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 DateFormatter object.

However, I am currently working with an API that returns both date strings (yyyy-MM-dd) and datetime strings (yyyy-MM-dd HH:mm:ss), depending on the property. Is there a way to have the JSONDecoder handle this, since the provided DateFormatter object can only deal with a single dateFormat at a time?

One ham-handed solution is to rewrite the accompanying Decodable models to just accept strings as their properties and to provide public Date getter/setter variables, but that seems like a poor solution to me. Any thoughts?

Answer 1

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 2

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 3

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
}