Add onChanged callback

fixes #2

example/lib/main.dart

@@ -52,14 +52,28 @@ Widget createScaffold() {
                       return 'False!';
                     }
                   },
+                  onChanged: (value) {
+                    if (value) {
+                      print("ListTile Checked :)");
+                    } else {
+                      print("ListTile Not Checked :(");
+                    }
+                  },
                 ),
                 CheckboxIconFormField(
                   initialValue: true,
                   enabled: false,
                   iconSize: 32,
                   onSaved: (bool? value) {},
+                  onChanged: (value) {
+                    if (value) {
+                      print("Icon Checked :)");
+                    } else {
+                      print("Icon Not Checked :(");
+                    }
+                  },
                 ),
-                RaisedButton(
+                ElevatedButton(
                   onPressed: () {
                     if (_formKey.currentState!.validate()) {
                       _formKey.currentState!.save();

lib/checkbox_icon_formfield.dart

@@ -7,6 +7,7 @@ class CheckboxIconFormField extends FormField<bool> {
     BuildContext? context,
     FormFieldSetter<bool>? onSaved,
     bool initialValue = false,
+    ValueChanged<bool>? onChanged,
     AutovalidateMode autovalidateMode,
     bool enabled = true,
     IconData trueIcon = Icons.check,
@@ -29,19 +30,27 @@ class CheckboxIconFormField extends FormField<bool> {
             return Padding(
                 padding: EdgeInsets.all(padding),
                 child: state.value!
-                    ? _createTappableIcon(state, enabled, trueIcon,
+                    ? _createTappableIcon(state, enabled, trueIcon, onChanged,
                         trueIconColor, disabledColor, iconSize)
-                    : _createTappableIcon(state, enabled, falseIcon,
+                    : _createTappableIcon(state, enabled, falseIcon, onChanged,
                         falseIconColor, disabledColor, iconSize));
           },
         );
 
-  static Widget _createTappableIcon(FormFieldState<bool> state, bool enabled,
-      IconData icon, Color? iconColor, Color? disabledColor, double? iconSize) {
+  static Widget _createTappableIcon(
+    FormFieldState<bool> state,
+    bool enabled,
+    IconData icon,
+    ValueChanged<bool>? onChanged,
+    Color? iconColor,
+    Color? disabledColor,
+    double? iconSize,
+  ) {
     return IconButton(
       onPressed: enabled
           ? () {
               state.didChange(!state.value!);
+              if (onChanged != null) onChanged(state.value!);
             }
           : null,
       icon: Icon(icon,

lib/checkbox_list_tile_formfield.dart

@@ -9,6 +9,7 @@ class CheckboxListTileFormField extends FormField<bool> {
     FormFieldSetter<bool>? onSaved,
     FormFieldValidator<bool>? validator,
     bool initialValue = false,
+    ValueChanged<bool>? onChanged,
     AutovalidateMode autovalidateMode,
     bool enabled = true,
     bool dense = false,
@@ -33,7 +34,12 @@ class CheckboxListTileFormField extends FormField<bool> {
               activeColor: activeColor,
               checkColor: checkColor,
               value: state.value,
-              onChanged: enabled ? state.didChange : null,
+              onChanged: enabled
+                  ? (value) {
+                      state.didChange(value);
+                      if (onChanged != null) onChanged(value!);
+                    }
+                  : null,
               subtitle: state.hasError
                   ? Text(
                       state.errorText!,