ScrollViewer - Indication of child element scrolled into view

Question

Is there an event that is raised when a child is scrolled into view and gives an indication of what child was realized?

Of course there is the ScrollChanged event, b

Answer 1

I think you should look at this article which gives a way of telling if a control is visible to the viewer.

If you were to hook up a call to that custom method in your ScrollChanged handler, thus having a checked every time you scrolled, I think that would do the trick. I'll try this out myself....

Edit: It works! Here's the method:

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

And my simple code call:

private void Scroll_Changed(object sender, ScrollChangedEventArgs e)
{
    Object o = sender;
    bool elementIsVisible = false;

    foreach (FrameworkElement child in this.stackPanel1.Children)
    {
        if (child != null)
        {
            elementIsVisible = this.IsUserVisible(child, this.scroller);

            if (elementIsVisible)
            {
                 // Your logic
            }
        }
    }
}

Edit: I took a look through the source code of the ScrollViewer from the link that dev hedgehog posted and found this interesting private function:

// Returns true only if element is partly visible in the current viewport
private bool IsInViewport(ScrollContentPresenter scp, DependencyObject element)
{
     Rect viewPortRect = KeyboardNavigation.GetRectangle(scp);
     Rect elementRect = KeyboardNavigation.GetRectangle(element);
     return viewPortRect.IntersectsWith(elementRect);
}

This obviously suggests that even the ScrollViewer itself is interested in knowing what's visible and, as I expected, essentially performs the same kind of calculation as in that helper method. It might be worthwhile to download this code and see who calls this method, where, and why.

Edit: Looks like its called by OnKeyDown() and used to determine focus behavior, and that's it. Interesting....