如何使用C＃停止计时器计数器为零？

Question

我用下面的代码：

DispatcherTimer sec = new DispatcherTimer(); 
sec.Interval = new TimeSpan(0, 0, 0, 1); 

sec.Tick += delegate 
{ 
    lblsec.Text = b--.ToString() + " Seconds."; 
}; 

sec.Start(); 
c--; 

该代码会显示从5计数器开始，并会降低下来，它会变为负。 我的问题是当它达到零时如何阻止它？

Answer 1

首先，您的计时器间隔太短。你永远不会从Windows获得单毫秒定时器间隔，而且出于UI的目的，用户永远不会很快察觉到定时器更新。对于这样的事情，100ms或更长时间更合适。

其次，你不能指望计时器是非常精确的。例如，如果您指定100毫秒的时间间隔，则您可能会在一秒钟内回拨十次，但通常您不会。它将取决于Windows线程调度程序的解析以及UI线程正在执行的其他活动。

考虑到这一点，并与你正试图在这里做的是设置一个五秒钟的定时器，显示倒计时到用户的假设，这样的事情应该工作：

TimeSpan total = TimeSpan.FromSeconds(5); 
DispatcherTimer timer = new DispatcherTimer(); 
Stopwatch sw = new Stopwatch(); 

timer.Interval = TimeSpan.FromMilliseconds(100); 
timer.Tick += (sender, e) => 
{ 
    double secondsLeft = (total - sw.Elapsed).TotalSeconds; 

    if (secondsLeft <= 0) 
    { 
     timer.Stop(); 
     secondsLeft = 0; 
    } 

    lblsec.Text = secondsLeft.ToString("0.0") + " Seconds"; 
}; 

sw.Start(); 
timer.Start(); 

附录：

这里是示出了如何在上面的代码可以用于一个完整的WPF程序：

C＃ ：

class TimerModel : INotifyPropertyChanged 
{ 
    private TimeSpan _timeLeft; 
    private readonly ICommand _startCommand; 

    public TimeSpan TimeLeft 
    { 
     get { return _timeLeft; } 
     set 
     { 
      if (value != _timeLeft) 
      { 
       _timeLeft = value; 
       OnPropertyChanged(); 
      } 
     } 
    } 

    public ICommand Start { get { return _startCommand; } } 

    public TimerModel() 
    { 
     _startCommand = new StartCommand(this); 
    } 

    private class StartCommand : ICommand 
    { 
     private bool _running; 
     private readonly TimerModel _timerModel; 

     public bool CanExecute(object parameter) 
     { 
      return !_running; 
     } 

     public event EventHandler CanExecuteChanged; 

     public StartCommand(TimerModel timerModel) 
     { 
      _timerModel = timerModel; 
     } 

     public void Execute(object parameter) 
     { 
      TimeSpan total = TimeSpan.FromSeconds(5); 
      DispatcherTimer timer = new DispatcherTimer(); 
      Stopwatch sw = new Stopwatch(); 

      timer.Interval = TimeSpan.FromMilliseconds(100); 
      timer.Tick += (sender, e) => 
      { 
       TimeSpan timeLeft = total - sw.Elapsed; 

       if (timeLeft <= TimeSpan.Zero) 
       { 
        timer.Stop(); 
        timeLeft = TimeSpan.Zero; 
        _running = false; 
        OnCanExecuteChanged(); 
       } 

       _timerModel.TimeLeft = timeLeft; 
      }; 

      sw.Start(); 
      timer.Start(); 
      _running = true; 
      OnCanExecuteChanged(); 
     } 

     private void OnCanExecuteChanged() 
     { 
      EventHandler handler = CanExecuteChanged; 

      if (handler != null) 
      { 
       handler(this, EventArgs.Empty); 
      } 
     } 
    } 

    public event PropertyChangedEventHandler PropertyChanged; 

    private void OnPropertyChanged([CallerMemberName]string propertyName = null) 
    { 
     PropertyChangedEventHandler handler = PropertyChanged; 

     if (handler != null) 
     { 
      handler(this, new PropertyChangedEventArgs(propertyName)); 
     } 
    } 
} 

XAML：

<Window x:Class="TestSO27333077CountdownTimer.MainWindow" 
     xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" 
     xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" 
     xmlns:l="clr-namespace:TestSO27333077CountdownTimer" 
     Title="MainWindow" Height="350" Width="525"> 

    <Window.DataContext> 
    <l:TimerModel/> 
    </Window.DataContext> 

    <StackPanel> 
    <Button Content="Start" Command="{Binding Start}" HorizontalAlignment="Left"/> 
    <TextBlock Text="{Binding TimeLeft.TotalSeconds, StringFormat={}{0:0.0} Seconds}"/> 
    </StackPanel> 
</Window> 

注意，计时也可以使用DateTime.UtcNow属性，而不是一个Stopwatch来完成。

 public void Execute(object parameter) 
     { 
      DateTime finishTime = DateTime.UtcNow + TimeSpan.FromSeconds(5); 
      DispatcherTimer timer = new DispatcherTimer(); 

      timer.Interval = TimeSpan.FromMilliseconds(100); 
      timer.Tick += (sender, e) => 
      { 
       TimeSpan timeLeft = finishTime - DateTime.UtcNow; 

       if (timeLeft <= TimeSpan.Zero) 
       { 
        timer.Stop(); 
        timeLeft = TimeSpan.Zero; 
        _running = false; 
        OnCanExecuteChanged(); 
       } 

       _timerModel.TimeLeft = timeLeft; 
      }; 

      timer.Start(); 
      _running = true; 
      OnCanExecuteChanged(); 
     }