diff --git a/Source/BuiltInBasicType.swift b/Source/BuiltInBasicType.swift
index 8e44c18..4ea131d 100644
--- a/Source/BuiltInBasicType.swift
+++ b/Source/BuiltInBasicType.swift
@@ -21,8 +21,8 @@ import Foundation
 
 protocol _BuiltInBasicType: _Transformable {
 
-    static func _transform(from object: Any) -> Self?
-    func _plainValue() -> Any?
+    static func _transform(from object: Any, transformer: _Transformer?) -> Self?
+    func _plainValue(transformer: _Transformer?) -> Any?
 }
 
 // Suppport integer type
@@ -34,7 +34,10 @@ protocol IntegerPropertyProtocol: FixedWidthInteger, _BuiltInBasicType {
 
 extension IntegerPropertyProtocol {
 
-    static func _transform(from object: Any) -> Self? {
+    static func _transform(from object: Any, transformer: _Transformer?) -> Self? {
+        if let result = transformer?.transform(from: object, type: self) {
+            return result
+        }
         switch object {
         case let str as String:
             return Self(str, radix: 10)
@@ -45,7 +48,10 @@ extension IntegerPropertyProtocol {
         }
     }
     
-    func _plainValue() -> Any? {
+    func _plainValue(transformer: _Transformer?) -> Any? {
+        if let result = transformer?.plainValue(from: self) {
+            return result
+        }
         return self
     }
 }
@@ -63,7 +69,10 @@ extension UInt64: IntegerPropertyProtocol {}
 
 extension Bool: _BuiltInBasicType {
 
-    static func _transform(from object: Any) -> Bool? {
+    static func _transform(from object: Any, transformer: _Transformer?) -> Bool? {
+        if let result = transformer?.transform(from: object, type: self) {
+            return result
+        }
         switch object {
         case let str as NSString:
             let lowerCase = str.lowercased
@@ -81,7 +90,10 @@ extension Bool: _BuiltInBasicType {
         }
     }
 
-    func _plainValue() -> Any? {
+    func _plainValue(transformer: _Transformer?) -> Any? {
+        if let result = transformer?.plainValue(from: self) {
+            return result
+        }
         return self
     }
 }
@@ -94,7 +106,10 @@ protocol FloatPropertyProtocol: _BuiltInBasicType, LosslessStringConvertible {
 
 extension FloatPropertyProtocol {
 
-    static func _transform(from object: Any) -> Self? {
+    static func _transform(from object: Any, transformer: _Transformer?) -> Self? {
+        if let result = transformer?.transform(from: object, type: self) {
+            return result
+        }
         switch object {
         case let str as String:
             return Self(str)
@@ -105,7 +120,10 @@ extension FloatPropertyProtocol {
         }
     }
 
-    func _plainValue() -> Any? {
+    func _plainValue(transformer: _Transformer?) -> Any? {
+        if let result = transformer?.plainValue(from: self) {
+            return result
+        }
         return self
     }
 }
@@ -123,7 +141,10 @@ fileprivate let formatter: NumberFormatter = {
 
 extension String: _BuiltInBasicType {
 
-    static func _transform(from object: Any) -> String? {
+    static func _transform(from object: Any, transformer: _Transformer?) -> String? {
+        if let result = transformer?.transform(from: object, type: self) {
+            return result
+        }
         switch object {
         case let str as String:
             return str
@@ -144,7 +165,10 @@ extension String: _BuiltInBasicType {
         }
     }
 
-    func _plainValue() -> Any? {
+    func _plainValue(transformer: _Transformer?) -> Any? {
+        if let result = transformer?.plainValue(from: self) {
+            return result
+        }
         return self
     }
 }
@@ -153,8 +177,11 @@ extension String: _BuiltInBasicType {
 
 extension Optional: _BuiltInBasicType {
 
-    static func _transform(from object: Any) -> Optional? {
-        if let value = (Wrapped.self as? _Transformable.Type)?.transform(from: object) as? Wrapped {
+    static func _transform(from object: Any, transformer: _Transformer?) -> Optional? {
+        if let result = transformer?.transform(from: object, type: self) {
+            return result
+        }
+        if let value = (Wrapped.self as? _Transformable.Type)?.transform(from: object, transformer: transformer) as? Wrapped {
             return Optional(value)
         } else if let value = object as? Wrapped {
             return Optional(value)
@@ -168,10 +195,13 @@ extension Optional: _BuiltInBasicType {
         })
     }
 
