Present null for property with nil value

README.md

@@ -683,7 +683,19 @@ class BasicTypes: HandyJSON {
 }
 ```
 
+## Q: How to present a `null` value of `nil` property when serialize to JSON?
 
+In `mapping(mapper: HelpingMapper)`, use `mapper.nullPresent` to apply null present for property
+
+```swift
+class Animal: HandyJSON {
+    var name: String?
+    var owner: String?
+    func mapping(mapper: HelpingMapper) {
+        mapper.nullPresent(self.owner)
+    }
+}
+```
 
 # License
 

Source/ExtendCustomModelType.swift

@@ -235,7 +235,7 @@ extension _ExtendCustomModelType {
         for (key, property) in properties {
             var realKey = key
             var realValue = property.0
-
+            var useNullValueIfNeeded = false
             if let info = property.1 {
                 if info.bridged, let _value = (instance as! NSObject).value(forKey: key) {
                     realValue = _value
@@ -245,6 +245,10 @@ extension _ExtendCustomModelType {
                     continue
                 }
 
+                if mapper.propertyPresentNullValue(key: Int(bitPattern: info.address)) {
+                    useNullValueIfNeeded = true
+                }
+
                 if let mappingHandler = mapper.getMappingHandler(key: Int(bitPattern: info.address)) {
                     // if specific key is set, replace the label
                     if let mappingPaths = mappingHandler.mappingPaths, mappingPaths.count > 0 {
@@ -266,6 +270,10 @@ extension _ExtendCustomModelType {
                     dict[realKey] = result
                     continue
                 }
+                else if useNullValueIfNeeded {
+                    dict[realKey] = NSNull()
+                    continue
+                }
             }
 
             InternalLogger.logDebug("The value for key: \(key) is not transformable type")

Source/HelpingMapper.swift

@@ -85,6 +85,7 @@ public class HelpingMapper {
 
     private var mappingHandlers = [Int: MappingPropertyHandler]()
     private var excludeProperties = [Int]()
+    private var nullPresentProperties = [Int]()
 
     internal func getMappingHandler(key: Int) -> MappingPropertyHandler? {
         return self.mappingHandlers[key]
@@ -94,6 +95,10 @@ public class HelpingMapper {
         return self.excludeProperties.contains(key)
     }
 
+    internal func propertyPresentNullValue(key: Int) -> Bool {
+        return self.nullPresentProperties.contains(key)
+    }
+
     public func specify<T>(property: inout T, name: String) {
         self.specify(property: &property, name: name, converter: nil)
     }
@@ -128,6 +133,14 @@ public class HelpingMapper {
         self._exclude(property: &property)
     }
 
+    public func nullPresent<T>(property: inout T) {
+        let pointer = withUnsafePointer(to: &property, { return $0 })
+        let iPointer = Int(bitPattern: pointer)
+        if !self.excludeProperties.contains(iPointer) && !nullPresentProperties.contains(iPointer) {
+            nullPresentProperties.append(iPointer)
+        }
+    }
+
     fileprivate func addCustomMapping(key: Int, mappingInfo: MappingPropertyHandler) {
         self.mappingHandlers[key] = mappingInfo
     }

Tests/HandyJSONTests/OtherFeaturesTest.swift

@@ -404,4 +404,19 @@ class StructObjectTest: XCTestCase {
         let a = A.deserialize(from: jsonString)!
         XCTAssertEqual(a.upperName, "HANDYJSON")
     }
+
+    func testNullPresentValue() {
+        class A: HandyJSON {
+            var name: String?
+            required init() {}
+
+            func mapping(mapper: HelpingMapper) {
+                mapper.nullPresent(property: &self.name)
+            }
+        }
+
+        let a = A()
+        let jsonString = a.toJSONString()!
+        XCTAssertEqual(jsonString, "{\"name\":null}")
+    }
 }