Flutter async and await

main.dart

import 'package:flutter/material.dart';

class SandGlass {
  int _sand = 100;

  time() {
    return _sand;
  }

  Future timeDecrease() async {
    _sand = _sand -10;
    await new Future.delayed(const Duration(milliseconds: 500));
    print('time_decrease()');
  }
}

class MyApp extends StatefulWidget {
  @override
  State<StatefulWidget> createState() => MyAppState();
}

class MyAppState extends State {
  SandGlass clock = SandGlass();

  @override
  void initState() {
    super.initState();
  }

  @override
  Widget build(BuildContext context) {
    _reDrawWidget();
    print('---------${clock.time()}');
    return Center(child: Text('time is: ${clock.time()}'));
  }

  _reDrawWidget() async {
    if (clock.time() == 0) return;
    await Future.delayed(const Duration(seconds: 1));
    print('reDrawWidget()\n');
    
    clock.timeDecrease();
    setState(() {});
  }
}

void main() => runApp(new MaterialApp(home: Scaffold(body: MyApp())));