[state] How to Set/Update State of StatefulWidget from other StatefulWidget in Flutter?

Here is the solution which worked for me.

OUTPUT: State of Cart Widget is updated, upon addition of items.

Create a globalKey for the widget you want to update by calling the trigger from anywhere

final GlobalKey<CartWidgetState> cartKey = GlobalKey();

Make sure it's saved in a file have global access such that, it can be accessed from anywhere. I save it in globalClass where is save commonly used variables through the app's state.

class CartWidget extends StatefulWidget {

  CartWidget({Key key}) : super(key: key);
  @override
  CartWidgetState createState() => CartWidgetState();
}

class CartWidgetState extends State<CartWidget> {
  @override
  Widget build(BuildContext context) {
    //return your widget
    return Container();
  }
}

Call your widget from some other class.

class HomeScreen extends StatefulWidget {

  HomeScreen ({Key key}) : super(key: key);
  @override
  HomeScreenState createState() => HomeScreen State();
}

class HomeScreen State extends State<HomeScreen> {
  @override
  Widget build(BuildContext context) {
    return ListView(
              children:[
                 ChildScreen(), 
                 CartWidget(key:cartKey)
              ]
            );
  }
}



class ChildScreen extends StatefulWidget {

  ChildScreen ({Key key}) : super(key: key);
  @override
  ChildScreenState createState() => ChildScreen State();
}

class ChildScreen State extends State<ChildScreen> {
  @override
  Widget build(BuildContext context) {
    return InkWell(
              onTap: (){
                // This will update the state of your inherited widget/ class
                if (cartKey.currentState != null)
                    cartKey.currentState.setState(() {});
              },
              child: Text("Update The State of external Widget"),
           );
  }
}