-    func _plainValue() -> Any? {
+    func _plainValue(transformer: _Transformer?) -> Any? {
+        if let result = transformer?.plainValue(from: self) {
+            return result
+        }
         if let value = _getWrappedValue() {
             if let transformable = value as? _Transformable {
-                return transformable.plainValue()
+                return transformable.plainValue(transformer: transformer)
             } else {
                 return value
             }
@@ -184,7 +214,7 @@ extension Optional: _BuiltInBasicType {
 
 extension Collection {
 
-    static func _collectionTransform(from object: Any) -> [Iterator.Element]? {
+    static func _collectionTransform(from object: Any, transformer: _Transformer?) -> [Iterator.Element]? {
         guard let arr = object as? [Any] else {
             InternalLogger.logDebug("Expect object to be an array but it's not")
             return nil
@@ -192,7 +222,7 @@ extension Collection {
         typealias Element = Iterator.Element
         var result: [Element] = [Element]()
         arr.forEach { (each) in
-            if let element = (Element.self as? _Transformable.Type)?.transform(from: each) as? Element {
+            if let element = (Element.self as? _Transformable.Type)?.transform(from: each, transformer: transformer) as? Element {
                 result.append(element)
             } else if let element = each as? Element {
                 result.append(element)
@@ -201,11 +231,14 @@ extension Collection {
         return result
     }
 
-    func _collectionPlainValue() -> Any? {
+    func _collectionPlainValue(transformer: _Transformer?) -> Any? {
+        if let result = transformer?.plainValue(from: self) {
+            return result
+        }
         typealias Element = Iterator.Element
         var result: [Any] = [Any]()
         self.forEach { (each) in
-            if let transformable = each as? _Transformable, let transValue = transformable.plainValue() {
+            if let transformable = each as? _Transformable, let transValue = transformable.plainValue(transformer: transformer) {
                 result.append(transValue)
             } else {
                 InternalLogger.logError("value: \(each) isn't transformable type!")
@@ -217,26 +250,32 @@ extension Collection {
 
 extension Array: _BuiltInBasicType {
 
-    static func _transform(from object: Any) -> [Element]? {
-        return self._collectionTransform(from: object)
+    static func _transform(from object: Any, transformer: _Transformer?) -> [Element]? {
+        if let result = transformer?.transform(from: object, type: self) {
+            return result
+        }
+        return self._collectionTransform(from: object, transformer: transformer)
     }
 
-    func _plainValue() -> Any? {
-        return self._collectionPlainValue()
+    func _plainValue(transformer: _Transformer?) -> Any? {
+        return self._collectionPlainValue(transformer: transformer)
     }
 }
 
 extension Set: _BuiltInBasicType {
 
-    static func _transform(from object: Any) -> Set<Element>? {
-        if let arr = self._collectionTransform(from: object) {
+    static func _transform(from object: Any, transformer: _Transformer?) -> Set<Element>? {
+        if let result = transformer?.transform(from: object, type: self) {
+            return result
+        }
+        if let arr = self._collectionTransform(from: object, transformer: transformer) {
             return Set(arr)
         }
         return nil
     }
 
-    func _plainValue() -> Any? {
-        return self._collectionPlainValue()
+    func _plainValue(transformer: _Transformer?) -> Any? {
+        return self._collectionPlainValue(transformer: transformer)
     }
 }
 
@@ -244,7 +283,10 @@ extension Set: _BuiltInBasicType {
 
 extension Dictionary: _BuiltInBasicType {
 
-    static func _transform(from object: Any) -> [Key: Value]? {
+    static func _transform(from object: Any, transformer: _Transformer?) -> [Key: Value]? {
+        if let result = transformer?.transform(from: object, type: self) {
+            return result
+        }
         guard let dict = object as? [String: Any] else {
             InternalLogger.logDebug("Expect object to be an NSDictionary but it's not")
             return nil
@@ -252,7 +294,7 @@ extension Dictionary: _BuiltInBasicType {
         var result = [Key: Value]()
         for (key, value) in dict {
             if let sKey = key as? Key {
-                if let nValue = (Value.self as? _Transformable.Type)?.transform(from: value) as? Value {
+                if let nValue = (Value.self as? _Transformable.Type)?.transform(from: value, transformer: transformer) as? Value {
                     result[sKey] = nValue
                 } else if let nValue = value as? Value {
                     result[sKey] = nValue
@@ -262,12 +304,15 @@ extension Dictionary: _BuiltInBasicType {
         return result
     }
 
