Help for an extension: switch workspace in overview

[Bazon Bloch <bazonbloch arcor de>]

Hello,

I'm about to write a Gnome-Shell-Extension and by now, everything is 
fine, except one thing:

I need to call a function when the workspaces switch in the overview.

I tried with Main.wm.actionMoveWorkspaceUp, but that is only for 
workspace change outside the overview.
So what can I do to get a function when switching workspaces IN the 
overview?

thanks!