如何 JSON 在同一结构中编码多种日期格式

Question

需要编码为 JSON，一个具有 2 个日期实例变量（日期和时间）的结构，但是，我需要使用不同的格式对每个日期实例变量进行编码，即。对于 "day":"yyyy-M-d" 和 "time":"H:m:s".

已经编写了一个没有问题的自定义解码器。但不确定如何编写所需的自定义编码器来解决这个问题。

例如，我可以解码以下 JSON 字符串： { "biometrics" : [ {"biometricId":1,"amount":2.1,"source":"Alderaan","day":"2019-1-3","time":" 11-3-3","unitId":2}, {"biometricId":10,"amount":3.1,"source":"Endoor","day":"2019-2-4","time":" 11-4-4","unitId":20}] }

但是，当我对它进行编码时，我只能将其编码为单一日期格式:( 帮助，将不胜感激。谢谢。

import UIKit

let biometricsJson = """
{ "biometrics" : [
    {"biometricId":1,"amount":2.1,"source":"Alderaan","day":"2019-1-3","time":"11-3-3","unitId":2},
    {"biometricId":10,"amount":3.1,"source":"Endoor","day":"2019-2-4","time":"11-4-4","unitId":20}]
}
"""

struct Biometrics: Codable {
    var biometrics: [Biometric]
}

struct Biometric: Codable {

    var biometricId: Int
    var unitId: Int
    var source: String?
    var amount: Double
    var day: Date
    var time: Date

    init(biometricId: Int, unitId: Int, source: String, amount: Double, day: Date, time: Date){
        self.biometricId = biometricId
        self.unitId = unitId
        self.source = source
        self.amount = amount
        self.day = day
        self.time = time
    }
}

extension Biometric {

    static let decoder: JSONDecoder = {
        let decoder = JSONDecoder()
        decoder.dateDecodingStrategy = .custom { decoder in
            let container = try decoder.singleValueContainer()
            let dateString = try container.decode(String.self)

            let formatter = DateFormatter()
            formatter.timeZone = TimeZone.current
            formatter.dateFormat = "H:m:s"
            if let date = formatter.date(from: dateString) {
                return date
            }

            formatter.dateFormat = "yyyy-M-d"
            if let date = formatter.date(from: dateString) {
                return date
            }
            throw DecodingError.dataCorruptedError(in: container,
                                                   debugDescription: "Cannot decode date string \(dateString)")
        }
        return decoder
    }()
}

let biometrics = try Biometric.decoder.decode(Biometrics.self, from:biometricsJson.data(using: .utf8)!)

let jsonEncoder = JSONEncoder()
let encodedJson = try jsonEncoder.encode(biometrics)
let jsonString = String(data: encodedJson, encoding: .utf8)
if biometricsJson != jsonString {
    print("error: decoding, then encoding does not give the same string")
    print("biometricsJson: \(biometricsJson)")
    print("jsonString: \(jsonString!)")
}

我希望编码后的 JSON 能够被解码器解码。即 biometricsJson == jsonString

Answer 1

在自定义 encode(to:) 中，只需使用所需的格式化程序将每个编码为字符串。 JSON 中没有 "date" 类型；它只是一个字符串。沿着这些线的东西：

enum CodingKeys: CodingKey {
    case biometricId, amount, source, day, time, unitId
}

func encode(to encoder: Encoder) throws {
    var container = encoder.container(keyedBy: CodingKeys.self)
    try container.encode(biometricId, forKey: .biometricId)
    try container.encode(unitId, forKey: .unitId)
    try container.encode(source, forKey: .source)
    try container.encode(amount, forKey: .amount)

    let formatter = DateFormatter()
    formatter.timeZone = TimeZone.current
    formatter.dateFormat = "H:m:s"
    let timeString = formatter.string(from: time)
    try container.encode(timeString, forKey: .time)

    formatter.dateFormat = "yyyy-M-d"
    let dayString = formatter.string(from: day)
    try container.encode(dayString, forKey: .day)
}

但请注意，您无法测试等效字符串。 JSON 词典不保留顺序，因此无法保证逐个字符匹配。

请注意，如果您真的想要日期和时间，您应该考虑 DateComponents 而不是 Date。日期是时间的特定实例；它不在任何时区，而且不能只是一小时、一分钟和一秒。

此外，您对 Double 的使用会导致舍入差异。所以 2.1 将被编码为 2.1000000000000001。如果这是一个问题，您应该使用 Decimal 作为 amount 而不是 Double.

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