Stopwatch 是一個(gè)秒表接口，用于測量經(jīng)過的時(shí)間。結(jié)合 Timer 更新 UI

import 'package:flutter/material.dart';
import 'dart:async';

void main() => runApp(new MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
      home: new MyHomePage(),
    );
  }
}

class MyHomePage extends StatefulWidget {
  @override
  _MyHomePageState createState() => new _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {

  Stopwatch watch = new Stopwatch();
  Timer timer;

  String elapsedTime = '';

  updateTime(Timer timer) {
    if (watch.isRunning) {

      setState(() {
        elapsedTime = transformMilliSeconds(watch.elapsedMilliseconds);
      });
    }
  }

  @override
  Widget build(BuildContext context) {

    return new Scaffold(
        appBar: new AppBar(
          centerTitle: true,
          title: new Text('StopWatch'),
        ),
        body: new Container(
            padding: EdgeInsets.all(20.0),
            child: new Column(
              children: <Widget>[
                new Text(elapsedTime, style: new TextStyle(fontSize: 25.0)),
                SizedBox(height: 20.0),
                new Row(
                  mainAxisAlignment: MainAxisAlignment.center,
                  children: <Widget>[
                    new FloatingActionButton(
                        backgroundColor: Colors.green,
                        onPressed: startWatch,
                        child: new Icon(Icons.play_arrow)),
                    SizedBox(width: 20.0),
                    new FloatingActionButton(
                        backgroundColor: Colors.red,
                        onPressed: stopWatch,
                        child: new Icon(Icons.stop)),
                    SizedBox(width: 20.0),
                    new FloatingActionButton(
                        backgroundColor: Colors.blue,
                        onPressed: resetWatch,
                        child: new Icon(Icons.refresh)),
                  ],
                )
              ],
            )));
  }

  startWatch() {
    watch.start();
    timer = new Timer.periodic(new Duration(milliseconds: 100), updateTime);
  }

  stopWatch() {
    watch.stop();
    setTime();
  }

  resetWatch() {
    watch.reset();
    setTime();
  }

  setTime() {
    var timeSoFar = watch.elapsedMilliseconds;
    setState(() {
      elapsedTime = transformMilliSeconds(timeSoFar);
    });
  }

  transformMilliSeconds(int milliseconds) {
    //Thanks to Andrew
    int hundreds = (milliseconds / 10).truncate();
    int seconds = (hundreds / 100).truncate();
    int minutes = (seconds / 60).truncate();

    String minutesStr = (minutes % 60).toString().padLeft(2, '0');
    String secondsStr = (seconds % 60).toString().padLeft(2, '0');

    return "$minutesStr:$secondsStr";
  }
}

https://tphangout.com/flutter-charts-and-stopwatches/