Change State from one stateful class to another stateful class

This is my code. I want Change State from one stateful class to another stateful class

here a cart on navigation and I want change from button class

import 'package:flutter/material.dart';

void main() => runApp(MyApp());

int cart = 10;

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      debugShowCheckedModeBanner: false,
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: Scaffold(
        appBar: AppBar(
          title: Text("data"),
          actions: <Widget>[
            TopCart(),
          ],
        ),
        body: Container(
          child: Center(
            child: Buttons(),
          ),
        ),
      ),
    );
  }
}

class TopCart extends StatefulWidget {
  @override
  _TopCartState createState() => _TopCartState();
}

class _TopCartState extends State<TopCart> {
  @override
  Widget build(BuildContext context) {
    return Container(
      margin: EdgeInsets.all(10),
      decoration: BoxDecoration(
        color: Color(0xffffffff),
        borderRadius: BorderRadius.circular(8),
      ),
      child: Row(
        children: <Widget>[
          Icon(
            Icons.card_travel,
            color: Colors.blue,
          ),
          Text(
            cart.toString(),
            style: TextStyle(
              color: Colors.blue,
              fontSize: 25,
            ),
          )
        ],
      ),
    );
  }
}

class Buttons extends StatefulWidget {
  @override
  _ButtonsState createState() => _ButtonsState();
}

class _ButtonsState extends State<Buttons> {
  @override
  Widget build(BuildContext context) {
    return Padding(
      padding: EdgeInsets.symmetric(horizontal: 16),
      child: Row(
        mainAxisAlignment: MainAxisAlignment.spaceBetween,
        crossAxisAlignment: CrossAxisAlignment.start,
        children: <Widget>[
          GestureDetector(
            onTap: () {
              cart = cart - 1;
              print(cart);
            },
            child: Container(
              width: MediaQuery.of(context).size.width / 2 - 24,
              height: MediaQuery.of(context).size.width / 2 - 24,
              color: Colors.green,
              child: Icon(
                Icons.add,
                color: Colors.white,
              ),
            ),
          ),
          GestureDetector(
            onTap: () {
              cart = cart - 1;
              print(cart);
            },
            child: Container(
              width: MediaQuery.of(context).size.width / 2 - 24,
              height: MediaQuery.of(context).size.width / 2 - 24,
              color: Colors.red,
              child: Icon(
                Icons.remove,
                color: Colors.white,
              ),
            ),
          ),
        ],
      ),
    );
  }
}

Add a function which you want to perform in another class like,

testData(){
    print("Test");
}

Now, to use this function to another class like in Buttons class. Pass it as an arguments like,

Container(
    child:Buttons(testData);
)

In Buttons Class follow the code like,

// Buttons construtor
Buttons(Function myfunction);

You can use this function now in your second class like on any button tap,

RaisedButton(
      child:Text("Save")
      onPress:myfunction(),
)

This way you can pass functions as arguments in dart.