How to get the viewport element index

I use flex4 list:

 <component:SmoothScrollingList x="200"
           y="180"
           dataProvider="{myProvider}"
           itemRenderer="myitemdrender.FriendPageItemRender"
           id="friendPageList"
           mouseDown="friendPageList_mouseDownHandler(event)">
    <component:layout>
        <s:HorizontalLayout requestedColumnCount="6"/>
    </component:layout>
 </component:SmoothScrollingList>

as you can see, there is 6 element can see in the viewport.

so how can i know the current vie开发者_StackOverflow中文版wport element index?

ex:

currentViewPortIndex is : [7][8][9][10][11][12]

as far as i know , i can only know the the index that i already select by function

list.selectedIndex

did flex4 proivde some other function that i can use?

thanks.

HorizontalLayout has a firstIndexInView property.