pomodoro timer example

main.dart

      import 'package:flutter/material.dart';

      void main() {
        runApp(new Container(
          color: Colors.white,
        ));
        runApp(new MaterialApp(
          theme: new ThemeData(primarySwatch: Colors.red),
          home: new HomePage(),
        ));
      }

      class PomodoroTimer extends AnimatedWidget {
        PomodoroTimer({ this.timeRemainingInSeconds })
          : super(listenable: timeRemainingInSeconds);

        Animation<int> timeRemainingInSeconds;

        Widget build(BuildContext context) {
          String minutes = '${(timeRemainingInSeconds.value / 60).floor()}';
          String seconds = '${(timeRemainingInSeconds.value % 60)}'.padLeft(2, '0');
          return new Text(
            '$minutes:$seconds',
            style: Theme.of(context).textTheme.display2,
          );
        }
      }

      class HomePage extends StatefulWidget {
        HomePageState createState() => new HomePageState();
      }

      class HomePageState extends State<HomePage> with TickerProviderStateMixin {

        AnimationController _controller;

        @override void initState() {
          _controller = new AnimationController(
            vsync: this,
            duration: const Duration(minutes: 25),
          );
        }

        @override
        Widget build(BuildContext context) {
          return new Scaffold(
            appBar: new AppBar(
              title: new Text('Pomodoro Timer'),
            ),
            floatingActionButton: new FloatingActionButton(
              child: new Icon(Icons.play_arrow),
              onPressed: () {
                _controller.forward(from: 0.0);
              },
            ),
            body: new Center(
              child: new PomodoroTimer(
                timeRemainingInSeconds: new IntTween(
                  begin: _controller.duration.inSeconds,
                  end: 0,
                ).animate(_controller),
              ),
            ),
          );
        }
      }

Please, could you say why from parameter is 0.0 in onPressed here? Why it is not _controller.duration.inSeconds like begin value?

If i want to press that button the timer should pause and while clicking again it have to play from remaining time . how i can achieve this