How can i parse an Json array of a list of different object using Codable?

Question

I have an json array with a list of item, that has different property.

{
    "items": [ 
        {
            "id": "1",
            "name": "name",
            "propertyOfA": "1243",
            "productype": "a"

        },
        {
            "id": "2",
            "name": "name",
            "propertyOfA": "12",
            "productype": "a"
        },
        {
            "id": "3",
            "name": "name",
            "propertyOfA": "1243",
            "productype": "a"
        },
        {
            "id": "1",
            "name": "name",
            "propertyOfB": "1243",
            "productype": "b"
        },
        {
            "id": "1",
            "name": "name",
            "propertyOfC": "1243",
            "propertyOfC2": "1243",
            "productype": "c"
        }
        ]
}

What i usually do is parse it into something like :


struct RequestData: Codable {
    let items: [Item]
}

enum ProductType: String, Codable {
    case a = "A"
    case b = "B"
    case c = "C"
}

struct Item: Codable {
    let id, name: String
    let propertyOfA: String?
    let productype: String
    let propertyOfB: String?
    let propertyOfC: String?
    let propertyOfC2: String?
}

But if this array keep growing, its gonna end up with a Item with a huge amount of optional property, so i want each object have its own dedicated class

adding some kind of bridge inside codable parsings,

what i want to archive is something like :

struct RequestData: Codable {
    let items: Items
}

struct items: Codable {
    let arrayA: [A]
    let arrayB = [B]
    let arrayC = [C]
}

struct A: Codable {
    let id: String,
    let name: String,
    let propertyOfA: String,
    let producttype: Productype
}

struct B {
...
}

struct C {
...
}

can i do this with Codable ?

vadian · Accepted Answer · 2021-06-13 10:04:25Z

A reasonable solution is an enum with associated values because the type can be determined by the productype key. The init method first decodes the productype with a CodingKey then in a switch it decodes (from a singleValueContainer) and associates the proper type/value to the corresponding case.

enum ProductType: String, Codable {
    case a, b, c
}

struct Root : Codable {
    let items : [Product]
}

struct ProductA : Codable {
    let id, name: String
    let productype: ProductType
    let propertyOfA : String
}

struct ProductB : Codable {
    let id, name: String
    let productype: ProductType
    let propertyOfB : String
}

struct ProductC : Codable {
    let id, name: String
    let productype: ProductType
    let propertyOfC, propertyOfC2 : String
}

enum Product : Codable {
    
    case a(ProductA), b(ProductB), c(ProductC)
    
    enum CodingKeys : String, CodingKey { case productype }
    
    init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self)
        let type = try  container.decode(ProductType.self, forKey: .productype)
        let singleContainer = try decoder.singleValueContainer()
        switch type {
            case .a : self = .a(try singleContainer.decode(ProductA.self))
            case .b : self = .b(try singleContainer.decode(ProductB.self))
            case .c : self = .c(try singleContainer.decode(ProductC.self))
        }
    }
    
    func encode(to encoder: Encoder) throws {
        var container = encoder.singleValueContainer()
        switch self {
            case .a(let productA): try container.encode(productA)
            case .b(let productB): try container.encode(productB)
            case .c(let productC): try container.encode(productC)
        }
    }
}

And decode

let jsonString = """
{
    "items": [
        {
            "id": "1",
            "name": "name",
            "propertyOfA": "1243",
            "productype": "a"

        },
        {
            "id": "2",
            "name": "name",
            "propertyOfA": "12",
            "productype": "a"
        },
        {
            "id": "3",
            "name": "name",
            "propertyOfA": "1243",
            "productype": "a"
        },
        {
            "id": "1",
            "name": "name",
            "propertyOfB": "1243",
            "productype": "b"
        },
        {
            "id": "1",
            "name": "name",
            "propertyOfC": "1243",
            "propertyOfC2": "1243",
            "productype": "c"
        }
        ]
}
"""
do {
    let result = try JSONDecoder().decode(Root.self, from: Data(jsonString.utf8))
    print(result)
} catch { print(error)}

To read the enum values use also a switch.

Hello, i appreciated this solution, it help a lot, is that possible to you to complete this with encoding part to make it conform to Codable ?
Please see the edit. To be able to encode the different types the key productype – by the way please consolidate the spelling of the key – must be added and decoded in ProductA-C

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