Pushing read-only GUI properties back into ViewModel

Question

I want to write a ViewModel that always knows the current state of some read-only dependency properties from the View.

Specifically, my GUI contains a FlowDocumentPa

Answer 1

I like Dmitry Tashkinov's solution! However it crashed my VS in design mode. That's why I added a line to OnSourceChanged method:

    private static void OnSourceChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        if (!((bool)DesignerProperties.IsInDesignModeProperty.GetMetadata(typeof(DependencyObject)).DefaultValue))
            ((DataPipe)d).OnSourceChanged(e);
    }