WP7上的计时器分派更改顺序混乱

Question

I'm trying to make a countdown timer on Windows Phone 7, which is really important for my application. But I can't find any method to update text in UI regullary every one second.

Timer dt = new System.Threading.Timer(delegate
{
 Dispatcher.BeginInvoke(() =>
   {
      newtime = oldtime--;
      System.Diagnostics.Debug.WriteLine("#" + counter.ToString() + 
                                         " new: " + newtime.ToString() + 
                                         " old: " + oldtime.ToString());
      counter++;
      oldtime = newtime;
   }
}, null, 0, 1000);

After running my app console output seems like this:

 #1 new: 445 old: 446 #2 new: 444 old: 445 #3 new: 445 old: 446 #4 new: 443 old: 444 #5 new: 444 old: 445 #6 new: 442 old: 443 #7 new: 443 old: 444 #8 new: 441 old: 442 

I can't figure out how to get rid of that unwanted invokes (#3, #5, #7, etc.)

Thank you for any advice.

Answer 1

You should use the DispatcherTimer instead. The following example shows a timer doing a countdown from ten.

DispatcherTimer _timer = new DispatcherTimer { Interval = TimeSpan.FromSeconds(1) };
int _timeLeft = 10;

public MyClass()
{
    InitializeComponent();
    _timer.Tick += TimerTick;
    _timer.Start();
    MyTextBox.Text = _timeLeft.ToString();
}

void TimerTick(object sender, EventArgs e)
{
    _timeLeft--;
    if (_timeLeft == 0)
    {
        _timer.Stop();
        MyTextBox.Text = null;
    }
    else
    {
        MyTextBox.Text = _timeLeft.ToString();
    }
}

Answer 2

Try the following mod:

DispatcherTimer _timer = new DispatcherTimer { Interval = TimeSpan.FromMilliseconds(200) };
int _timeLeft = 50;
Stopwatch watch = new Stopwatch();
public MainPage()
{
InitializeComponent();
_timer.Tick += TimerTick;
_timer.Start();
textBlock1.Text = _timeLeft.ToString();
watch.Start();
}
void TimerTick(object sender, EventArgs e)
{

    if ((_timeLeft - (int)watch.Elapsed.TotalSeconds) <= 0)
    {
        watch.Stop();
        _timer.Stop();
        textBlock1.Text = null;
    }
    else
    {
        textBlock1.Text = (_timeLeft - (int)watch.Elapsed.TotalSeconds).ToString();
    }
}

By the way Shawn's code just works fine on my device but if you're experiencing problems, just use a Stopwatch and subtract the elapsed time from your time variable. Also, run DispatcherTimer a little faster (of course for this technique) like 200ms for greater accuracy (everything has been implemented above). Hope that helps.

Answer 3

Looking at the code and at the comments, I suspect that the fault with your application is not with the Timer code, but instead with whatever initialises the Timer - I suspect the timer is being constructed twice.

It's hard to debug this without seeing the code outside of the block you've posted, but the symptoms you describe suggest that you are initialising multiple Timers and multiple stack/closure variables oldTime and newTime

At a simple level, you could try protecting the Timer construction - eg with something like:

public class MyClass
{

// existing code...

private bool _timerStarted;

private void StartTimer()
{

if (_timerStarted)
{
    Debug.WriteLine("Timer already started - ignoring");
    return;
}

_timerStarted = true;

var newTime = 500;
var oldTime = 500;
var counter = 1;

Timer dt = new System.Threading.Timer(delegate
{
 Dispatcher.BeginInvoke(() =>
   {
      newtime = oldtime--;
      System.Diagnostics.Debug.WriteLine("#" + counter.ToString() + 
                                         " new: " + newtime.ToString() + 
                                         " old: " + oldtime.ToString());
      counter++;
      oldtime = newtime;
   }
}, null, 0, 1000);
}
}