如何在Swift4中使用Codable将日期字符串转换为可选的小数秒

Question

我正在用Swift的Codable替换旧的JSON解析代码，并且遇到了一些麻烦。我想这不是一个Codable问题，而是一个DateFormatter问题。

从结构开始

 struct JustADate: Codable {
    var date: Date
 }

和一个json字符串

let json = """
  { "date": "2017-06-19T18:43:19Z" }
"""

现在让我们解码

let decoder = JSONDecoder()
decoder.dateDecodingStrategy = .iso8601

let data = json.data(using: .utf8)!
let justADate = try! decoder.decode(JustADate.self, from: data) //all good

但是，如果我们更改日期，使其具有小数秒，例如：

let json = """
  { "date": "2017-06-19T18:43:19.532Z" }
"""

现在坏了。日期有时会以小数秒返回，有时却不会。我用来解决该问题的方法是在映射代码中，我有一个转换函数，该函数尝试使用带和不带小数秒的dateFormats。我不太确定如何使用Codable处理它。有什么建议？

Answer 1

您可以使用两个不同的日期格式器（带和不带小数秒）并创建自定义的DateDecodingStrategy。如果在解析API返回的日期时失败，则可以按@PauloMattos的建议在注释中引发DecodingError：

iOS 9，macOS 10.9，tvOS 9，watchOS 2，Xcode 9或更高版本

自定义ISO8601DateFormatter：

extension Formatter {
    static let iso8601withFractionalSeconds: DateFormatter = {
        let formatter = DateFormatter()
        formatter.calendar = Calendar(identifier: .iso8601)
        formatter.locale = Locale(identifier: "en_US_POSIX")
        formatter.timeZone = TimeZone(secondsFromGMT: 0)
        formatter.dateFormat = "yyyy-MM-dd'T'HH:mm:ss.SSSXXXXX"
        return formatter
    }()
    static let iso8601: DateFormatter = {
        let formatter = DateFormatter()
        formatter.calendar = Calendar(identifier: .iso8601)
        formatter.locale = Locale(identifier: "en_US_POSIX")
        formatter.timeZone = TimeZone(secondsFromGMT: 0)
        formatter.dateFormat = "yyyy-MM-dd'T'HH:mm:ssXXXXX"
        return formatter
    }()
}

---

习惯DateDecodingStrategy：

extension JSONDecoder.DateDecodingStrategy {
    static let customISO8601 = custom {
        let container = try $0.singleValueContainer()
        let string = try container.decode(String.self)
        if let date = Formatter.iso8601withFractionalSeconds.date(from: string) ?? Formatter.iso8601.date(from: string) {
            return date
        }
        throw DecodingError.dataCorruptedError(in: container, debugDescription: "Invalid date: \(string)")
    }
}

---

习惯DateEncodingStrategy：

extension JSONEncoder.DateEncodingStrategy {
    static let customISO8601 = custom {
        var container = $1.singleValueContainer()
        try container.encode(Formatter.iso8601withFractionalSeconds.string(from: $0))
    }
}

---

编辑/更新 ：

Xcode 10•Swift 4.2或更高版本•iOS 11.2.1或更高版本

ISO8601DateFormatter现在支持formatOptions .withFractionalSeconds：

extension Formatter {
    static let iso8601withFractionalSeconds: ISO8601DateFormatter = {
        let formatter = ISO8601DateFormatter()
        formatter.formatOptions = [.withInternetDateTime, .withFractionalSeconds]
        return formatter
    }()
    static let iso8601: ISO8601DateFormatter = {
        let formatter = ISO8601DateFormatter()
        formatter.formatOptions = [.withInternetDateTime]
        return formatter
    }()
}

---

习惯DateDecodingStrategy和DateEncodingStrategy将与上面显示的相同。

---

// Playground testing
struct ISODates: Codable {
    let dateWith9FS: Date
    let dateWith3FS: Date
    let dateWith2FS: Date
    let dateWithoutFS: Date
}

---

let isoDatesJSON = """
{
"dateWith9FS": "2017-06-19T18:43:19.532123456Z",
"dateWith3FS": "2017-06-19T18:43:19.532Z",
"dateWith2FS": "2017-06-19T18:43:19.53Z",
"dateWithoutFS": "2017-06-19T18:43:19Z",
}
"""

---

let isoDatesData = Data(isoDatesJSON.utf8)

let decoder = JSONDecoder()
decoder.dateDecodingStrategy = .customISO8601

do {
    let isoDates = try decoder.decode(ISODates.self, from: isoDatesData)
    print(Formatter.iso8601withFractionalSeconds.string(from: isoDates.dateWith9FS))   // 2017-06-19T18:43:19.532Z
    print(Formatter.iso8601withFractionalSeconds.string(from: isoDates.dateWith3FS))   // 2017-06-19T18:43:19.532Z
    print(Formatter.iso8601withFractionalSeconds.string(from: isoDates.dateWith2FS))   // 2017-06-19T18:43:19.530Z
    print(Formatter.iso8601withFractionalSeconds.string(from: isoDates.dateWithoutFS)) // 2017-06-19T18:43:19.000Z
} catch {
    print(error)
}