是应该易于修复.

Note that I didn't use the enum ContentType in the init because it didn't match the sample json data but that should be easily fixed.

使用此解决方案的标准代码

Standard code for using this solution

do {
    let decoder = JSONDecoder()
    decoder.keyDecodingStrategy = .convertFromSnakeCase

    let result = try decoder.decode([Catalog].self, from: data)
    print(result)
} catch {
    print(error)
}

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