How to use computed property in a codable struct (swift)

Question

I've created a "codable" struct to serialize a data set and encode it to Json. Everything is working great except the computed properties don't show in the json string. How can I include computed properties during the encode phase.

Ex:

struct SolidObject:Codable{

    var height:Double                      = 0
    var width:Double                       = 0
    var length:Double                      = 0

    var volume:Double {
        get{
            return height * width * length
        }
    }
}

var solidObject = SolidObject()
solidObject.height = 10.2
solidObject.width = 7.3
solidObject.length = 5.0

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

prints out "{"width":7.2999999999999998,"length":5,"height":10.199999999999999}"

I am also curious about having 7.29999.. instead of 7.3 but my main question is "how can I include "volume" to this json string too"?

score 22 · Accepted Answer · answered Mar 15 '18 at 23:29

You need to manually encode/decode instead of letting the automated stuff do it for you. This works as expected in a Swift playground.

struct SolidObject: Codable {

    var height:Double                      = 0
    var width:Double                       = 0
    var length:Double                      = 0

    var volume:Double {
        get{
            return height * width * length
        }
    }

    enum CodingKeys: String, CodingKey {
        case height
        case width
        case length

        case volume
    }

    init() { }

    init(from decoder: Decoder) throws {
        let values = try decoder.container(keyedBy: CodingKeys.self)
        height = try values.decode(Double.self, forKey: .height)
        width = try values.decode(Double.self, forKey: .width)
        length = try values.decode(Double.self, forKey: .length)
    }

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

        try container.encode(volume, forKey: .volume)
    }

}

var solidObject = SolidObject()
solidObject.height = 10.2
solidObject.width = 7.3
solidObject.length = 5.0

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

Thank you... Can you also explain the empty init block there? How come it doesn't cause "var solidObject = SolidObject()" type of initialization problems? — monroo, Mar 16 '18 at 09:18
If you don't have the empty init() block, you can't spin up a new instance with just SolidObject() because the other init will be the only visible signature. If you throw the above code in a playground, you can see the error with 'var solidObject = SolidObject()' if you remove the empty init() — Mark Thormann, Mar 16 '18 at 19:15

How to use computed property in a codable struct (swift)

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