问题
I am replacing my old JSON parsing code with Swift\'s Codable and am running into a bit of a snag. I guess it isn\'t as much a Codable question as it is a DateFormatter question.
Start with a struct
struct JustADate: Codable {
var date: Date
}
and a json string
let json = \"\"\"
{ \"date\": \"2017-06-19T18:43:19Z\" }
\"\"\"
now lets decode
let decoder = JSONDecoder()
decoder.dateDecodingStrategy = .iso8601
let data = json.data(using: .utf8)!
let justADate = try! decoder.decode(JustADate.self, from: data) //all good
But if we change the date so that it has fractional seconds, for example:
let json = \"\"\"
{ \"date\": \"2017-06-19T18:43:19.532Z\" }
\"\"\"
Now it breaks. The dates sometimes come back with fractional seconds and sometimes do not. The way I used to solve it was in my mapping code I had a transform function that tried both dateFormats with and without the fractional seconds. I am not quite sure how to approach it using Codable however. Any suggestions?
回答1:
You can use two different date formatters (with and without fraction seconds) and create a custom DateDecodingStrategy. In case of failure when parsing the date returned by the API you can throw a DecodingError as suggested by @PauloMattos in comments:
iOS 9, macOS 10.9, tvOS 9, watchOS 2, Xcode 9 or later
The custom ISO8601 DateFormatter:
extension Formatter {
static let iso8601: DateFormatter = {
let formatter = DateFormatter()
formatter.calendar = Calendar(identifier: .iso8601)
formatter.locale = Locale(identifier: "en_US_POSIX")
formatter.timeZone = TimeZone(secondsFromGMT: 0)
formatter.dateFormat = "yyyy-MM-dd'T'HH:mm:ss.SSSXXXXX"
return formatter
}()
static let iso8601noFS: DateFormatter = {
let formatter = DateFormatter()
formatter.calendar = Calendar(identifier: .iso8601)
formatter.locale = Locale(identifier: "en_US_POSIX")
formatter.timeZone = TimeZone(secondsFromGMT: 0)
formatter.dateFormat = "yyyy-MM-dd'T'HH:mm:ssXXXXX"
return formatter
}()
}
The custom DateDecodingStrategy and Error:
extension JSONDecoder.DateDecodingStrategy {
static let customISO8601 = custom {
let container = try $0.singleValueContainer()
let string = try container.decode(String.self)
if let date = Formatter.iso8601.date(from: string) ?? Formatter.iso8601noFS.date(from: string) {
return date
}
throw DecodingError.dataCorruptedError(in: container, debugDescription: "Invalid date: \(string)")
}
}
The custom DateEncodingStrategy:
extension JSONEncoder.DateEncodingStrategy {
static let customISO8601 = custom {
var container = $1.singleValueContainer()
try container.encode(Formatter.iso8601.string(from: $0))
}
}
edit/update:
Xcode 10 • Swift 4.2 or later • iOS 11.2.1 or later
ISO8601DateFormatter now supports formatOptions .withFractionalSeconds in iOS11 or later:
extension Formatter {
static let iso8601: ISO8601DateFormatter = {
let formatter = ISO8601DateFormatter()
formatter.formatOptions = [.withInternetDateTime, .withFractionalSeconds]
return formatter
}()
static let iso8601noFS = ISO8601DateFormatter()
}
The customs DateDecodingStrategy and DateEncodingStrategy would be the same as shown above.
