1-) FLUTTER - GlobalKey kullanımı

import 'package:flutter/material.dart';

class GlobalPage extends StatefulWidget {
  const GlobalPage({Key? key}) : super(key: key);

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

class _GlobalPageState extends State<GlobalPage> {
  final sayacWidgetKey = GlobalKey<SayacWidgetState>();

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: null,
      body: Center(
        child: Container(
          child: Column(
            mainAxisAlignment: MainAxisAlignment.center,
            children: [
              Text(
                "BUTTONA BASILMA MİKTARI",
                style: TextStyle(fontSize: 25, fontWeight: FontWeight.bold),
              ),
              SayacWidget(
                key: sayacWidgetKey,
              ),
              Text(sayacWidgetKey.currentState?.count.toString() ?? "0"), // nul değilse count yaz nul ise sıfır yaz
            ],
          ),
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: () {
          sayacWidgetKey.currentState!.arttir();
        },
        child: Icon(Icons.add),
      ),
    );
  }
}

class SayacWidget extends StatefulWidget {
  const SayacWidget({Key? key}) : super(key: key);

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

class SayacWidgetState extends State<SayacWidget> {
  int count = 0;

  void arttir() {
    setState(() {
      count++;
    });
  }

  @override
  Widget build(BuildContext context) {
    return Text(count.toString(),
        style: TextStyle(fontSize: 25, fontWeight: FontWeight.bold));
  }
}