WPF forcing GUI update using Dipatcher

Question

Hi everyone!

I need only to show a custom control (a clock with rotating hands) and with this to replace the mouse cursor, the problem is that if I write:

Me.gridScreen.Visibility = Visibility.Visible
' some operations that takes about 1 second
Me.gridScreen.Visibility = Visibility.Hidden
(gridScreen is the grid that contains the user-control)

Obviously I can see nothing, because the update of the UI happens at the end of the procedure. I have tried Me.UpdateLayout(), but it doesn't work.

I have tryed to use the dispacker in many way but none that works :-(

This is my lost attempt:

(uCurClock is the usercontrol, gridScreen a Grid placed at the top-level in the window, with trasparent background, that contains the usercontrol)

Private Sub showClock()
    Dim thread = New System.Threading.Thread(AddressOf showClockIntermediate)
    thread.Start()
End Sub

Private Sub hideClock()
    Dim thread = New System.Threading.Thread(AddressOf hideClockIntermediate)
    thread.Start()
End Sub

Private Sub showClockIntermediate()
    Me.Dispatcher.BeginInvoke(DispatcherPriority.Normal, _
        New Action(AddressOf showClockFinale))
End Sub

Private Sub hideClockIntermediate()
    Me.Dispatcher.BeginInvoke(DispatcherPriority.Normal, _
        New Action(AddressOf hideClockFinale))
End Sub

Private Sub showClockFinale()
    Dim pt As Point = Mouse.GetPosition(Nothing)
    Me.uCurClock.Margin = New Thickness(pt.X - 9, pt.Y - 9, 0, 0)
    Me.gridScreen.Visibility = Visibility.Visible
    Me.Cursor = Cursors.None
    Me.UpdateLayout()
End Sub

Private Sub hideClockFinale()
    Me.gridScreen.Visibility = Visibility.Hidden
    Me.Cursor = Cursors.Arrow
    Me.UpdateLayout()
End Sub

Private Sub u_MouseMove(ByVal sender As System.Object, _
    ByVal e As MouseEventArgs) Handles gridScreen.MouseMove

    Dim pt As Point = e.GetPosition(Nothing)
    Me.uCurClock.Margin = New Thickness(pt.X - 9, pt.Y - 9, 0, 0)

    e.Handled = True
End Sub

Private Sub u_MouseEnter(ByVal sender As System.Object, _
    ByVal e As MouseEventArgs) Handles gridScreen.MouseEnter

    Me.uCurClock.Visibility = Visibility.Visible

    e.Handled = True
End Sub

Private Sub u_MouseLeave(ByVal sender As System.Object, _
    ByVal e As MouseEventArgs) Handles gridScreen.MouseLeave

    Me.uCurClock.Visibility = Visibility.Hidden

    e.Handled = True
End Sub

PIleggi

Answer 1

The problem is not to do with the composition of messages being executed on the dispatcher, it's that you're executing long-running work on the dispatcher at all. You must ensure that long-running/potentially blocking operations are executed on a background thread. The easiest way to do so is with the BackgroundWorker component.

Excuse the C#:

var backgroundWorker = new BackgroundWorker();

backgroundWorker.DoWork += delegate
{
    // long running work goes here
};
backgroundWorker.RunWorkerCompleted += delegate
{
    // change cursor back to normal here
};

// change cursor to busy here

// kick off the background task
backgroundWorker.RunWorkerAsync();

HTH,
Kent