Decoding custom date formats using Decodable

KeyedDecodingContainer.swift

extension KeyedDecodingContainer where Key: CodingKey {
    func decodeDate(from key: Key, using formats: String...) throws -> Date {
        let dateAsString = try decode(String.self, forKey: key)
        let dateFormatter = DateFormatter()
        for format in formats {
            dateFormatter.dateFormat = format
            guard let date = dateFormatter.date(from: dateAsString) else {
                continue
            }
            return date
        }
        let context = DecodingError.Context(codingPath: [], debugDescription: "Cannot decode date: \(dateAsString)")
        throw DecodingError.dataCorrupted(context)
    }
}

String.swift

extension String {
    static let iso8601format = "yyyy-MM-dd'T'HH:mm:ss'Z'"
    static let iso8601fractionalFormat = "yyyy-MM-dd'T'HH:mm:ss.S'Z'"
    static let customDate = "MM/dd/yyyy HH:mm:ss"
}

Usage.md

Usage

struct DateWrapper {
    let dateISO8601: Date
    let dateISO8601withFractionalSeconds: Date
    let customDate: Date
}

extension DateWrapper {
    enum CodingKeys: String, CodingKey {
        case dateISO8601, dateISO8601withFractionalSeconds, customDate
    }
}

extension DateWrapper: Decodable {
    init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self)
        dateISO8601 = try container.decodeDate(from: .dateISO8601, using: .iso8601format)
        dateISO8601withFractionalSeconds = try container.decodeDate(from: .dateISO8601withFractionalSeconds, using: .iso8601fractionalFormat)
        customDate = try container.decodeDate(from: .customDate, using: .customDate)
    }
}

let json: String = """
{
  "dateISO8601": "2019-03-14T12:53:26",
  "dateISO8601withFractionalSeconds": "2019-03-14T12:54:58.28",
  "customDate": "03/12/2019 11:13:00"
}
"""

let jsonDecoder = JSONDecoder()
let jsonData = Data(json.utf8)
do {
    let parsedDate = try jsonDecoder.decode(DateWrapper.self, from: jsonData)
    print(parsedDate)
} catch {
    print(error)
}