-    func _plainValue() -> Any? {
+    func _plainValue(transformer: _Transformer?) -> Any? {
+        if let result = transformer?.plainValue(from: self) {
+            return result
+        }
         var result = [String: Any]()
         for (key, value) in self {
             if let key = key as? String {
                 if let transformable = value as? _Transformable {
-                    if let transValue = transformable.plainValue() {
+                    if let transValue = transformable.plainValue(transformer: transformer) {
                         result[key] = transValue
                     }
                 }
diff --git a/Source/BuiltInBridgeType.swift b/Source/BuiltInBridgeType.swift
index 53fadd6..9cb1d32 100644
--- a/Source/BuiltInBridgeType.swift
+++ b/Source/BuiltInBridgeType.swift
@@ -10,27 +10,33 @@ import Foundation
 
 protocol _BuiltInBridgeType: _Transformable {
 
-    static func _transform(from object: Any) -> _BuiltInBridgeType?
-    func _plainValue() -> Any?
+    static func _transform(from object: Any, transformer: _Transformer?) -> _BuiltInBridgeType?
+    func _plainValue(transformer: _Transformer?) -> Any?
 }
 
 extension NSString: _BuiltInBridgeType {
 
-    static func _transform(from object: Any) -> _BuiltInBridgeType? {
-        if let str = String.transform(from: object) {
+    static func _transform(from object: Any, transformer: _Transformer?) -> _BuiltInBridgeType? {
+        if let str = String.transform(from: object, transformer: transformer) {
             return NSString(string: str)
         }
         return nil
     }
 
-    func _plainValue() -> Any? {
+    func _plainValue(transformer: _Transformer?) -> Any? {
+        if let result = transformer?.plainValue(from: self) {
+            return result
+        }
         return self
     }
 }
 
 extension NSNumber: _BuiltInBridgeType {
 
-    static func _transform(from object: Any) -> _BuiltInBridgeType? {
+    static func _transform(from object: Any, transformer: _Transformer?) -> _BuiltInBridgeType? {
+        if let result = transformer?.transform(from: object, type: self) {
+            return result
+        }
         switch object {
         case let num as NSNumber:
             return num
@@ -51,29 +57,38 @@ extension NSNumber: _BuiltInBridgeType {
         }
     }
 
-    func _plainValue() -> Any? {
+    func _plainValue(transformer: _Transformer?) -> Any? {
+        if let result = transformer?.plainValue(from: self) {
+            return result
+        }
         return self
     }
 }
 
 extension NSArray: _BuiltInBridgeType {
     
-    static func _transform(from object: Any) -> _BuiltInBridgeType? {
+    static func _transform(from object: Any, transformer: _Transformer?) -> _BuiltInBridgeType? {
+        if let result = transformer?.transform(from: object, type: self) {
+            return result
+        }
         return object as? NSArray
     }
 
-    func _plainValue() -> Any? {
-        return (self as? Array<Any>)?.plainValue()
+    func _plainValue(transformer: _Transformer?) -> Any? {
+        return (self as? Array<Any>)?.plainValue(transformer: transformer)
     }
 }
 
 extension NSDictionary: _BuiltInBridgeType {
     
-    static func _transform(from object: Any) -> _BuiltInBridgeType? {
+    static func _transform(from object: Any, transformer: _Transformer?) -> _BuiltInBridgeType? {
+        if let result = transformer?.transform(from: object, type: self) {
+            return result
+        }
         return object as? NSDictionary
     }
 
