Scrolling a LongListSelector by its Vertical Offse

Question

So finally, I get the ViewportControl that I've been looking for the LongListSelector from so long but when I try to scroll to a specific offset using the below methods, I'm not able to scroll to the offset. I'm using a public static class GlobalVars (Global Variables) to pass data between pages.

So when a user selects an item from the LongListSelector -> Navigates to ItemDetail Page by this:

 private void llsArtists_SelectionChanged(object sender, SelectionChangedEventArgs e)
    {
        GetFirstVisibleItem(llsArtists);
        var artist = (Artist)e.AddedItems[0];
        GlobalVars._artistName = artist.ArtistName;
        GetSelectedArtist(artist.ArtistName);

        NavigationService.Navigate(new Uri("/ArtistDetail.xaml", UriKind.Relative));
    }

and then comes back to the same page by hitting Back Key, I would like the user to be on the same item as the last one (currently it jumps to the first index.) Therefore, I get the Viewport and the vertical offset:

        private void GetFirstVisibleItem(LongListSelector lls)
    {     
        var offset = FindViewport(lls).Viewport.Top;
        GlobalVars._artistScrollOffset = offset;
    }


   private static ViewportControl FindViewport(DependencyObject parent)
    {
        var childCount = VisualTreeHelper.GetChildrenCount(parent);
        for (var i = 0; i < childCount; i++)
        {
            var elt = VisualTreeHelper.GetChild(parent, i);
            if (elt is ViewportControl)

                return (ViewportControl)elt;

            var result = FindViewport(elt);

            if (result != null) 
                return result;
        }
        return null;
    }

and finally ScrollToOffset

        void ScrollToOffset()
    {
        Point loc = new Point();
        loc.X = 0;
        //loc.Y = Math.Round(GlobalVars._artistScrollOffset, 2);
        loc.Y = GlobalVars._artistScrollOffset;
        FindViewport(llsArtists).SetViewportOrigin(loc);
    }

The problem is - If I explicitly set the value of Y for the Point loc like I set the loc.X = 0, it just works fine! The LongListSelector works perfectly! But, if I would like it to be a variable which is exactly what I would like it to be - IT JUST DOESN'T WORK! I find this very strange? What could be causing this? Any help would be appreciated!

Answer 1

I think you're over thinking this one quite a bit. You shouldn't need to do anything, it should return to back to the page where it left off at the exact position.

The only thing I can think is that you're "changing"/"resetting" the DataContext or ItemsSource when you NavigateTo or NavigateFrom. Other possibility is that you set the LongListSelector DataContent/ItemsSource on the PhoneApplicationPage_Loaded event.

If you did the last one, it will reset the list.

Either set the ItemsSource in the Constructor or change the PhoneApplicationPage_Loaded to something like this:

private void PhoneApplicationPage_Loaded(object sender, RoutedEventArgs e)
{
    if(myLongListSelector.ItemsSource == null)
    {
        // myLongListSelector.ItemsSource = YOUR_DATASOURCE;
    }        
}

This way it will only set it once so returning to the page doesn't reassign it.