[flexcoders] ChangeWatcher watch ArrayCollection

[Fu Di]

hi  every one

i used ChangeWatcher   to watch an  AC,  code like this:   
ChangeWatcher.watch(model, "positionInfoProvider", dataListener);
"positionInfoProvider" is the AC.  when  one element of  "positionInfoProvider" 
was changed,  the ChangeWatcher  can't  listen it.
 unless i give  AC a new value. i  do't want to do this.  any idea??    

 extend problem, i  used  an AC   as  DataGrid 's provider , i just want to  
change a element of the AC to make DataGrid update. how to do it??

 thanks