Call setState outside widgets

Question

Can I call setState from outside the widgets. I try to register the setState of the children widgets - and call it from outside. It don't seem to work - why is that?

import 'package:flutter/material.dart';
import 'package:linkimager/sizes.dart';
import 'package:linkimager/store.dart';

void main() {
  runApp(const MyApp());
}

class MyApp extends StatefulWidget {
  const MyApp({super.key});

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

class MyAppState extends State<MyApp> {

  @override
  Widget build(BuildContext context) {
    ScreenSizes.init(MediaQuery.of(context).size);
    int number = 0;
    return MaterialApp(
      home: Scaffold(
        body: ChildWidget(number),
        floatingActionButton: FloatingActionButton(
          onPressed: () {
            number++;
            Renderer.setStateChild();
          },
          child: const Icon(Icons.add),
        ),
      ),
    );
  }
}

class ChildWidget extends StatefulWidget {
  final int number;

  const ChildWidget(this.number, {super.key});

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

class ChildWidgetState extends State<ChildWidget> {

  @override
  void initState() {
    print("registered setStateChild");
    Renderer.setStateChild = () => setState;
    super.initState();
  }

  @override
  Widget build(BuildContext context) {

    return Center(child: Text(widget.number.toString()));
  }
}

class Renderer {
  static Function setStateChild = () {}; 
}

Im not looking for state management tools but want to understand what is blocking the rendering from outside

Answer 1

I made this example for you:

    import 'package:flutter/material.dart';

void main() {
  runApp(const MyApp());
}

class MyApp extends StatefulWidget {
  const MyApp({super.key});

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

class MyAppState extends State<MyApp> {
  @override
  Widget build(BuildContext context) {
    // ScreenSizes.init(MediaQuery.of(context).size);
    int number = 0;
    return MaterialApp(
      home: Scaffold(
        body: ChildWidget(),
        floatingActionButton: FloatingActionButton(
          onPressed: () {
            number++;
            Renderer.setStateChild();
          },
          child: const Icon(Icons.add),
        ),
      ),
    );
  }
}

class ChildWidget extends StatefulWidget {
  const ChildWidget({super.key});

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

class ChildWidgetState extends State<ChildWidget> {
  int number = 0;

  @override
  void initState() {
    print("registered setStateChild");
    Renderer.setStateChild = () {
      setState(() {
        print("setStateChild called");
        number++;
      });
    };
    super.initState();
  }

  @override
  Widget build(BuildContext context) {
    return Center(child: Text(number.toString()));
  }
}

class Renderer {
  static Function setStateChild = () {};
}

I hope its close to what youre looking for.