Binding dependency property - set in custom user control, get in viewmodel

Question

The aim is to instantiate an object in a custom UserControl and then access that instance from a viewmodel. But the following is not working, the UpRatings property in the viewmodel is null.

Custom UserControl with Dependency Property

public partial class Scale:UserControl
{

    public Scale()
    {
        InitializeComponent();
    }

    public static readonly DependencyProperty MouseRatingsProperty = 
        DependencyProperty.Register(
        "MouseRatings",
        typeof(IObservable<double>),
        typeof(Scale));

    public IObservable<double> MouseRatings
    {
        get
        {
            return (IObservable<double>)GetValue(MouseRatingsProperty);
        }
        set
        {
            SetValue(MouseRatingsProperty, value);                
        }
    }

    // requires System.Reactive NuGet
    private ISubject<double> _mouseRatings = new Subject<double>();

    protected override void OnInitialized(EventArgs e)
    {
        base.OnInitialized(e);
        // _mouseRatings is the object instance I need to access in the viewmodel
        MouseRatings = _mouseRatings;
    }
}

View

<StackPanel>
    <scale:Scale MouseRatings="{Binding Path=UpRatings, Mode=TwoWay}"/>
</StackPanel>

ViewModel

// requires Prism.Wpf NuGet
public class MyViewModel : BindableBase
{
    public IObservable<double> UpRatings { get; set; }

    // called after the DataContext is set, so the UpRatings binding should work here by now
    // see https://github.com/PrismLibrary/Prism/blob/master/Documentation/WPF/60-Navigation.md
    public void OnNavigatedTo(NavigationContext navigationContext)
    {
        // but UpRatings is unfortunately null here
        Trace.WriteLine(UpRatings == null ? "null" : "not null");
    }

}

Note that the DataContext is not set directly on the custom usercontrol but a parent of hers (and I can confirm that inside the custom usercontrol).

Answer 1

It started to work when MouseRatings = _mouseRatings; is called later as follows, while MouseRatings lost its value for some mysterious reason if setting it OnInitialized.

public Scale()
    {
        InitializeComponent();
        IsVisibleChanged += Scale_IsVisibleChanged;
    }

private void Scale_IsVisibleChanged(object sender, DependencyPropertyChangedEventArgs e)
    {
        if (IsVisible)
        {
            MouseRatings = _mouseRatings;
        }
    }

It would also work to set it in the UserControl Loaded event, but then the viewmodel has to wait extra time before being able to access the object.