c# - Get ListView Visible items

Question

I have a ListView which might contains a lot of items, so it is virtualized and recycling items. It does not use sort. I need to refresh some value display, but when there are too many items, it is too slow to update everything, so I would like to refresh only the visible items.

How could I get a list of all currently displayed items ? I tried to look into the ListView or in the ScrollViewer, but I still have no idea how to achieve this. The solution must NOT go through all items to test if they can be seen, because this would be too slow.

I'm not sure code or xaml would be useful, it is just a Virtualized/Recycling ListView with its ItemSource bound to an Array.

Edit : Answer :
thanks to akjoshi, I found the way :

• get the ScrollViewer of the ListView (with a FindDescendant method, that you can do yourself with the VisualTreeHelper ).

• read its ScrollViewer.VerticalOffset : it is the number of the first item shown

• read its ScrollViewer.ViewportHeight : it is the count of items shown.
  Rq : CanContentScroll must be true.

Answer 1

After trying to figure out something similar, I thought I would share my result here (as it seems easier than the other responses):

Simple visibility test I got from here.

private static bool IsUserVisible(FrameworkElement element, FrameworkElement container)
{
    if (!element.IsVisible)
        return false;

    Rect bounds =
        element.TransformToAncestor(container).TransformBounds(new Rect(0.0, 0.0, element.ActualWidth, element.ActualHeight));
    var rect = new Rect(0.0, 0.0, container.ActualWidth, container.ActualHeight);
    return rect.Contains(bounds.TopLeft) || rect.Contains(bounds.BottomRight);
}

Afterwards you can loop through the listboxitems and use that test to determine which are visible. Since the listboxitems are always ordered the same the first visible one in this list would be the first visible one to the user.

private List<object> GetVisibleItemsFromListbox(ListBox listBox, FrameworkElement parentToTestVisibility)
{
    var items = new List<object>();

    foreach (var item in PhotosListBox.Items)
    {
        if (IsUserVisible((ListBoxItem)listBox.ItemContainerGenerator.ContainerFromItem(item), parentToTestVisibility))
        {
            items.Add(item);
        }
        else if (items.Any())
        {
            break;
        }
    }

    return items;
}