vb.net - How do I detect if a panel is in the viewable window? -

using winforms , vb.net vs 2012:

if have large scrollable panel , put bunch of smaller panels inside larger one. when scroll larger panel or down, of smaller panels scroll out of view. question is, can put listener on smaller panels smaller panel knows when has scrolled out of view?

i can't use .visible property because set true when panel not in window. know can set .focus property bring view, can still lose focus when in viewable window else takes focus.

so there property or can listen detects when panel has scrolled out of view? , if not, there else can detect this?

thanks.

from scroll() , mousewheel() events of panel, use code this:

private sub panel1_scroll(sender object, e scrolleventargs) handles panel1.scroll     checkvisibility(sender) end sub  private sub panel1_mousewheel(sender object, e mouseeventargs) handles panel1.mousewheel     checkvisibility(sender) end sub  private sub checkvisibility(byval pnl panel)     dim results new list(of string)      dim pnlrectscreen rectangle = pnl.parent.rectangletoscreen(pnl.bounds)     each child control in pnl.controls         dim childrectscreen rectangle = pnl.rectangletoscreen(child.bounds)         if pnlrectscreen.intersectswith(childrectscreen)             if pnlrectscreen.contains(childrectscreen)                 results.add(child.name & ": visible")             else                 results.add(child.name & ": partially visible")             end if         else             results.add(child.name & ": not visible")         end if     next      listbox1.datasource = nothing     listbox1.datasource = results end sub 

if want know when becomes hidden only, track current visibility dictionary() , when changes hidden. here's more robust example:

public enum controlvisibility     hidden     partialyvisible     fullyvisible end enum  private visibiltystate new dictionary(of control, controlvisibility)  private sub form1_shown(sender object, e eventargs) handles me.shown     dim pnl panel = panel1     dim pnlrectscreen rectangle = pnl.parent.rectangletoscreen(pnl.bounds)     each child control in pnl.controls         dim childrectscreen rectangle = panel2.rectangletoscreen(child.bounds)         if pnlrectscreen.intersectswith(childrectscreen)             if pnlrectscreen.contains(childrectscreen)                 visibiltystate.add(child, controlvisibility.fullyvisible)             else                 visibiltystate.add(child, controlvisibility.partialyvisible)             end if         else             visibiltystate.add(child, controlvisibility.hidden)         end if     next end sub  private sub panel1_scroll(sender object, e scrolleventargs) handles panel1.scroll     checkvisibility(sender) end sub  private sub panel1_mousewheel(sender object, e mouseeventargs) handles panel1.mousewheel     checkvisibility(sender) end sub  private sub checkvisibility(byval pnl panel)     dim newvisibility controlvisibility     dim pnlrectscreen rectangle = pnl.parent.rectangletoscreen(pnl.bounds)     each child control in pnl.controls         dim childrectscreen rectangle = pnl.rectangletoscreen(child.bounds)         if pnlrectscreen.intersectswith(childrectscreen)             if pnlrectscreen.contains(childrectscreen)                 newvisibility = controlvisibility.fullyvisible             else                 newvisibility = controlvisibility.partialyvisible             end if         else             newvisibility = controlvisibility.hidden         end if          ' has changed?         if visibiltystate(child) <> newvisibility andalso newvisibility = controlvisibility.hidden             debug.print(child.name & " has become hidden: " & datetime.now.tostring("h:mm:ss.ffff"))         end if          visibiltystate(child) = newvisibility     next end sub