Enhancement: form.Marshaler and form.Unmarshaler

README.md

@@ -23,7 +23,7 @@ It has the following features:
 
 Common Questions
 
-- Does it support encoding.TextUnmarshaler? No because TextUnmarshaler only accepts []byte but posted values can have multiple values, so is not suitable.
+- Does it support encoding.TextUnmarshaler? No, instead we have `form.Unmarshaler` because TextUnmarshaler only accepts []byte but posted values can have multiple values, so is not suitable.
 - Mixing `array/slice` with `array[idx]/slice[idx]`, in which order are they parsed? `array/slice` then `array[idx]/slice[idx]`
 
 Supported Types ( out of the box )
@@ -230,6 +230,33 @@ encoder.RegisterCustomTypeFunc(func(x interface{}) ([]string, error) {
 }, time.Time{})
 ```
 
+Implementing Marshaler and Unmarshaler
+--------------
+Marshaler
+```go
+type CustomStruct struct {
+    A string
+    B string
+}
+
+func (c CustomStruct) MarshalForm() ([]string, error) {
+    return []string{ c.A, c.B }, nil
+}
+```
+
+Unmarshaler
+```go
+type CustomStruct struct {
+    A string
+    B string
+}
+
+func (c *CustomStruct) UnmarshalForm(ss []string) error {
+    c.A = ss[0]
+    c.B = ss[1]
+}
+```
+
 Ignoring Fields
 --------------
 you can tell form to ignore fields using `-` in the tag

decoder.go

@@ -63,7 +63,6 @@ func (d *decoder) parseMapData() {
 		}
 
 		for i = 0; i < len(k); i++ {
-
 			switch k[i] {
 			case '[':
 				idx = i
@@ -117,7 +116,6 @@ func (d *decoder) parseMapData() {
 
 					if ke.ivalue > rd.sliceLen {
 						rd.sliceLen = ke.ivalue
-
 					}
 				}
 
@@ -140,7 +138,6 @@ func (d *decoder) parseMapData() {
 }
 
 func (d *decoder) traverseStruct(v reflect.Value, typ reflect.Type, namespace []byte) (set bool) {
-
 	l := len(namespace)
 	first := l == 0
 
@@ -177,14 +174,12 @@ func (d *decoder) traverseStruct(v reflect.Value, typ reflect.Type, namespace []
 }
 
 func (d *decoder) setFieldByType(current reflect.Value, namespace []byte, idx int) (set bool) {
-
 	var err error
 	v, kind := ExtractType(current)
 
 	arr, ok := d.values[string(namespace)]
 
 	if d.d.customTypeFuncs != nil {
-
 		if ok {
 			if cf, ok := d.d.customTypeFuncs[v.Type()]; ok {
 				val, err := cf(arr[idx:])
@@ -199,6 +194,27 @@ func (d *decoder) setFieldByType(current reflect.Value, namespace []byte, idx in
 			}
 		}
 	}
+	{
+		v := v // deliberately shadow v as to not modify
+		t := v.Type()
+		if t.Kind() != reflect.Ptr && v.CanAddr() {
+			v = v.Addr()
+		}
+		if um, ok := v.Interface().(Unmarshaler); ok {
+			// if receiver is a nil pointer, set before calling function.
+			if t.Kind() == reflect.Ptr && v.IsNil() {
+				t = t.Elem()
+				v.Set(reflect.New(t))
+				um = v.Interface().(Unmarshaler)
+			}
+			if err = um.UnmarshalForm(arr[idx:]); err != nil {
+				d.setError(namespace, err)
+				return
+			}
+			set = true
+			return
+		}
+	}
 	switch kind {
 	case reflect.Interface:
 		if !ok || idx == len(arr) {
@@ -602,7 +618,6 @@ func (d *decoder) setFieldByType(current reflect.Value, namespace []byte, idx in
 }
 
 func (d *decoder) getMapKey(key string, current reflect.Value, namespace []byte) (err error) {
-
 	v, kind := ExtractType(current)
 
 	if d.d.customTypeFuncs != nil {