Adds ISO8601WithFractionalSecondsStrategy

Sources/BetterCodable/ISO8601WithFractionalSecondsStrategy.swift

@@ -0,0 +1,29 @@
+import Foundation
+
+/// Decodes `String` values as an ISO8601 `Date`.
+///
+/// `@ISO8601Date` relies on an `ISO8601DateFormatter` in order to decode `String` values into `Date`s. Encoding the `Date`
+/// will encode the value into the original string value.
+///
+/// For example, decoding json data with a `String` representation  of `"1996-12-19T16:39:57-08:00"` produces a valid `Date`
+/// representing 39 minutes and 57 seconds after the 16th hour of December 19th, 1996 with an offset of -08:00 from UTC
+/// (Pacific Standard Time).
+@available(iOS 11.0, tvOS 11.0, watchOS 4.0, macOS 10.13, *)
+public struct ISO8601WithFractionalSecondsStrategy: DateValueCodableStrategy {
+    private static let formatter: ISO8601DateFormatter = {
+        let formatter = ISO8601DateFormatter()
+        formatter.formatOptions = [.withInternetDateTime, .withFractionalSeconds]
+        return formatter
+    }()
+
+    public static func decode(_ value: String) throws -> Date {
+        guard let date = Self.formatter.date(from: value) else {
+            throw DecodingError.dataCorrupted(.init(codingPath: [], debugDescription: "Invalid Date Format!"))
+        }
+        return date
+    }
+
+    public static func encode(_ date: Date) -> String {
+        return  Self.formatter.string(from: date)
+    }
+}

Tests/BetterCodableTests/DateValueTests.swift

@@ -11,7 +11,25 @@ class CustomDateCodableValueTests: XCTestCase {
         let fixture = try JSONDecoder().decode(Fixture.self, from: jsonData)
         XCTAssertEqual(fixture.iso8601, Date(timeIntervalSince1970: 851042397))
     }
-
+
+    @available(iOS 11.0, tvOS 11.0, watchOS 4.0, macOS 10.13, *)
+    func testDecodingAndEncodingISO8601DateStringWithFractionalSeconds() throws {
+        struct Fixture: Codable {
+            @DateValue<ISO8601WithFractionalSecondsStrategy> var iso8601: Date
+            @DateValue<ISO8601WithFractionalSecondsStrategy> var iso8601Short: Date
+        }
+        let jsonData = """
+        {
+          "iso8601": "1996-12-19T16:39:57.123456Z",
+          "iso8601Short": "1996-12-19T16:39:57.000Z-08:00"
+        }
+        """.data(using: .utf8)!
+
+        let fixture = try JSONDecoder().decode(Fixture.self, from: jsonData)
+        XCTAssertEqual(fixture.iso8601Short, Date(timeIntervalSince1970: 851013597.0))
+        XCTAssertEqual(fixture.iso8601, Date(timeIntervalSince1970: 851013597.123))
+    }
+
     func testDecodingAndEncodingRFC3339DateString() throws {
          struct Fixture: Codable {
             @DateValue<RFC3339Strategy> var rfc3339Date: Date