Test case and example for using date with a propertyWrapper. (#17)

Author: fabianfett

README.md

@@ -35,7 +35,7 @@ Add `pure-swift-json` as dependency to your `Package.swift`:
 
 ```swift
   dependencies: [
-    .package(url: "https://github.com/fabianfett/pure-swift-json.git", .upToNextMajore(from: "0.1.0")),
+    .package(url: "https://github.com/fabianfett/pure-swift-json.git", .upToNextMajor(from: "0.1.0")),
   ],
 ```
 
@@ -125,7 +125,7 @@ struct MyEvent: Decodable {
     guard let timestamp = MyEvent.dateFormatter.date(from: dateString) else {
       let dateFormat = String(describing: MyEvent.dateFormatter.dateFormat)
       throw DecodingError.dataCorruptedError(forKey: .eventTime, in: container, debugDescription:
-        "Expected date to be in format `\(dateFormat)`, but `\(dateFormat) does not forfill format`")
+        "Expected date to be in format `\(dateFormat)`, but `\(dateString) does not forfill format`")
     }
     self.eventTime = timestamp
   }
@@ -143,6 +143,48 @@ struct MyEvent: Decodable {
 
 You can find more information about [encoding and decoding custom types in Apple's documentation](https://developer.apple.com/documentation/foundation/archives_and_serialization/encoding_and_decoding_custom_types).
 
+Of course you can use `@propertyWrapper`s to make this more elegant:
+
+```swift
+import Foundation
+
+@propertyWrapper
+struct DateStringCoding: Decodable {
+  var wrappedValue: Date
+  
+  init(wrappedValue: Date) {
+    self.wrappedValue = wrappedValue
+  }
+
+  init(from decoder: Decoder) throws {
+    let container = try decoder.singleValueContainer()
+    let dateString = try container.decode(String.self)
+    guard let date = Self.dateFormatter.date(from: dateString) else {
+      let dateFormat = String(describing: Self.dateFormatter.dateFormat)
+      throw DecodingError.dataCorruptedError(in: container, debugDescription:
+            "Expected date to be in format `\(dateFormat)`, but `\(dateString) does not forfill format`")
+    }
+    self.wrappedValue = date
+  }
+
+  private static let dateFormatter: DateFormatter = Self.createDateFormatter()
+  private static func createDateFormatter() -> DateFormatter {
+    let formatter = DateFormatter()
+    formatter.dateFormat = "yyyy-MM-dd'T'HH:mm:ss.SSSZ"
+    formatter.timeZone   = TimeZone(secondsFromGMT: 0)
+    formatter.locale     = Locale(identifier: "en_US_POSIX")
+    return formatter
+  }
+}
+
+struct MyEvent: Decodable {
+  @DateStringCoding
+  var eventTime: Date
+}
+```
+
+Checkout the a full example in the test file [DateCodingTests](https://github.com/fabianfett/pure-swift-json/blob/master/Tests/JSONCodingTests/DateCodingTests.swift).
+
 ### UTF-16 and UTF-32
 
 If your input is [UTF-16](https://en.wikipedia.org/wiki/UTF-16) or [UTF-32](https://en.wikipedia.org/wiki/UTF-32) encoded, you can easily convert it to UTF-8: 
@@ -173,3 +215,4 @@ Focus areas for the time being:
 - [@weissi](https://github.com/weissi) thanks for answering all my questions and for opening tickets [SR-12125](https://bugs.swift.org/browse/SR-12125) and [SR-12126](https://bugs.swift.org/browse/SR-12126)
 - [@dinhhungle](https://github.com/dinhhungle) thanks for your quality assurance. It helped a lot! 
 - [@Ro-M](https://github.com/Ro-M) thanks for checking my README.md
+- [@Trzyipolkostkicukru](https://github.com/Trzyipolkostkicukru) thanks for your advice on `@propertyWrappers` and for finding typos.

Tests/JSONCodingTests/DateCodingTests.swift

@@ -0,0 +1,79 @@
+import XCTest
+@testable import PureSwiftJSONCoding
+
+class DateCodingTests: XCTestCase {
+
+  @propertyWrapper
+  struct DateStringCoding: Codable {
+    var wrappedValue: Date
+    
+    init(wrappedValue: Date) {
+      self.wrappedValue = wrappedValue
+    }
+
+    init(from decoder: Decoder) throws {
+      let container = try decoder.singleValueContainer()
+      let dateString = try container.decode(String.self)
+      guard let date = Self.dateFormatter.date(from: dateString) else {
+        let dateFormat = String(describing: Self.dateFormatter.dateFormat)
+        throw DecodingError.dataCorruptedError(in: container, debugDescription:
+              "Expected date to be in format `\(dateFormat)`, but `\(dateString) does not forfill format`")
+      }
+      self.wrappedValue = date
+    }
+    
+    func encode(to encoder: Encoder) throws {
+      var container = encoder.singleValueContainer()
+      let dateString = Self.dateFormatter.string(from: self.wrappedValue)
+      try container.encode(dateString)
+    }
+
+    private static let dateFormatter: DateFormatter = Self.createDateFormatter()
+    private static func createDateFormatter() -> DateFormatter {
+      let formatter = DateFormatter()
+      formatter.dateFormat = "yyyy-MM-dd'T'HH:mm:ss.SSS'Z'"
+      formatter.timeZone   = TimeZone(secondsFromGMT: 0)
+      formatter.locale     = Locale(identifier: "en_US_POSIX")
+      return formatter
+    }
+  }
+  
+  struct MyEvent: Codable {
+    @DateStringCoding
+    var eventTime: Date
+  }
+  
+  func testDecodeDatePropertyWrapper() {
+    do {
+      let dateString = "2020-03-18T13:11:10.000Z"
+      let json = #"{"eventTime": "\#(dateString)"}"#
+      let result = try PureSwiftJSONCoding.JSONDecoder().decode(MyEvent.self, from: [UInt8](json.utf8))
+      
+      let components = DateComponents(
+        calendar: Calendar(identifier: .gregorian), timeZone: TimeZone(secondsFromGMT: 0),
+        year: 2020, month: 03, day: 18, hour: 13, minute: 11, second: 10, nanosecond: 0)
+      XCTAssertEqual(result.eventTime, components.date)
+    }
+    catch {
+      XCTFail("Unexpected error: \(error)")
+    }
+  }
+  
+  func testEncodeDatePropertyWrapper() {
+    do {
+      let dateString = "2020-03-18T13:11:10.000Z"
+      let json = #"{"eventTime":"\#(dateString)"}"#
+      
+      let components = DateComponents(
+        calendar: Calendar(identifier: .gregorian), timeZone: TimeZone(secondsFromGMT: 0),
+        year: 2020, month: 03, day: 18, hour: 13, minute: 11, second: 10, nanosecond: 0)
+      
+      let event = MyEvent(eventTime: components.date!)
+      let bytes = try PureSwiftJSONCoding.JSONEncoder().encode(event)
+      XCTAssertEqual(String(decoding: bytes, as: Unicode.UTF8.self), json)
+    }
+    catch {
+      XCTFail("Unexpected error: \(error)")
+    }
+  }
+}