selection callback function

Hey Budi G thanks for the help.
I ended up figuring it out and using implementing them into a handful of scripts these past few days. They are great. Thanks.

first keep your mind on register callback fn ,
and after using it you need unregister callback fn.
check out the mxs reference for details.

I try to make one example:
in this forum I've made a script, there's the update buttons as manually update for user.
http://www.scriptspot.com/forums/3ds-max/general-scripting/dropdown-list…

now I will remove the update buttons and convert it to call back function.

rollout drop_roll "Drop Down List"
(
 local obj_array = #()
 dropdownlist obj_dd "Objects :" offset:[0,-20] items:#()
 
	fn update_objs =
	 ( 
	   obj_array = objects --as array
	   obj_dd.items = for i in obj_array collect  i.name
	)
 on drop_roll open do 
 ( 
   update_objs()
   callbacks.addScript #selectionSetChanged "drop_roll.update_objs()" id:#testcall  -- register call back fn for automatically update the listbox when some object has been deleted or added
 )
 on obj_dd selected i do select obj_array[i]

 on drop_roll close do callbacks.removeScripts id:#testcall -- unregister call back scripts when closing the dialog
 
)
 
createDialog drop_roll