WPF Ensure RenderTargetBitmap has updated Binding values from background thread changes

Question

I'm having a problem with RenderTargetBitmap, in that I can't get it consistently grab an updated render after I change bound properties on a background thread.

Here is what I have:

        // Update a property on an INotifyPropertyChanged view model
        // This runs on a background thread
        viewModel.SomeBoundProperty += 10;

        // Flush dispatcher queue (from http://stackoverflow.com/a/2596035/612510)
        _lcd.Dispatcher.Invoke(() => {}, System.Windows.Threading.DispatcherPriority.Loaded);

        // Render the updated control
        _lcd.Dispatcher.Invoke(() =>
        {
            _lcd.Measure(new System.Windows.Size(240, 160));
            _lcd.Arrange(new System.Windows.Rect(0, 0, 240, 160));
            _lcd.UpdateLayout();

            _renderTarget.Render(_lcd);
        }

Alas, approximately half the time I'm getting the render before the control is updated with the new value, and the other half it updates correctly.

From what I understand WPF dispatches property change notifications automatically to the UI thread. How can I ensure that these dispatched notifications are all processed before doing the render? This code works fine if I make sure SomeBoundProperty is updated on the Dispatcher thread, but that is less than ideal for this particular application.

Any suggestions?

Answer 1

Some trial and error has me figuring that property change notifications are dispatched at the DispatcherPriority.Background priority, so changing the flushing line to this:

// Flush dispatcher queue 
_lcd.Dispatcher.Invoke(() => {}, System.Windows.Threading.DispatcherPriority.ContextIdle);

...looks like it fixed the problem. DispatcherPriority.ContextIdle is one level below DispatcherPriority.Backgound. The render now updates consistently every time.