-    func _plainValue() -> Any? {
-        return (self as? Dictionary<String, Any>)?.plainValue()
+    func _plainValue(transformer: _Transformer?) -> Any? {
+        return (self as? Dictionary<String, Any>)?.plainValue(transformer: transformer)
     }
 }
diff --git a/Source/Custom/Transformer.swift b/Source/Custom/Transformer.swift
new file mode 100644
index 0000000..286eac5
--- /dev/null
+++ b/Source/Custom/Transformer.swift
@@ -0,0 +1,22 @@
+//
+//  Transformer.swift
+//  HandyJSON
+//
+//  Created by 熊朝伟 on 2021/1/15.
+//
+
+import Foundation
+
+public protocol _Transformer {
+    func transform<K,V>(from object: Any, type: [K:V].Type) -> [K:V]?
+    func transform<T>(from object: Any, type: [T].Type) -> [T]?
+    func transform<T>(from object: Any, type: T.Type) -> T?
+    func plainValue<T>(from object: T) -> Any?
+}
+
+extension _Transformer {
+    public func transform<K,V>(from object: Any, type: [K:V].Type) -> [K:V]? { nil }
+    public func transform<T>(from object: Any, type: [T].Type) -> [T]? { nil }
+    public func transform<T>(from object: Any, type: T.Type) -> T? { nil }
+    public func plainValue<T>(from object: T) -> Any? { nil }
+}
diff --git a/Source/Deserializer.swift b/Source/Deserializer.swift
index 150c929..289d24e 100644
--- a/Source/Deserializer.swift
+++ b/Source/Deserializer.swift
@@ -63,33 +63,33 @@ public class JSONDeserializer<T: HandyJSON> {
 
     /// Finds the internal dictionary in `dict` as the `designatedPath` specified, and map it to a Model
     /// `designatedPath` is a string like `result.data.orderInfo`, which each element split by `.` represents key of each layer, or nil
-    public static func deserializeFrom(dict: NSDictionary?, designatedPath: String? = nil) -> T? {
-        return deserializeFrom(dict: dict as? [String: Any], designatedPath: designatedPath)
+    public static func deserializeFrom(dict: NSDictionary?, designatedPath: String? = nil, transformer: _Transformer? = nil) -> T? {
+        return deserializeFrom(dict: dict as? [String: Any], designatedPath: designatedPath, transformer: transformer)
     }
 
     /// Finds the internal dictionary in `dict` as the `designatedPath` specified, and map it to a Model
     /// `designatedPath` is a string like `result.data.orderInfo`, which each element split by `.` represents key of each layer, or nil
-    public static func deserializeFrom(dict: [String: Any]?, designatedPath: String? = nil) -> T? {
+    public static func deserializeFrom(dict: [String: Any]?, designatedPath: String? = nil, transformer: _Transformer? = nil) -> T? {
         var targetDict = dict
         if let path = designatedPath {
             targetDict = getInnerObject(inside: targetDict, by: path) as? [String: Any]
         }
         if let _dict = targetDict {
-            return T._transform(dict: _dict) as? T
+            return T._transform(dict: _dict, transformer: transformer) as? T
         }
         return nil
     }
 
     /// Finds the internal JSON field in `json` as the `designatedPath` specified, and converts it to Model
     /// `designatedPath` is a string like `result.data.orderInfo`, which each element split by `.` represents key of each layer, or nil
-    public static func deserializeFrom(json: String?, designatedPath: String? = nil) -> T? {
+    public static func deserializeFrom(json: String?, designatedPath: String? = nil, transformer: _Transformer? = nil) -> T? {
         guard let _json = json else {
             return nil
         }
         do {
             let jsonObject = try JSONSerialization.jsonObject(with: _json.data(using: String.Encoding.utf8)!, options: .allowFragments)
             if let jsonDict = jsonObject as? NSDictionary {
-                return self.deserializeFrom(dict: jsonDict, designatedPath: designatedPath)
+                return self.deserializeFrom(dict: jsonDict, designatedPath: designatedPath, transformer: transformer)
             }
         } catch let error {
             InternalLogger.logError(error)
@@ -99,13 +99,13 @@ public class JSONDeserializer<T: HandyJSON> {
 
     /// Finds the internal dictionary in `dict` as the `designatedPath` specified, and use it to reassign an exist model
     /// `designatedPath` is a string like `result.data.orderInfo`, which each element split by `.` represents key of each layer, or nil
-    public static func update(object: inout T, from dict: [String: Any]?, designatedPath: String? = nil) {
+    public static func update(object: inout T, from dict: [String: Any]?, designatedPath: String? = nil, transformer: _Transformer? = nil) {
         var targetDict = dict
         if let path = designatedPath {
             targetDict = getInnerObject(inside: targetDict, by: path) as? [String: Any]
         }
         if let _dict = targetDict {
-            T._transform(dict: _dict, to: &object)
+            T._transform(dict: _dict, to: &object, transformer: transformer)
         }
     }
 
diff --git a/Source/EnumType.swift b/Source/EnumType.swift
index 378546f..a35c116 100644
--- a/Source/EnumType.swift
+++ b/Source/EnumType.swift
@@ -10,22 +10,28 @@ import Foundation
 
