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

Question

1. For Example in the below code plus button works and able to update the text but the minus button does not.
2. But if we press FloatingActionButton then the State is refreshed .
3. The minus button is changing the value of the variable but not updating the state of parent widget .

here is code .....

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
      title: 'Flutter Demo',
      theme: new ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: new MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

int number;

EdgeInsets globalMargin = const EdgeInsets.symmetric(horizontal: 20.0, vertical: 20.0);
TextStyle textStyle = const TextStyle(
  fontSize: 100.0,
  color: Colors.black,
);

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);

  final String title;

  @override
  _MyHomePageState createState() => new _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  @override
  void initState() {
    super.initState();
    number = number ?? 0;
  }

  @override
  Widget build(BuildContext context) {
    return new Scaffold(
      appBar: new AppBar(
        title: new Text(widget.title),
      ),
      body: new Column(
        children: <Widget>[
          new Text(
            number.toString(),
            style: textStyle,
          ),
          new GridView.count(
            crossAxisCount: 2,
            shrinkWrap: true,
            scrollDirection: Axis.vertical,
            children: <Widget>[
              new InkResponse(
                child: new Container(
                    margin: globalMargin,
                    color: Colors.green,
                    child: new Center(
                      child: new Text(
                        "+",
                        style: textStyle,
                      ),
                    )),
                onTap: () {
                  setState(() {
                    number = number + 1;
                  });
                },
              ),
              new Sub(),
            ],
          ),
        ],
      ),
      floatingActionButton: new FloatingActionButton(
        onPressed: () {
          setState(() {});
        },
        child: new Icon(Icons.update),
      ),
    );
  }
}

class Sub extends StatefulWidget {
  @override
  _SubState createState() => new _SubState();
}

class _SubState extends State<Sub> {
  @override
  Widget build(BuildContext context) {
    return new InkResponse(
      child: new Container(
          margin: globalMargin,
          color: Colors.red,
          child: new Center(
            child: new Text(
              "-",
              style: textStyle,
            ),
          )),
      onTap: () {
        setState(() {
          number = number - 1;
        });
      },
    );
  }
}

Answer 1

1.On Child Widget : add parameter Function paramter

class ChildWidget extends StatefulWidget {
  final Function() notifyParent;
  ChildWidget({Key key, @required this.notifyParent}) : super(key: key);
}

2.On Parent Widget : create a Function for the child to callback

refresh() {
  setState(() {});
}

3.On Parent Widget : pass parentFunction to Child Widget

new ChildWidget( notifyParent: refresh );  

4.On Child Widget : call the Parent Function

  widget.notifyParent();

Answer 2

Screenshot (Parent to child, Child to parent):

This examples shows calling a method

1. Defined in Child widget from Parent widget.
2. Defined in Parent widget from Child widget.

---

Code:

class ParentPage extends StatelessWidget {
  final GlobalKey<ChildPageState> _key = GlobalKey();

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: Text("Parent")),
      body: Center(
        child: Column(
          children: <Widget>[
            Expanded(
              child: Container(
                color: Colors.grey,
                width: double.infinity,
                alignment: Alignment.center,
                child: ElevatedButton(
                  child: Text("Call method in child"),
                  onPressed: () => _key.currentState!.methodInChild(), // calls method in child
                ),
              ),
            ),
            Text("Above = Parent\nBelow = Child"),
            Expanded(
              child: ChildPage(
                key: _key,
                function: methodInParent,
              ),
            ),
          ],
        ),
      ),
    );
  }

  methodInParent() => Fluttertoast.showToast(msg: "Method called in parent", gravity: ToastGravity.CENTER);
}

class ChildPage extends StatefulWidget {
  final VoidCallback function;

  ChildPage({Key? key, required this.function}) : super(key: key);

  @override
  ChildPageState createState() => ChildPageState();
}

class ChildPageState extends State<ChildPage> {
  @override
  Widget build(BuildContext context) {
    return Container(
      color: Colors.teal,
      width: double.infinity,
      alignment: Alignment.center,
      child: ElevatedButton(
        child: Text("Call method in parent"),
        onPressed: () => widget.function(), // calls method in parent
      ),
    );
  }

  methodInChild() => Fluttertoast.showToast(msg: "Method called in child");
}

Answer 3

Old Solutions:

1. Create a global instance of _MyHomePageState. Use this instance in _SubState as _myHomePageState.setState
2. No need to create a global instance. Instead, just pass the parent instance to the child widget

New Solution: Passing a callback seems the better solution than above solutions

CODE UPDATED AS PER FLUTTER 2.0:

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
      title: 'Flutter Demo',
      theme: new ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: new MyHomePage(),
    );
  }
}

EdgeInsets globalMargin =
    const EdgeInsets.symmetric(horizontal: 20.0, vertical: 20.0);
TextStyle textStyle = const TextStyle(
  fontSize: 100.0,
  color: Colors.black,
);

class MyHomePage extends StatefulWidget {
  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  int number = 0;

  @override
  Widget build(BuildContext context) {
    return new Scaffold(
      appBar: new AppBar(
        title: new Text('SO Help'),
      ),
      body: new Column(
        children: <Widget>[
          new Text(
            number.toString(),
            style: textStyle,
          ),
          new GridView.count(
            crossAxisCount: 2,
            shrinkWrap: true,
            scrollDirection: Axis.vertical,
            children: <Widget>[
              new InkResponse(
                child: new Container(
                    margin: globalMargin,
                    color: Colors.green,
                    child: new Center(
                      child: new Text(
                        "+",
                        style: textStyle,
                      ),
                    )),
                onTap: () {
                  setState(() {
                    this.number++;
                  });
                },
              ),
              new Sub(onTap: () {
                setState(() {
                  this.number--;
                });
              }),
            ],
          ),
        ],
      ),
      floatingActionButton: new FloatingActionButton(
        onPressed: () {
          setState(() {});
        },
        child: new Icon(Icons.update),
      ),
    );
  }
}

class Sub extends StatelessWidget {
  final Function onTap;
  Sub({this.onTap});

  @override
  Widget build(BuildContext context) {
    return new InkResponse(
      child: new Container(
        margin: globalMargin,
        color: Colors.red,
        child: new Center(
          child: new Text(
            "-",
            style: textStyle,
          ),
        ),
      ),
      onTap: this.onTap,
    );
  }
}

Just let me know if it works.