Provider

ChangeNotifierProvider

import 'package:flutter/material.dart';
import 'package:controller/aaa_state.dart';
import 'package:provider/provider.dart';

class AAA extends StatefulWidget {
  @override
  _AAAState createState() => _AAAState();
}

class _AAAState extends State<AAA> {

  AAASate _aaaSate = AAASate();

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text("AAA"),
      ),
      body: ChangeNotifierProvider(
        create: (_) => _aaaSate,
        child: Consumer(builder: (BuildContext context, AAASate value, Widget child) {
          return Column(
            children: [
              Text("num==  ${_aaaSate.num}"),
              GestureDetector(
                onTap: () {
                  _aaaSate.setNum(n: _aaaSate.num+1);
                },
                child: Container(
                  width: 100,
                  height: 100,
                  color: Colors.red,
                ),
              )
            ],
          );
        }),
      ),
    );
  }
}

import 'package:flutter/cupertino.dart';

class AAASate extends ChangeNotifier {

  int _num = 0;
  int get num => _num;
  void setNum({int n}) {
    _num = n;
    notifyListeners();
  }


}