 public protocol _RawEnumProtocol: _Transformable {
 
-    static func _transform(from object: Any) -> Self?
-    func _plainValue() -> Any?
+    static func _transform(from object: Any, transformer: _Transformer?) -> Self?
+    func _plainValue(transformer: _Transformer?) -> Any?
 }
 
 extension RawRepresentable where Self: _RawEnumProtocol {
 
-    public static func _transform(from object: Any) -> Self? {
+    public static func _transform(from object: Any, transformer: _Transformer?) -> Self? {
+        if let result = transformer?.transform(from: object, type: self) {
+            return result
+        }
         if let transformableType = RawValue.self as? _Transformable.Type {
-            if let typedValue = transformableType.transform(from: object) {
+            if let typedValue = transformableType.transform(from: object, transformer: transformer) {
                 return Self(rawValue: typedValue as! RawValue)
             }
         }
         return nil
     }
 
-    public func _plainValue() -> Any? {
+    public func _plainValue(transformer: _Transformer?) -> Any? {
+        if let result = transformer?.plainValue(from: self) {
+            return result
+        }
         return self.rawValue
     }
 }
diff --git a/Source/Export.swift b/Source/Export.swift
index cef2a10..91db958 100644
--- a/Source/Export.swift
+++ b/Source/Export.swift
@@ -13,3 +13,5 @@ public protocol HandyJSONCustomTransformable: _ExtendCustomBasicType {}
 public protocol HandyJSON: _ExtendCustomModelType {}
 
 public protocol HandyJSONEnum: _RawEnumProtocol {}
+
+public protocol HandyJSONTransformer: _Transformer {}
diff --git a/Source/ExtendCustomBasicType.swift b/Source/ExtendCustomBasicType.swift
index 9ee9610..fbc1bfa 100644
--- a/Source/ExtendCustomBasicType.swift
+++ b/Source/ExtendCustomBasicType.swift
@@ -23,6 +23,6 @@
 
 public protocol _ExtendCustomBasicType: _Transformable {
 
-    static func _transform(from object: Any) -> Self?
-    func _plainValue() -> Any?
+    static func _transform(from object: Any, transformer: _Transformer?) -> Self?
+    func _plainValue(transformer: _Transformer?) -> Any?
 }
diff --git a/Source/ExtendCustomModelType.swift b/Source/ExtendCustomModelType.swift
index 781b007..3bb6c9f 100644
--- a/Source/ExtendCustomModelType.swift
+++ b/Source/ExtendCustomModelType.swift
@@ -52,13 +52,13 @@ fileprivate func getRawValueFrom(dict: [String: Any], property: PropertyInfo, ma
     return dict[property.key]
 }
 
-fileprivate func convertValue(rawValue: Any, property: PropertyInfo, mapper: HelpingMapper) -> Any? {
+fileprivate func convertValue(rawValue: Any, property: PropertyInfo, mapper: HelpingMapper, transformer: _Transformer?) -> Any? {
     if rawValue is NSNull { return nil }
     if let mappingHandler = mapper.getMappingHandler(key: Int(bitPattern: property.address)), let transformer = mappingHandler.assignmentClosure {
         return transformer(rawValue)
     }
     if let transformableType = property.type as? _Transformable.Type {
-        return transformableType.transform(from: rawValue)
+        return transformableType.transform(from: rawValue, transformer: transformer)
     } else {
         return extensions(of: property.type).takeValue(from: rawValue)
     }
@@ -112,15 +112,18 @@ extension NSObject {
 
 extension _ExtendCustomModelType {
 
-    static func _transform(from object: Any) -> Self? {
+    static func _transform(from object: Any, transformer: _Transformer?) -> Self? {
+        if let result = transformer?.transform(from: object, type: self) {
+            return result
+        }
         if let dict = object as? [String: Any] {
             // nested object, transform recursively
-            return self._transform(dict: dict) as? Self
+            return self._transform(dict: dict, transformer: transformer) as? Self
         }
         return nil
     }
 
-    static func _transform(dict: [String: Any]) -> _ExtendCustomModelType? {
+    static func _transform(dict: [String: Any], transformer: _Transformer?) -> _ExtendCustomModelType? {
 
         var instance: Self
         if let _nsType = Self.self as? NSObject.Type {
@@ -129,12 +132,12 @@ extension _ExtendCustomModelType {
             instance = Self.init()
         }
         instance.willStartMapping()
-        _transform(dict: dict, to: &instance)
+        _transform(dict: dict, to: &instance, transformer: transformer)
         instance.didFinishMapping()
         return instance
     }
 
