ScrollViewer and ScrollToVerticalOffset with mvvm

I would like to use the ScrollToVerticalOffset method of a ScrollViewer to go to the top of the scrollviewer.

But with a MVVM approch.

I think I have to create a dependency property to take this behavior.

EDIT : The behavior is :

public class ScrollPositionBehavior : Behavior<FrameworkElement>
{
    public double Position
    {
        get { return (double)GetValue(PositionProperty); }
        set { SetValue(PositionProperty, value); }
    }

    public static readonly DependencyProperty PositionProperty = DependencyProperty.Register("Position", typeof(double), typeof(ScrollPositionBehavior), new PropertyMetadata((double)0, new PropertyChangedCallback(OnPositionChanged)));

    protected override void OnAttached()
    {
        base.OnAttached();
    }

    private static void OnPositionChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        ScrollPositionBehavior behavior = d as ScrollPositionBehavior;
        double value = (double)e.NewValue;
        ((ScrollViewer)(behavior.AssociatedObject)).ScrollToVerticalOffset(value);
    }

    protected override void OnDetaching()
    {
      开发者_JAVA百科  base.OnDetaching();
    }
}

used like :

    <ScrollViewer>
        <Interactivity:Interaction.Behaviors>
            <fxBehavior:ScrollPositionBehavior Position="{Binding Position}" />
        </Interactivity:Interaction.Behaviors>
        <other things ...>
    </ScrollViewer>

with

xmlns:Interactivity="clr-namespace:System.Windows.Interactivity;assembly=System.Windows.Interactivity"
xmlns:fxBehavior="clr-namespace:MyNamespace.Behavior;assembly=MyAssembly"

I have a parser xaml exception :

this is a : AG_E_PARSER_BAD_PROPERTY_VALUE

Note that i'm using the behavior based on a FrameworkElement, as I'm using silverlight 3 (in fact, this is SL for WP7). I've seen that the binding should work only for FrameworkElement.

Thanks in advance for your help

You were on the right way. First of all you need to change your OnPositionChanged method to find out which instance of the behavior had its Position changed:

private static void OnPositionChanged(DependencyObject d,
                                      DependencyPropertyChangedEventArgs e)
{
    ScrollPositionBehavior behavior = d as ScrollPositionBehavior;
    double value = (double)e.NewValue;
    behavior.AssociatedObject.ScrollToVerticalOffset(value);
}

Then, you'll get the ScrollViewer as associated object when you attach the behavior to it:

<ScrollViewer>
    <i:Interaction.Behaviors>
        <my:ScrollPositionBehavior Position="{what you need, e.g. Binding}" />
    </i:Interaction.Behaviors>
</ScrollViewer>

Note that if you use a Binding it can be a OneWay binding, because the Position will never get updated by the behavior itself.