selected ??????

Dear Sir!

I would like to select object..

I would like to select the following.

When I call dialog and i select button,

I select face of object. and Immediately, i would like to display the result(normal vector of face) information immetedly.

but In open cascade, The object have to select previosly.

I would like to display the result to select immediately(interactively) without select object previously

How do i ?

Thanks in advanced..