-    static func _transform(dict: [String: Any], to instance: inout Self) {
+    static func _transform(dict: [String: Any], to instance: inout Self, transformer: _Transformer?) {
         guard let properties = getProperties(forType: Self.self) else {
             InternalLogger.logDebug("Failed when try to get properties from type: \(type(of: Self.self))")
             return
@@ -168,7 +171,7 @@ extension _ExtendCustomModelType {
             InternalLogger.logVerbose("field: ", property.key, "  offset: ", property.offset, "  isBridgeProperty: ", isBridgedProperty)
 
             if let rawValue = getRawValueFrom(dict: _dict, property: propertyDetail, mapper: mapper) {
-                if let convertedValue = convertValue(rawValue: rawValue, property: propertyDetail, mapper: mapper) {
+                if let convertedValue = convertValue(rawValue: rawValue, property: propertyDetail, mapper: mapper, transformer: transformer) {
                     assignProperty(convertedValue: convertedValue, instance: instance, property: propertyDetail)
                     continue
                 }
@@ -180,16 +183,19 @@ extension _ExtendCustomModelType {
 
 extension _ExtendCustomModelType {
 
-    func _plainValue() -> Any? {
-        return Self._serializeAny(object: self)
+    func _plainValue(transformer: _Transformer?) -> Any? {
+        if let result = transformer?.plainValue(from: self) {
+            return result
+        }
+        return Self._serializeAny(object: self, transformer: transformer)
     }
 
-    static func _serializeAny(object: _Transformable) -> Any? {
+    static func _serializeAny(object: _Transformable, transformer: _Transformer?) -> Any? {
 
         let mirror = Mirror(reflecting: object)
 
         guard let displayStyle = mirror.displayStyle else {
-            return object.plainValue()
+            return object.plainValue(transformer: transformer)
         }
 
         // after filtered by protocols above, now we expect the type is pure struct/class
@@ -222,13 +228,13 @@ extension _ExtendCustomModelType {
 
             let requiredInfo = merge(children: children, propertyInfos: propertyInfos)
 
-            return _serializeModelObject(instance: mutableObject, properties: requiredInfo, mapper: mapper) as Any
+            return _serializeModelObject(instance: mutableObject, properties: requiredInfo, mapper: mapper, transformer: transformer) as Any
         default:
-            return object.plainValue()
+            return object.plainValue(transformer: transformer)
         }
     }
 
-    static func _serializeModelObject(instance: _ExtendCustomModelType, properties: [String: (Any, PropertyInfo?)], mapper: HelpingMapper) -> [String: Any] {
+    static func _serializeModelObject(instance: _ExtendCustomModelType, properties: [String: (Any, PropertyInfo?)], mapper: HelpingMapper, transformer: _Transformer?) -> [String: Any] {
 
         var dict = [String: Any]()
         for (key, property) in properties {
@@ -261,7 +267,7 @@ extension _ExtendCustomModelType {
             }
 
             if let typedValue = realValue as? _Transformable {
-                if let result = self._serializeAny(object: typedValue) {
+                if let result = self._serializeAny(object: typedValue, transformer: transformer) {
                     dict[realKey] = result
                     continue
                 }
diff --git a/Source/HelpingMapper.swift b/Source/HelpingMapper.swift
index 209f1e5..a5f8207 100644
--- a/Source/HelpingMapper.swift
+++ b/Source/HelpingMapper.swift
@@ -102,7 +102,7 @@ public class HelpingMapper {
         self.specify(property: &property, name: nil, converter: converter)
     }
     
-    public func specify<T>(property: inout T, name: String?, converter: ((String) -> T)?) {
+    public func specify<T>(property: inout T, name: String?, converter: ((String) -> T)?, transformer: _Transformer? = nil) {
         let pointer = withUnsafePointer(to: &property, { return $0 })
         let key = Int(bitPattern: pointer)
         let names = (name == nil ? nil : [name!])
@@ -111,7 +111,7 @@ public class HelpingMapper {
             let assignmentClosure = { (jsonValue: Any?) -> Any? in
                 if let _value = jsonValue{
                     if let object = _value as? NSObject {
-                        if let str = String.transform(from: object){
+                        if let str = String.transform(from: object, transformer: transformer){
                             return _converter(str)
                         }
                     }
diff --git a/Source/Serializer.swift b/Source/Serializer.swift
index e05f563..9820baf 100644
--- a/Source/Serializer.swift
+++ b/Source/Serializer.swift
@@ -25,16 +25,16 @@ import Foundation
 
 public extension HandyJSON {
 
-    func toJSON() -> [String: Any]? {
-        if let dict = Self._serializeAny(object: self) as? [String: Any] {
+    func toJSON(transformer: _Transformer? = nil) -> [String: Any]? {
+        if let dict = Self._serializeAny(object: self, transformer: transformer) as? [String: Any] {
             return dict
         }
         return nil
     }
 
