popup with arrow placement when near screen edge

Hello All,

 

I have a question is about WPF.

 

First at all, I did a popup with arrow  and there placement on right but arrow is pointing at left. When near screen edge, then placement will auto change to opposite which is left. Now my problem is how can I change my arrow to opposite when near screen edge?

 

Have any ideas doing in WPF?  

http://stackoverflow.com/questions/18497029/how-to-identify-the-validationerror-tooltip-placement-in-wpf-textbox

 

Similar question at above link.

 

Thanks.

 

Best regards,