How to convert a date string with optional fractional seconds using Codable in Swift4

Question

I am replacing my old JSON parsing code with Swift's Codable and am running into a bit of a snag. I guess it isn't as much a Codable question as it is a DateFormatter question.

Start with a struct

 struct JustADate: Codable {     var date: Date  } 

and a json string

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

now lets decode

let decoder = JSONDecoder() decoder.dateDecodingStrategy = .iso8601  let data = json.data(using: .utf8)! let justADate = try! decoder.decode(JustADate.self, from: data) //all good 

But if we change the date so that it has fractional seconds, for example:

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

Now it breaks. The dates sometimes come back with fractional seconds and sometimes do not. The way I used to solve it was in my mapping code I had a transform function that tried both dateFormats with and without the fractional seconds. I am not quite sure how to approach it using Codable however. Any suggestions?

Answer 1

You can use two different date formatters (with and without fraction seconds) and create a custom DateDecodingStrategy. In case of failure when parsing the date returned by the API you can throw a DecodingError as suggested by @PauloMattos in comments:

iOS 9, macOS 10.9, tvOS 9, watchOS 2, Xcode 9 or later

The custom ISO8601 DateFormatter:

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     }() } 

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The custom 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)")     } } 

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The custom DateEncodingStrategy:

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

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edit/update:

Xcode 10 • Swift 4.2 or later • iOS 11.2.1 or later

ISO8601DateFormatter now supports 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     }() } 

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The customs DateDecodingStrategy and DateEncodingStrategy would be the same as shown above.

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

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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", } """ 

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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) }