-    func toJSONString(prettyPrint: Bool = false) -> String? {
+    func toJSONString(prettyPrint: Bool = false, transformer: _Transformer? = nil) -> String? {
 
-        if let anyObject = self.toJSON() {
+        if let anyObject = self.toJSON(transformer: transformer) {
             if JSONSerialization.isValidJSONObject(anyObject) {
                 do {
                     let jsonData: Data
@@ -57,13 +57,13 @@ public extension HandyJSON {
 
 public extension Collection where Iterator.Element: HandyJSON {
 
-    func toJSON() -> [[String: Any]?] {
-        return self.map{ $0.toJSON() }
+    func toJSON(transformer: _Transformer? = nil) -> [[String: Any]?] {
+        return self.map{ $0.toJSON(transformer: transformer) }
     }
 
-    func toJSONString(prettyPrint: Bool = false) -> String? {
+    func toJSONString(prettyPrint: Bool = false, transformer: _Transformer? = nil) -> String? {
 
-        let anyArray = self.toJSON()
+        let anyArray = self.toJSON(transformer: transformer)
         if JSONSerialization.isValidJSONObject(anyArray) {
             do {
                 let jsonData: Data
@@ -77,7 +77,7 @@ public extension Collection where Iterator.Element: HandyJSON {
                 InternalLogger.logError(error)
             }
         } else {
-            InternalLogger.logDebug("\(self.toJSON()) is not a valid JSON Object")
+            InternalLogger.logDebug("\(self.toJSON(transformer: transformer)) is not a valid JSON Object")
         }
         return nil
     }
diff --git a/Source/Transformable.swift b/Source/Transformable.swift
index d03bb31..c996632 100644
--- a/Source/Transformable.swift
+++ b/Source/Transformable.swift
@@ -12,38 +12,38 @@ public protocol _Transformable: _Measurable {}
 
 extension _Transformable {
 
-    static func transform(from object: Any) -> Self? {
+    public static func transform(from object: Any, transformer: _Transformer?) -> Self? {
         if let typedObject = object as? Self {
             return typedObject
         }
         switch self {
         case let type as _ExtendCustomBasicType.Type:
-            return type._transform(from: object) as? Self
+            return type._transform(from: object, transformer: transformer) as? Self
         case let type as _BuiltInBridgeType.Type:
-            return type._transform(from: object) as? Self
+            return type._transform(from: object, transformer: transformer) as? Self
         case let type as _BuiltInBasicType.Type:
-            return type._transform(from: object) as? Self
+            return type._transform(from: object, transformer: transformer) as? Self
         case let type as _RawEnumProtocol.Type:
-            return type._transform(from: object) as? Self
+            return type._transform(from: object, transformer: transformer) as? Self
         case let type as _ExtendCustomModelType.Type:
-            return type._transform(from: object) as? Self
+            return type._transform(from: object, transformer: transformer) as? Self
         default:
             return nil
         }
     }
 
-    func plainValue() -> Any? {
+    public func plainValue(transformer: _Transformer?) -> Any? {
         switch self {
         case let rawValue as _ExtendCustomBasicType:
-            return rawValue._plainValue()
+            return rawValue._plainValue(transformer: transformer)
         case let rawValue as _BuiltInBridgeType:
-            return rawValue._plainValue()
+            return rawValue._plainValue(transformer: transformer)
         case let rawValue as _BuiltInBasicType:
-            return rawValue._plainValue()
+            return rawValue._plainValue(transformer: transformer)
         case let rawValue as _RawEnumProtocol:
-            return rawValue._plainValue()
+            return rawValue._plainValue(transformer: transformer)
         case let rawValue as _ExtendCustomModelType:
-            return rawValue._plainValue()
+            return rawValue._plainValue(transformer: transformer)
         default:
             return nil
         }