marksands · September 11, 2020 15:53 · Sep 11, 2020
diff --git a/OptionalDateValue.swift b/OptionalDateValue.swift
@@ -0,0 +1,59 @@
+
+public protocol OptionalDateValueCodableStrategy {
+  associatedtype RawValue: Codable
+
+  static func decode(_ value: RawValue?) throws -> Date?
+  static func encode(_ date: Date?) -> RawValue?
+}
+
+@propertyWrapper
+public struct OptionalDateValue<Formatter: OptionalDateValueCodableStrategy>: Codable {
+  private let value: Formatter.RawValue?
+  public var wrappedValue: Date?
+
+  public init(wrappedValue: Date?) {
+    self.wrappedValue = wrappedValue
+    self.value = Formatter.encode(wrappedValue)
+  }
+
+  public init(from decoder: Decoder) throws {
+    self.value = try? Formatter.RawValue(from: decoder)
+    self.wrappedValue = try? Formatter.decode(value)
+  }
+
+  public func encode(to encoder: Encoder) throws {
+    try value.encode(to: encoder)
+  }
+}
+
+public struct OptionalTimestampStrategy: OptionalDateValueCodableStrategy {
+    public static func decode(_ value: TimeInterval?) throws -> Date? {
+      return value.map { Date(timeIntervalSince1970: $0) }
+    }
+
+    public static func encode(_ date: Date?) -> TimeInterval? {
+        return date?.timeIntervalSince1970
+    }
+}
+
+struct Fixture: Codable {
+  @OptionalDateValue<OptionalTimestampStrategy> var timestamp: Date?
+}
+
+func testDecodingTimestamp() {
+  let jsonData = #"{"timestamp": 851042397.0}"#.data(using: .utf8)!
+
+  let fixture = try! JSONDecoder().decode(Fixture.self, from: jsonData)
+  assert(fixture.timestamp == Date(timeIntervalSince1970: 851042397))
+  assert(fixture.timestamp?.timeIntervalSince1970 == 851042397)
+}
+
+func testDecodingNil() {
+  let jsonData = #"{"timestamp": null}"#.data(using: .utf8)!
+
+  let fixture = try! JSONDecoder().decode(Fixture.self, from: jsonData)
+  assert(fixture.timestamp == nil)
+}
+
+testDecodingTimestamp()
+testDecodingNil()