Flutter setState to another class?

Screenshot (Null safe):

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Full code:

This is the ParentPage:

class ParentPage extends StatefulWidget {
  @override
  _ParentPageState createState() => _ParentPageState();
}

class _ParentPageState extends State<ParentPage> {
  int _count = 0;

  // Pass this method to the child page.
  void _update(int count) {
    setState(() => _count = count);
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Column(
        children: [
          Text('Value (in parent) = $_count'),
          ChildPage(update: _update),
        ],
      ),
    );
  }
}

This is the ChildPage:

class ChildPage extends StatelessWidget {
  final ValueChanged<int> update;
  ChildPage({required this.update});

  @override
  Widget build(BuildContext context) {
    return ElevatedButton(
      onPressed: () => update(100), // Passing value to the parent widget.
      child: Text('Update (in child)'),
    );
  }
}

You can use callbacks functions to achieve this. Please refer to the below code.

import 'package:flutter/material.dart';

class RootPage extends StatefulWidget {
  @override
  _RootPageState createState() => new _RootPageState();
}
class _RootPageState extends State<RootPage> {
  FeedPage feedPage;

  Widget currentPage;

  @override
  void initState() {
    super.initState();
    feedPage = FeedPage(this.callback);

    currentPage = feedPage;
  }

  void callback(Widget nextPage) {
    setState(() {
      this.currentPage = nextPage;
    });
  }

  @override
  Widget build(BuildContext context) {
    return new Scaffold(
      //Current page to be changed from other classes too?
        body: currentPage
    );
  }
}



class FeedPage extends StatefulWidget {
  Function callback;

  FeedPage(this.callback);

  @override
  _feedPageState createState() => new _feedPageState();
}
class _feedPageState extends State<FeedPage> {

  @override
  Widget build(BuildContext context) {
    return new FlatButton(
      onPressed: () {
        this.widget.callback(new NextPage());
//        setState(() {
//          //change the currentPage in RootPage so it switches FeedPage away and gets a new class that I'll make
//        });
      },
      child: new Text('Go to a new page but keep root, just replace this feed part'),
    );
  }
}

This is very similar to this problem and you could refer 3rd point in my answer.

I know that you need a solution for your specific code but I recommed to you to have a look on BloC pattern as it the recommended aproch when you want to pass state changes from widget to another especially multiple pages

the idea is simple although the implementation is a "little bit" more complicated