如何在可编码结构中使用计算属性（swift）

Question

我创建了一个“可编码”结构来序列化数据集并将其编码为 Json。除了计算的属性没有显示在 json 字符串中之外，一切都很好。如何在编码阶段包含计算属性。

例如：

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)
}

打印出 "{"width":7.2999999999999998,"length":5,"height":10.199999999999999}"

我也很好奇有 7.29999.. 而不是 7.3，但我的主要问题是“我怎样才能在这个 json 字符串中也包含“音量”？

Answer 1

您需要手动编码/解码，而不是让自动化的东西为您完成。这在 Swift 操场上按预期工作。

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)
}