DataService Fill Operation Completion

How can you determine exactly when a Flex DataService fill operation has completed?

ex:

dsDataService.fill(arrayList, 'flex:hql', 'From com.package.Object);
trace(arrayList.length); // <-- This returns 0

***

while(arrayList.length < 1) {} // <--- This never ends


The trace returns 0 but if I wait a second or two before calling it it returns with the correct size of 30.
The arraylist will also successfully bind to a datagrid.

Can I do something with the AsyncToken that is returned from the fill to know when the operation is completed?

Thanks for your help!