当密钥根据用户输入发生变化时，如何从 json 解码变量？

Question

我正在尝试使用 Swift 4 中的 JSONDecoder() 解析来自 CoinmarketCap 的一些 JSON 响应。但问题是来自 json 的响应会根据用户输入而变化。例如，如果用户想要以 eur 为单位的价格，则输出如下：

[
    {
        "price_eur": "9022.9695444"
    }
]

但如果用户想要以 gbp 为单位的价格：

[
    {
        "price_gbp": "7906.8032145"
    }
]

所以问题是如果变量（json 键）名称发生变化，我应该如何制作从 Decodable 继承的结构？

Answer 1

如果您有少量可能的键，您可以执行以下操作

struct Price: Decodable {

    var value: String

    enum CodingKeys: String, CodingKey {
        case price_eur
        case price_gbp
        case price_usd
    }

    init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self)
        do {
            value = try container.decode(String.self, forKey: .price_eur)
        } catch {
            do {
                value = try container.decode(String.self, forKey: .price_gbp)
            } catch {
                value = try container.decode(String.self, forKey: .price_usd)
            }
        }
    }
}

let data = try! JSONSerialization.data(withJSONObject: ["price_gbp": "10.12"], options: [])
let price = try JSONDecoder().decode(Price.self, from: data)

否则，您将需要手动解析数据。

Answer 2

您可以通过为您的结构创建自定义 init(from:) 方法来解码动态密钥，然后使用两组编码密钥，一个 enum 包含编译时已知的所有密钥，另一个 struct 您初始化使用通过用户输入生成的动态键（包含货币名称）。

在您的自定义 init(from:) 方法中，您只需使用它们各自的键解码每个属性。

let chosenCurrency = "gbp"

struct CurrencyResponse: Decodable {
    let name:String
    let symbol:String
    let price:String
    private static var priceKey:String {
        return "price_\(chosenCurrency)"
    }

    private enum SimpleCodingKeys: String, CodingKey {
        case name, symbol
    }

    private struct PriceCodingKey : CodingKey {
        var stringValue: String
        init?(stringValue: String) {
            self.stringValue = stringValue
        }
        var intValue: Int?
        init?(intValue: Int) {
            return nil
        }
    }

    init(from decoder:Decoder) throws {
        let values = try decoder.container(keyedBy: SimpleCodingKeys.self)
        name = try values.decode(String.self, forKey: .name)
        symbol = try values.decode(String.self, forKey: .symbol)
        let priceValue = try decoder.container(keyedBy: PriceCodingKey.self)
        price = try priceValue.decode(String.self, forKey: PriceCodingKey(stringValue:CurrencyResponse.priceKey)!)
    }
}

do {
    let cryptoCurrencies = try JSONDecoder().decode([CurrencyResponse].self, from: priceJSON.data(using: .utf8)!)
} catch {
    print(error)
}

测试 JSON：

let priceJSON = """
    [
    {
    "id": "bitcoin",
    "name": "Bitcoin",
    "symbol": "BTC",
    "rank": "1",
    "price_\(chosenCurrency)": "573.137",
    "price_btc": "1.0",
    "24h_volume_\(chosenCurrency)": "72855700.0",
    "market_cap_\(chosenCurrency)": "9080883500.0",
    "available_supply": "15844176.0",
    "total_supply": "15844176.0",
    "percent_change_1h": "0.04",
    "percent_change_24h": "-0.3",
    "percent_change_7d": "-0.57",
    "last_updated": "1472762067"
    },
    {
    "id": "ethereum",
    "name": "Ethereum",
    "symbol": "ETH",
    "rank": "2",
    "price_\(chosenCurrency)": "12.1844",
    "price_btc": "0.021262",
    "24h_volume_\(chosenCurrency)": "24085900.0",
    "market_cap_\(chosenCurrency)": "1018098455.0",
    "available_supply": "83557537.0",
    "total_supply": "83557537.0",
    "percent_change_1h": "-0.58",
    "percent_change_24h": "6.34",
    "percent_change_7d": "8.59",
    "last_updated": "1472762062"
}
]
"""