颤振：保持时间答案

【问题标题】：Flutter: Keeping Time颤振：保持时间
【发布时间】：2018-07-20 23:57:52
【问题描述】：

试图显示一个基于当前东部时间每周重置自身的静态计时器，例如屏幕显示计时器Days : Hours : Minutes : seconds（例如：7: 00 : 00 : 00），倒计时并在每个星期日结束时自行重置，或者在星期一开始。我有一个一般概念，它涉及DateTime、Timer 和Stopwatch，但很难将它们放在一起。

【问题讨论】：

虽然更多关于性能的信息 this article 会很有用

标签： datetime timer dart flutter stopwatch

【解决方案1】：

创建一个StatefulWidget，在您的Timer 更新时获取并存储当前时间。在build方法中，计算并格式化时间差：

import 'dart:async';

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Week Countdown',
      home: TestPage(),
    );
  }
}

class TestPage extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: Text('Week Countdown')),
      body: Center(
        child: Column(
          mainAxisSize: MainAxisSize.min,
          children: <Widget>[
            Text(
              'Time until next week:',
              style: Theme.of(context).textTheme.headline,
            ),
            WeekCountdown()
          ],
        ),
      ),
    );
  }
}

class WeekCountdown extends StatefulWidget {
  @override
  State<StatefulWidget> createState() => _WeekCountdownState();
}

class _WeekCountdownState extends State<WeekCountdown> {
  Timer _timer;
  DateTime _currentTime;

  @override
  void initState() {
    super.initState();
    _currentTime = DateTime.now();
    _timer = Timer.periodic(Duration(seconds: 1), _onTimeChange);
  }

  @override
  void dispose() {
    _timer.cancel();
    super.dispose();
  }

  void _onTimeChange(Timer timer) {
    setState(() {
      _currentTime = DateTime.now();
    });
  }

  @override
  Widget build(BuildContext context) {
    final startOfNextWeek = calculateStartOfNextWeek(_currentTime);
    final remaining = startOfNextWeek.difference(_currentTime);

    final days = remaining.inDays;
    final hours = remaining.inHours - remaining.inDays * 24;
    final minutes = remaining.inMinutes - remaining.inHours * 60;
    final seconds = remaining.inSeconds - remaining.inMinutes * 60;

    final formattedRemaining = '$days : $hours : $minutes : $seconds';

    return Text(formattedRemaining);
  }
}

DateTime calculateStartOfNextWeek(DateTime time) {
  final daysUntilNextWeek = 8 - time.weekday;
  return DateTime(time.year, time.month, time.day + daysUntilNextWeek);
}

【讨论】：