Know when UIElement is shown on screen

Question

Hi there,
So I have a wrap panel with a scrollbar and I'm putting a bunch of controls in them.  I would like to perform actions when the controls become visible (are scrolled into view), either just for the first time or any time.
Is there a way to detect when a UI element is either on screen and visible or off screen and not visible?
Many thanks in advance.
Somewhere out there...

Answer 1

Hi Barry,
Unfortunately that code is also not useful for my requirement as it specifically requires that I pass the object in that I want to know is visible or not.  If I have a wrappanel with 2000 items in, that would be unusable.  I have found an alternative
method of getting the top left item and bottom right item that are in view every time the scroll changes of the parent scroll viewer.
private Int32 FindTopLeftImageViewerIndex()
ImageViewer pIVrTopLeft = null;
Int32 pIntX = 0;
Int32 pIntY = 0;
for (pIntY = 0; pIntY <= 20; pIntY+=2)
for (pIntX = 0; pIntX <= 200; pIntX+=10)
IInputElement pIEtElement = ThumbnailScrollViewer.InputHitTest(new Point(pIntX, pIntY));
if (pIEtElement != null)
if (pIEtElement.GetType() != typeof(ScrollViewer))
pIVrTopLeft = GetParentImageViewer(pIEtElement);
if (pIVrTopLeft != null)
return (cLisImages.IndexOf(pIVrTopLeft));
return (-1);
private Int32 FindBottomRightImageViewerIndex()
//Get top left item
ImageViewer pIVrBottomRight = null;
Int32 pIntX = 0;
Int32 pIntY = 0;
Int32 pIntMaxX = (Int32)ThumbnailScrollViewer.ActualWidth;
Int32 pIntMaxY = (Int32)ThumbnailScrollViewer.ActualHeight;
for (pIntY = pIntMaxY; pIntY >= (pIntMaxY - 100); pIntY -= 10)
for (pIntX = pIntMaxX; pIntX >= 2; pIntX-=2)
IInputElement pIEtElement = ThumbnailScrollViewer.InputHitTest(new Point(pIntX, pIntY));
if (pIEtElement != null)
if (pIEtElement.GetType() != typeof(ScrollViewer))
pIVrBottomRight = GetParentImageViewer(pIEtElement);
if (pIVrBottomRight != null)
return (cLisImages.IndexOf(pIVrBottomRight));
return (cLisImages.Count - 1); //Assume the last thumbnail is in view
private ImageViewer GetParentImageViewer(IInputElement iElement)
FrameworkElement pFEtCurrent = (FrameworkElement)iElement;
while (pFEtCurrent.GetType() != typeof(ImageViewer))
pFEtCurrent = (FrameworkElement)pFEtCurrent.Parent;
if (pFEtCurrent == null)
return (null);
ImageViewer pIVrViewer = (ImageViewer)pFEtCurrent;
return (pIVrViewer);
The user control I am inserting is a custom one called an ImageViewer, as in this context I am displaying thumbnails.  The code to find an input control at a specific point returns the child controls of this usercontrol so I have a method that walks
the parent stack to find the owner control.
Once I have the indexes of the first and last item in view I can then perform tasks on them as opposed to doing it the second they are added to the control which just adds unnecessary overhead.
Somewhere out there...