[Swift4] Allow for custom dateformatter to be used (#6672)

* Add a custom dateformat to be set

* Samples updated

* Update CodableHelper.swift

Change to correct format: decoder.dataDecodingStrategy = .base64

* Update CodableHelper.mustache

Add `decoder.dataDecodingStrategy = .base64` back

* Update CodableHelper.swift

Fix `decoder.dataDecodingStrategy = .base64`

* Update CodableHelper.swift

Fix `decoder.dataDecodingStrategy = .base64`

Author: DawidvanGraan

modules/swagger-codegen/src/main/resources/swift4/CodableHelper.mustache

@@ -11,14 +11,20 @@ public typealias EncodeResult = (data: Data?, error: Error?)
 
 open class CodableHelper {
 
+    open static var dateformatter: DateFormatter?
+
     open class func decode<T>(_ type: T.Type, from data: Data) -> (decodableObj: T?, error: Error?) where T : Decodable {
         var returnedDecodable: T? = nil
         var returnedError: Error? = nil
 
         let decoder = JSONDecoder()
-        decoder.dataDecodingStrategy = .base64
-        if #available(iOS 10.0, *) {
-            decoder.dateDecodingStrategy = .iso8601
+        if let df = self.dateformatter {
+            decoder.dateDecodingStrategy = .formatted(df)
+        } else {
+            decoder.dataDecodingStrategy = .base64
+            if #available(iOS 10.0, *) {
+                decoder.dateDecodingStrategy = .iso8601
+            }
         }
 
         do {

samples/client/petstore/swift4/default/PetstoreClient/Classes/Swaggers/CodableHelper.swift

@@ -11,14 +11,20 @@ public typealias EncodeResult = (data: Data?, error: Error?)
 
 open class CodableHelper {
 
+    open static var dateformatter: DateFormatter?
+
     open class func decode<T>(_ type: T.Type, from data: Data) -> (decodableObj: T?, error: Error?) where T : Decodable {
         var returnedDecodable: T? = nil
         var returnedError: Error? = nil
 
         let decoder = JSONDecoder()
-        decoder.dataDecodingStrategy = .base64
-        if #available(iOS 10.0, *) {
-            decoder.dateDecodingStrategy = .iso8601
+        if let df = self.dateformatter {
+            decoder.dateDecodingStrategy = .formatted(df)
+        } else {
+            decoder.dataDecodingStrategy = .base64
+            if #available(iOS 10.0, *) {
+                decoder.dateDecodingStrategy = .iso8601
+            }
         }
 
         do {

samples/client/petstore/swift4/promisekit/PetstoreClient/Classes/Swaggers/CodableHelper.swift

@@ -11,14 +11,20 @@ public typealias EncodeResult = (data: Data?, error: Error?)
 
 open class CodableHelper {
 
+    open static var dateformatter: DateFormatter?
+
     open class func decode<T>(_ type: T.Type, from data: Data) -> (decodableObj: T?, error: Error?) where T : Decodable {
         var returnedDecodable: T? = nil
         var returnedError: Error? = nil
 
         let decoder = JSONDecoder()
-        decoder.dataDecodingStrategy = .base64
-        if #available(iOS 10.0, *) {
-            decoder.dateDecodingStrategy = .iso8601
+        if let df = self.dateformatter {
+            decoder.dateDecodingStrategy = .formatted(df)
+        } else {
+            decoder.dataDecodingStrategy = .base64
+            if #available(iOS 10.0, *) {
+                decoder.dateDecodingStrategy = .iso8601
+            }
         }
 
         do {

samples/client/petstore/swift4/rxswift/PetstoreClient/Classes/Swaggers/CodableHelper.swift

@@ -11,14 +11,20 @@ public typealias EncodeResult = (data: Data?, error: Error?)
 
 open class CodableHelper {
 
+    open static var dateformatter: DateFormatter?
+
     open class func decode<T>(_ type: T.Type, from data: Data) -> (decodableObj: T?, error: Error?) where T : Decodable {
         var returnedDecodable: T? = nil
         var returnedError: Error? = nil
 
         let decoder = JSONDecoder()
-        decoder.dataDecodingStrategy = .base64
-        if #available(iOS 10.0, *) {
-            decoder.dateDecodingStrategy = .iso8601
+        if let df = self.dateformatter {
+            decoder.dateDecodingStrategy = .formatted(df)
+        } else {
+            decoder.dataDecodingStrategy = .base64
+            if #available(iOS 10.0, *) {
+                decoder.dateDecodingStrategy = .iso8601
+            }
         }
 
         do {