Swift's JSONDecoder with multiple date formats in a JSON string?

Question

Swift\'s JSONDecoder offers a dateDecodingStrategy property, which allows us to define how to interpret incoming date strings in accordance with a

Answer 1

Swift 5

Actually based on @BrownsooHan version using a JSONDecoder extension

JSONDecoder+dateDecodingStrategyFormatters.swift

extension JSONDecoder {

    /// Assign multiple DateFormatter to dateDecodingStrategy
    ///
    /// Usage :
    ///
    ///      decoder.dateDecodingStrategyFormatters = [ DateFormatter.standard, DateFormatter.yearMonthDay ]
    ///
    /// The decoder will now be able to decode two DateFormat, the 'standard' one and the 'yearMonthDay'
    ///
    /// Throws a 'DecodingError.dataCorruptedError' if an unsupported date format is found while parsing the document
    var dateDecodingStrategyFormatters: [DateFormatter]? {
        @available(*, unavailable, message: "This variable is meant to be set only")
        get { return nil }
        set {
            guard let formatters = newValue else { return }
            self.dateDecodingStrategy = .custom { decoder in

                let container = try decoder.singleValueContainer()
                let dateString = try container.decode(String.self)

                for formatter in formatters {
                    if let date = formatter.date(from: dateString) {
                        return date
                    }
                }

                throw DecodingError.dataCorruptedError(in: container, debugDescription: "Cannot decode date string \(dateString)")
            }
        }
    }
}

It is a bit of a hacky way to add a variable that can only be set, but you can easily transform var dateDecodingStrategyFormatters by func setDateDecodingStrategyFormatters(_ formatters: [DateFormatter]? )

Usage

lets say that you have already defined several DateFormatters in your code like so :

extension DateFormatter {
    static let standardT: DateFormatter = {
        var dateFormatter = DateFormatter()
        dateFormatter.dateFormat = "yyyy-MM-dd'T'HH:mm:ss"
        return dateFormatter
    }()

    static let standard: DateFormatter = {
        var dateFormatter = DateFormatter()
        dateFormatter.dateFormat = "yyyy-MM-dd HH:mm:ss"
        return dateFormatter
    }()

    static let yearMonthDay: DateFormatter = {
        var dateFormatter = DateFormatter()
        dateFormatter.dateFormat = "yyyy-MM-dd"
        return dateFormatter
    }()
}

you can now just assign these to the decoder straight away by setting dateDecodingStrategyFormatters :

// Data structure
struct Dates: Codable {
    var date1: Date
    var date2: Date
    var date3: Date
}

// The Json to decode 
let jsonData = """
{
    "date1": "2019-05-30 15:18:00",
    "date2": "2019-05-30T05:18:00",
    "date3": "2019-04-17"
}
""".data(using: .utf8)!

// Assigning mutliple DateFormatters
let decoder = JSONDecoder()
decoder.dateDecodingStrategyFormatters = [ DateFormatter.standardT,
                                           DateFormatter.standard,
                                           DateFormatter.yearMonthDay ]


do {
    let dates = try decoder.decode(Dates.self, from: jsonData)
    print(dates)
} catch let err as DecodingError {
    print(err.localizedDescription)
}

Sidenotes

Once again I am aware that setting the dateDecodingStrategyFormatters as a var is a bit hacky, and I dont recommend it, you should define a function instead. However it is a personal preference to do so.

Answer 2

There is no way to do this with a single encoder. Your best bet here is to customize the encode(to encoder:) and init(from decoder:) methods and provide your own translation for one these values, leaving the built-in date strategy for the other one.

It might be worthwhile looking into passing one or more formatters into the userInfo object for this purpose.