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

There are a few ways to deal with this:

• You can create a DateFormatter subclass which first attempts the date-time string format, then if it fails, attempts the plain date format
• You can give a .custom Date decoding strategy wherein you ask the Decoder for a singleValueContainer(), decode a string, and pass it through whatever formatters you want before passing the parsed date out
• You can create a wrapper around the Date type which provides a custom init(from:) and encode(to:) which does this (but this isn't really any better than a .custom strategy)
• You can use plain strings, as you suggest
• You can provide a custom init(from:) on all types which use these dates and attempt different things in there

All in all, the first two methods are likely going to be the easiest and cleanest — you'll keep the default synthesized implementation of Codable everywhere without sacrificing type safety.

Answer 2

Facing this same issue, I wrote the following extension:

extension JSONDecoder.DateDecodingStrategy {
    static func custom(_ formatterForKey: @escaping (CodingKey) throws -> DateFormatter?) -> JSONDecoder.DateDecodingStrategy {
        return .custom({ (decoder) -> Date in
            guard let codingKey = decoder.codingPath.last else {
                throw DecodingError.dataCorrupted(DecodingError.Context(codingPath: decoder.codingPath, debugDescription: "No Coding Path Found"))
            }

            guard let container = try? decoder.singleValueContainer(),
                let text = try? container.decode(String.self) else {
                    throw DecodingError.dataCorrupted(DecodingError.Context(codingPath: decoder.codingPath, debugDescription: "Could not decode date text"))
            }

            guard let dateFormatter = try formatterForKey(codingKey) else {
                throw DecodingError.dataCorruptedError(in: container, debugDescription: "No date formatter for date text")
            }

            if let date = dateFormatter.date(from: text) {
                return date
            } else {
                throw DecodingError.dataCorruptedError(in: container, debugDescription: "Cannot decode date string \(text)")
            }
        })
    }
}

This extension allows you to create a DateDecodingStrategy for the JSONDecoder that handles multiple different date formats within the same JSON String. The extension contains a function that requires the implementation of a closure that gives you a CodingKey, and it is up to you to provide the correct DateFormatter for the provided key.

Lets say that you have the following JSON:

{
    "publication_date": "2017-11-02",
    "opening_date": "2017-11-03",
    "date_updated": "2017-11-08 17:45:14"
}

The following Struct:

struct ResponseDate: Codable {
    var publicationDate: Date
    var openingDate: Date?
    var dateUpdated: Date

    enum CodingKeys: String, CodingKey {
        case publicationDate = "publication_date"
        case openingDate = "opening_date"
        case dateUpdated = "date_updated"
    }
}

Then to decode the JSON, you would use the following code:

let dateFormatterWithTime: DateFormatter = {
    let formatter = DateFormatter()

    formatter.dateFormat = "yyyy-MM-dd HH:mm:ss"

    return formatter
}()

let dateFormatterWithoutTime: DateFormatter = {
    let formatter = DateFormatter()

    formatter.dateFormat = "yyyy-MM-dd"

    return formatter
}()

let decoder = JSONDecoder()

decoder.dateDecodingStrategy = .custom({ (key) -> DateFormatter? in
    switch key {
    case ResponseDate.CodingKeys.publicationDate, ResponseDate.CodingKeys.openingDate:
        return dateFormatterWithoutTime
    default:
        return dateFormatterWithTime
    }
})

let results = try? decoder.decode(ResponseDate.self, from: data)

Answer 3

It is a little verbose, but more flexible approach: wrap date with another Date class, and implement custom serialize methods for it. For example:

let dateFormatter = DateFormatter()
dateFormatter.dateFormat = "yyyy-MM-dd"

class MyCustomDate: Codable {
    var date: Date

    required init?(_ date: Date?) {
        if let date = date {
            self.date = date
        } else {
            return nil
        }
    }

    public func encode(to encoder: Encoder) throws {
        var container = encoder.singleValueContainer()
        let string = dateFormatter.string(from: date)
        try container.encode(string)
    }

    required public init(from decoder: Decoder) throws {
        let container = try decoder.singleValueContainer()
        let raw = try container.decode(String.self)
        if let date = dateFormatter.date(from: raw) {
            self.date = date
        } else {
            throw DecodingError.dataCorruptedError(in: container, debugDescription: "Cannot parse date")
        }
    }
}

So now you are independent of .dateDecodingStrategy and .dateEncodingStrategy and your MyCustomDate dates will parsed with specified format. Use it in class:

class User: Codable {
    var dob: MyCustomDate
}

Instantiate with

user.dob = MyCustomDate(date)

Answer 4

Please try decoder configurated similarly to this:

lazy var decoder: JSONDecoder = {
    let decoder = JSONDecoder()
    decoder.dateDecodingStrategy = .custom({ (decoder) -> Date in
        let container = try decoder.singleValueContainer()
        let dateStr = try container.decode(String.self)
        // possible date strings: "2016-05-01",  "2016-07-04T17:37:21.119229Z", "2018-05-20T15:00:00Z"
        let len = dateStr.count
        var date: Date? = nil
        if len == 10 {
            date = dateNoTimeFormatter.date(from: dateStr)
        } else if len == 20 {
            date = isoDateFormatter.date(from: dateStr)
        } else {
            date = self.serverFullDateFormatter.date(from: dateStr)
        }
        guard let date_ = date else {
            throw DecodingError.dataCorruptedError(in: container, debugDescription: "Cannot decode date string \(dateStr)")
        }
        print("DATE DECODER \(dateStr) to \(date_)")
        return date_
    })
    return decoder
}()

Answer 5

If you have multiple dates with different formats in single model, its bit difficult to apply .dateDecodingStrategy for each dates.

Check here https://gist.github.com/romanroibu/089ec641757604bf78a390654c437cb0 for a handy solution

Answer 6

try this. (swift 4)

let formatter = DateFormatter()

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

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