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

由 穿精又带淫゛_ 提交于 2019-11-27 02:07:46

问题


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?


回答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.




回答2:


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
}()



回答3:


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)



回答4:


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
}



回答5:


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.




回答6:


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.




回答7:


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




回答8:


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)


来源：https://stackoverflow.com/questions/44682626/swifts-jsondecoder-with-multiple-date-formats-in-a-json-string

标签
swift
json-deserialization
codable
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