// Playground testing
struct ISODates: Codable {
let dateWith9FS: Date
let dateWith3FS: Date
let dateWith2FS: Date
let dateWithoutFS: Date
}
let isoDatesJSON = """
{
"dateWith9FS": "2017-06-19T18:43:19.532123456Z",
"dateWith3FS": "2017-06-19T18:43:19.532Z",
"dateWith2FS": "2017-06-19T18:43:19.53Z",
"dateWithoutFS": "2017-06-19T18:43:19Z",
}
"""
let isoDatesData = Data(isoDatesJSON.utf8)
let decoder = JSONDecoder()
decoder.dateDecodingStrategy = .customISO8601
do {
let isoDates = try decoder.decode(ISODates.self, from: isoDatesData)
print(Formatter.iso8601.string(from: isoDates.dateWith9FS)) // 2017-06-19T18:43:19.532Z
print(Formatter.iso8601.string(from: isoDates.dateWith3FS)) // 2017-06-19T18:43:19.532Z
print(Formatter.iso8601.string(from: isoDates.dateWith2FS)) // 2017-06-19T18:43:19.530Z
print(Formatter.iso8601.string(from: isoDates.dateWithoutFS)) // 2017-06-19T18:43:19.000Z
} catch {
print(error)
}
回答2:
Alternatively to @Leo's answer, and if you need to provide support for older OS'es (ISO8601DateFormatter is available only starting with iOS 10, mac OS 10.12), you can write a custom formatter that uses both formats when parsing the string:
class MyISO8601Formatter: DateFormatter {
static let formatters: [DateFormatter] = [
iso8601Formatter(withFractional: true),
iso8601Formatter(withFractional: false)
]
static func iso8601Formatter(withFractional fractional: Bool) -> DateFormatter {
let formatter = DateFormatter()
formatter.calendar = Calendar(identifier: .iso8601)
formatter.locale = Locale(identifier: "en_US_POSIX")
formatter.timeZone = TimeZone(secondsFromGMT: 0)
formatter.dateFormat = "yyyy-MM-dd'T'HH:mm:ss\(fractional ? ".SSS" : "")XXXXX"
return formatter
}
override public func getObjectValue(_ obj: AutoreleasingUnsafeMutablePointer<AnyObject?>?,
for string: String,
errorDescription error: AutoreleasingUnsafeMutablePointer<NSString?>?) -> Bool {
guard let date = (type(of: self).formatters.flatMap { $0.date(from: string) }).first else {
error?.pointee = "Invalid ISO8601 date: \(string)" as NSString
return false
}
obj?.pointee = date as NSDate
return true
}
override public func string(for obj: Any?) -> String? {
guard let date = obj as? Date else { return nil }
return type(of: self).formatters.flatMap { $0.string(from: date) }.first
}
}
, which you can use it as date decoding strategy:
let decoder = JSONDecoder()
decoder.dateDecodingStrategy = .formatted(MyISO8601Formatter())
Although a little bit uglier in implementation, this has the advantage of being consistent with the decoding errors that Swift throws in case of malformed data, as we don't alter the error reporting mechanism).
For example:
struct TestDate: Codable {
let date: Date
}
// I don't advocate the forced unwrap, this is for demo purposes only
let jsonString = "{\"date\":\"2017-06-19T18:43:19Z\"}"
let jsonData = jsonString.data(using: .utf8)!
let decoder = JSONDecoder()
decoder.dateDecodingStrategy = .formatted(MyISO8601Formatter())
do {
print(try decoder.decode(TestDate.self, from: jsonData))
} catch {
print("Encountered error while decoding: \(error)")
}
will print TestDate(date: 2017-06-19 18:43:19 +0000)
Adding the fractional part
let jsonString = "{\"date\":\"2017-06-19T18:43:19.123Z\"}"
will result in the same output: TestDate(date: 2017-06-19 18:43:19 +0000)
However using an incorrect string:
let jsonString = "{\"date\":\"2017-06-19T18:43:19.123AAA\"}"
will print the default Swift error in case of incorrect data:
Encountered error while decoding: dataCorrupted(Swift.DecodingError.Context(codingPath: [__lldb_expr_84.TestDate.(CodingKeys in _B178608BE4B4E04ECDB8BE2F689B7F4C).date], debugDescription: "Date string does not match format expected by formatter.", underlyingError: nil))
回答3:
A new option (as of Swift 5.1) is a Property Wrapper. The CodableWrappers library has an easy way to deal with this.
For default ISO8601
@ISO8601DateCoding
struct JustADate: Codable {
var date: Date
}
If you want a custom version:
// Custom coder
@available(macOS 10.12, iOS 10.0, watchOS 3.0, tvOS 10.0, *)
public struct FractionalSecondsISO8601DateStaticCoder: StaticCoder {
private static let iso8601Formatter: ISO8601DateFormatter = {
let formatter = ISO8601DateFormatter()
formatter.formatOptions = .withFractionalSeconds
return formatter
}()
public static func decode(from decoder: Decoder) throws -> Date {
let stringValue = try String(from: decoder)
guard let date = iso8601Formatter.date(from: stringValue) else {
throw DecodingError.dataCorrupted(DecodingError.Context(codingPath: decoder.codingPath, debugDescription: "Expected date string to be ISO8601-formatted."))
}
return date
}
public static func encode(value: Date, to encoder: Encoder) throws {
try iso8601Formatter.string(from: value).encode(to: encoder)
}
}
// Property Wrapper alias
public typealias ISO8601FractionalDateCoding = CodingUses<FractionalSecondsISO8601DateStaticCoder>
// Usage
@ISO8601FractionalDateCoding
struct JustADate: Codable {
var date: Date
}
来源：https://stackoverflow.com/questions/46458487/how-to-convert-a-date-string-with-optional-fractional-seconds-using-codable-in-s