Binding to a ScrollViewer's ViewportWidth and ViewportHeight

Question

I am using the Model-View-ViewModel architecture in a WPF application I am building, and I would like a specific ViewModel to actually be reactive to the size of the view (not a normal use-case of the MVVM approach, I know).

Essentially, I have a ScrollViewer object and I want the viewmodel to observe the width and height of the scrollviewer and then be able to do things accordingly depending on what that width and height are.

I'd like to do something like this:

<ScrollViewer ViewportWidth="{Binding Path=MyViewportWidth, Mode=OneWayToSource}" ViewportHeight="{Binding Path=MyViewportHeight, Mode=OneWayToSource}" />

But of course this is impossible to do because "ViewportWidth" and "ViewportHeight" cannot be "bound to" (a.k.a. act as binding targets) because they are read-only dependency properties (even though I am not writing to them at all in this binding since it is OneWayToSource).

Anyone know of a good method to be able to do something like this?

Answer 1

You could try running something OnLoaded or OnResizeChanged that updates the viewmodel

private void ScrollViewer_Loaded(object sender, RoutedEventArgs e)
{
   ScrollViewer sv = sender as ScrollViewer;
   ViewModel vm = sv.DataContext as ViewModel;

   vm.ScrollViewerHeight = sv.ViewportHeight;
   vm.ScrollViewerWidth = sv.ViewportWidth;
}