Resuming futter lifecycle

life_cycle.dart

import 'package:flutter/material.dart';

class CountdownTimer extends StatefulWidget {
  @override
  _CountdownTimerState createState() => _CountdownTimerState();
}

class _CountdownTimerState extends State<CountdownTimer>
    with WidgetsBindingObserver, SingleTickerProviderStateMixin {
  double countdownTime;
  Duration _duration;
  Animation _animation;
  @override
  void initState() {
    super.initState();
    WidgetsBinding.instance.addObserver(this);
    WidgetsBinding.instance.addPostFrameCallback((timeStamp) {
      watch.start();
      setState(() {
        _duration = Duration(
            days: 1); //Duration set to days 1 on initState for testing purposes
      });
    });
    _animationController = AnimationController(
      duration: Duration(seconds: 100),
      vsync: this,
    );

    _animation = new CurvedAnimation(
      parent: _animationController,
      curve: Curves.linear,
    )..addStatusListener((AnimationStatus status) {
        if (status == AnimationStatus.completed) {
          _animationController.reset();
          _animationController.forward();
        }
      });
    _animationController.forward();
  }

  @override
  void dispose() {
    WidgetsBinding.instance.removeObserver(this);
    super.dispose();
  }

  @override
  void didChangeAppLifecycleState(AppLifecycleState state) {
    WidgetsBinding.instance.addPostFrameCallback((timeStamp) {
      //I want to update the timer when app is resumed
      if (state == AppLifecycleState.resumed) {
        print('App Resumed');
        setState(() {
          print('App life cycle resume duration $_duration}');
        });
      }
    });
    super.didChangeAppLifecycleState(state);
  }

  AnimationController _animationController;
  Stopwatch watch = Stopwatch();
  @override
  Widget build(BuildContext context) {
    print('Build run with duration $_duration');
    return Container(
      padding: const EdgeInsets.only(top: 10, bottom: 5),
      child: Column(
        children: [
          Container(
            width: 220,
            child: Row(
              mainAxisAlignment: MainAxisAlignment.spaceBetween,
              children: <Widget>[
                Text('DD', style: TextStyle(color: Colors.grey)),
                Text('HH', style: TextStyle(color: Colors.grey)),
                Text('MM', style: TextStyle(color: Colors.grey)),
                Text('SS', style: TextStyle(color: Colors.grey)),
              ],
            ),
          ),
          AnimatedBuilder(
              animation: _animation,
              builder: (_duration, Widget child) =>
                  Text(watch.elapsed.inSeconds.toString()))
        ],
      ),
    );
  }
}