call_method_from_child.dart

import 'package:flutter/material.dart';

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

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

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

  @override
  State<ParentWidget> createState() => _ParentWidgetState();
}

class _ParentWidgetState extends State<ParentWidget> {
  String parentTxt = "Initial Text";
  int count = 0;
  void updateParentTxt(String param) {
    count++;
    parentTxt = '$param $count times';
    setState(() {});
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: Text(parentTxt)),
      body: Center(
        child: Column(
          crossAxisAlignment: CrossAxisAlignment.center,
          mainAxisAlignment: MainAxisAlignment.center,
          children: [
            /// another child widget
            Child(methodFromParent: updateParentTxt),
          ],
        ), // pass the method as argument
      ),
    );
  }
}

class Child extends StatelessWidget {
  const Child({super.key, this.methodFromParent});
  final Function(String val)? methodFromParent;

  @override
  Widget build(BuildContext context) {
    return ElevatedButton(
      onPressed: () {
        methodFromParent?.call("Btn pressed from child: ");
      },
      child: const Text("Update parent"),
    );
  }
}