How can I get the VerticalOffset of a LongListSelector in WP8

Question

In WP7, the LongListSelector had a underlying ScrollViewer, from which I could recover the vertical offset of the list. But in Windows Phone 8, there\'s no underlying Scroll

Answer 1

This will give you the first visible item in the LLS.

private Dictionary<object, ContentPresenter> items;

private object GetFirstVisibleItem(LongListSelector lls)
{
    var offset = FindViewport(lls).Viewport.Top;
    return items.Where(x => Canvas.GetTop(x.Value) + x.Value.ActualHeight > offset)
        .OrderBy(x => Canvas.GetTop(x.Value)).First().Key;
}

private void LLS_ItemRealized(object sender, ItemRealizationEventArgs e)
{
    if (e.ItemKind == LongListSelectorItemKind.Item)
    {
        object o = e.Container.DataContext;
        items[o] = e.Container;
    }
}

private void LLS_ItemUnrealized(object sender, ItemRealizationEventArgs e)
{
    if (e.ItemKind == LongListSelectorItemKind.Item)
    {
        object o = e.Container.DataContext;
        items.Remove(o);
    }
}

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;
}