Question

In WPF when the MouseMove event is triggered, I want to check if the left mouse key is pressed, if it’s pressed I want to do some calculation.

I have tried this:

private void Window_MouseMove(object sender, MouseEventArgs e)
{
    if (e.LeftButton == MouseButtonState.Pressed)
    {
        Calculate();
    }

 }

But when I left-click and hold and move the mouse around the screen the Calculate() only gets called one time (i.e when the mouse starts moving), after that no matter where I move the mouse the method does not get called.

I want the Calculate() method to get called repeatedly while the mouse is moving and while the left mouse key is pressed.

Answer 1

A simple test for me shows this working:

private void MouseMoveEvent(object sender, MouseEventArgs e)
{
    if (e.LeftButton == MouseButtonState.Pressed)
    {
       _vm.MoveCount++;
    }
}

A would suspect that your Calculate method is a synchronous method that’s causing problems.

Try calling the method like below. This will start the calculate method in a new thread. Then the updating of the text box will happen in the original thread.

    private void WindowMouseMoveEvent(object sender, MouseEventArgs e)
    {
        if (e.LeftButton == MouseButtonState.Pressed)
        {
            var worker = new BackgroundWorker();
            worker.DoWork += Calculate;
            worker.RunWorkerCompleted += 
                (o, args) => myTextBox.Text = (string)args.Result;
            worker.RunWorkerAsync();
        }
    }


    private void Calculate(object sender, DoWorkEventArgs args)
    {
        //do work
        args.Result = "my new value"
    }