Dispatcher timer is firing twice :(

Question

i have problems with this timer, my function in the Tick event are appearing twice.. i want it to appear only once..

        public void timerStart()
        {
            DispatcherTimer updaterTimer = new DispatcherTimer();
            updaterTimer.Tick += new EventHandler(updaterTimer_Tick);
            updaterTimer.Interval = new TimeSpan(0,0,0,0,300);
            updaterTimer.Start();
        }

        private void updaterTimer_Tick(object sender, EventArgs e)
        {
           updaterTimer.Stop();
           checkSigningAvailable();
           updaterTimer.Start();
        }

This is the method that is checked every tick of the timer,

        public void checkSigningAvailable()
        {
           if (dt_signing_in.CompareTo(DateTime.Now) < 0)
           {
               if (!InPopAlready)
               {
                  InPopAlready = true;
                  disableSigningIn("False", this.event_id);
               }
           }
        }

And the messagebox in the bottom is appearing twice after calling this function above

        public void disableSigningIn(string Out,string event_id)
        {
           System.Console.WriteLine("POPED "+ InPopAlready);

           connection.Open();
           string sign = "True," + Out;
           string query = "update data_storage set data_details = '" + sign + "' where data_name = 'Signing';";
           NpgsqlCommand command = new NpgsqlCommand(query, connection);
           command.ExecuteNonQuery();
           connection.Close();
           sign_in.Content = "Sign-in Time : Over";
           string query2 = concatQuery(getIDnumberAttendance(event_id));
           updateAbsences(query2);

           MessageBox.Show("Signing in is over!", "No more signing in!", MessageBoxButton.OK, MessageBoxImage.Information);
}

Answer 1

You are adding " += new EventHandler " and adding and adding new EventHandlers all over new EventHandlers but never remove them..

All the previous one gets fired each Time the Timer Starts again. You can reproduce this behaviour if you implement a counter, then you will see that is doubles with each new added and raised Event. (Edit: Just was confused because the "new" keyword, but actually I will not delete the answer since I am pretty sure that in some cases it will exactly be the issue)

The following may help: How to remove all event handlers from an event

And here the most easy solution: (You may polish it by using delegates)

Declaration in Class:

private System.Windows.Threading.DispatcherTimer P5DispatcherHelpsystemTimer = new System.Windows.Threading.DispatcherTimer();
private EventHandler P5DispatcherTimerHandler;

Dispatcher Timer Method:

private void InitializeHelpsystemCronjobs(System.Windows.Controls.Canvas sub_CanvasElement)
{
        P5DispatcherTimerHandler = (sender, e) => P5DispatcherHelpsystemTimerTick(sender, e, sub_CanvasElement);
        P5DispatcherHelpsystemTimer.Tick += P5DispatcherTimerHandler;
        P5DispatcherHelpsystemTimer.Interval = new TimeSpan(0, 0, 1);
        P5DispatcherHelpsystemTimer.Start();
}

Dispatcher TimerTick Method:

private void P5DispatcherHelpsystemTimerTick(object sender, EventArgs e, System.Windows.Controls.Canvas sub_CanvasElement)
{
    P5DispatcherHelpsystemTimer.Stop();
    {
       // Do stuff
    }
    P5DispatcherHelpsystemTimer.Start();
}

// Somewhere when an Trigger Event or Action etc. happens and the Timer shall start:

InitializeHelpsystemCronjobs(HelpsystemHelpCanvas);

// Somewhere when it sahll stop:

P5DispatcherHelpsystemTimer.Stop();
P5DispatcherHelpsystemTimer.Tick -= P5DispatcherTimerHandler;

(If you have more complex situation you definitely need delegate List<>)

Answer 2

For me, it was not starting, stopping or the Timer at all. It was the DI/IOC container resolving two instances of the ViewModel which started a new refresh timer in the constructor. Check to make sure that you are always seeing the same ViewModel instance and not actually two separate ones, firing two separate Timers.

Answer 3

The code below fires the MessageBox only once:

public partial class MainWindow : Window
{
    public MainWindow()
    {
        InitializeComponent();

        this.timerStart();
    }

    DispatcherTimer updaterTimer;
    private bool InPopAlready;
    DateTime dt_signing_in;

    public void timerStart()
    {
        updaterTimer = new DispatcherTimer();
        updaterTimer.Tick += new EventHandler(updaterTimer_Tick);
        updaterTimer.Interval = new TimeSpan(0, 0, 0, 0, 300);
        updaterTimer.Start();
    }

    private void updaterTimer_Tick(object sender, EventArgs e)
    {
        updaterTimer.Stop();
        checkSigningAvailable();
        updaterTimer.Start();
    }



    public void checkSigningAvailable()
    {
        if (dt_signing_in.CompareTo(DateTime.Now) < 0)
        {
            if (!InPopAlready)
            {
                InPopAlready = true;

                // Calling your method and showing MessageBox
                MessageBox.Show("Signing in is over!", "No more signing in!", MessageBoxButton.OK, MessageBoxImage.Information);
            }
        }
    }
}

Answer 4

I tend to write my "once-off" timer code like this:

var updaterTimer = new DispatcherTimer();
updaterTimer.Interval = new TimeSpan(0, 0, 0, 0, 300);
EventHandler tick = null;
tick = (s, e) =>
{
    updaterTimer.Stop();
    updaterTimer.Tick -= tick;
    /* execute once-off code here */
};
updaterTimer.Tick += tick;
updaterTimer.Start();

Then I don't need to fluff with making new methods - it's all in one local code block.