在Swift 4中,如何使用Codable解码JSON并在解码对象之间创建引用？

Question

我如何使用Codable解码JSON并在创建对象(而不是结构)时交叉引用它们？在这个例子中,我希望Painting该类具有Color在JSON中定义的对象数组.(我也希望能够将它们编码回JSON.)

额外奖励:在这种情况下,我宁愿选择Painting.colors非可选let属性而不是 var.我不希望它在创作后改变,我不希望它永远是零.(我宁愿使用空数组的默认值而不是nil.)

class Art: Codable {
    var colors: [Color]?
    var Paintings: [Painting]?
}

class Color: Codable {
    var id: String?
    var hex: String?
}

class Painting: Codable {
    var name: String?
    var colors: [Color]?
}

let json = """
{
    "colors": [
        {"id": "black","hex": "000000"
        },
        {"id": "red", "hex": "FF0000"},
        {"id": "blue", "hex": "0000FF"},
        {"id": "green", "hex": "00FF00"},
        {"id": "yellow", "hex": "FFFB00"},
        {"id": "orange", "hex": "FF9300"},
        {"id": "purple", "hex": "FF00FF"}
    ],
    "paintings": [
        {
            "name": "Starry Night",
            "colorIds": ["blue", "black", "purple", "yellow"]
        },
        {
            "name": "The Scream",
            "colorIds": ["orange", "black", "blue"]
        },
        {
            "name": "Nighthawks",
            "colorIds": ["green", "orange", "blue", "yellow"]
        }
    ]
}
"""


let data = json.data(using: .utf8)
let art = try JSONDecoder().decode(Art.self, from: data!)

我考虑过的一些方法:

• 手动编码/解码 json.看起来像很多额外的工作但也许它给了我需要的控制权？

• 将JSON解码分解为多个步骤.将JSON反序列化为字典,首先拉出并解码颜色,然后绘制(可以访问上下文中的颜色).这感觉就像打架Codable,希望你一次解码所有Data,而不是Dictionary.

• 有Painting动态发现Color通过动态属性在运行时秒.但在开始真正的工作之前,我宁愿让所有的对象关系建立和验证,然后永远不会改变.但也许这会是最简单的？

• 不使用Codable

• 其他一些不好的想法

Answer 1

我投票重新打开你的问题,因为虽然JSON不方便Codable,但它可以完成.您将不得不手动解码JSON,以便问题变成:最难以实现的方法是什么？

我的经验法则是:不要与JSON作斗争.将其原样导入Swift值,然后您可以对其进行各种操作.为此,让我们定义一个RawArt紧跟JSON 的结构:

fileprivate struct RawArt: Decodable {
    struct RawPainting: Codable {
        var name: String
        var colorIds: [String]
    }

    var colors: [Color]             // the Color class matches the JSON so no need to define a new struct
    var paintings: [RawPainting]    // the Painting class does not so we need a substitute struct
}

现在将原始JSON对象转换为您的类:

class Art: Codable {
    var colors: [Color]
    var paintings: [Painting]

    required init(from decoder: Decoder) throws {
        let rawArt = try RawArt(from: decoder)

        self.colors = rawArt.colors
        self.paintings = rawArt.paintings.map { rawPainting in
            let name = rawPainting.name
            let colors = rawPainting.colorIds.flatMap { colorId in
                rawArt.colors.first(where: { $0.id == colorId })
            }

            return Painting(name: name, colors: colors)
        }
    }
}

class Color: Codable {
    var id: String
    var hex: String

    init(id: String, hex: String) {
        self.id = id
        self.hex = hex
    }
}

// It does not transform into the JSON you want so you may as well remove Codable conformance
class Painting: Codable {
    var name: String
    var colors: [Color]

    init(name: String, colors: [Color]) {
        self.name = name
        self.colors = colors
    }
}

要测试它实际引用一个Color对象:

let data = json.data(using: .utf8)
let art = try JSONDecoder().decode(Art.self, from: data!)

art.colors[0].id = "new_black"
print(art.paintings[0].colors[1].id)    // the second color in Starry Night: new_black

一切都是非可选的,从JSON中取消归档对象只需不到20行代码.