如何在同一结构中对多种日期格式进行JSON编码

Question

需要编码到JSON，一个有2个日期实例变量(日期和时间)的结构，但是，我需要用不同的格式编码每个日期实例变量，即。"day":"yyyy-M-d“和"time":"H:m:s”。

我写了一个自定义解码器，工作没有问题。但不确定如何编写所需的自定义编码器来解决此问题。

例如，我可以解码如下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} }

然而，当我对其进行编码时，我只能将其编码为单一的日期格式:(帮助，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!)")
}

我希望经过编码的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值编码为一个字符串。JSON中没有"date“类型；它只是一个字符串。大致是这样的：

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，您不能测试等价的字符串。JSON字典不能保持顺序，因此无法保证逐个字符匹配。

注意，如果你真的想要有日期和时间，你应该考虑DateComponents而不是Date。日期是时间上的特定实例；它不在任何时区，它不能只是小时、分钟和秒。

此外，您使用双精度将导致舍入差异。因此，2.1将被编码为2.1000000000000001。如果这是一个问题，您应该为amount使用Decimal，而不是Double。