[flexcoders] ArrayCollection vs XMLListCollection

[brad.bueche]

How come when I'm setting up a data grid using an ArrayCollection as 
the datasource I dont have to specify the contents of the data grid 
columns?  I can just do this:

<mx:DataGrid id="dg1" x="42" y="0" width="950" height="100" 
                 dataProvider="{bookData.lastResult.books.stock}" />

But in order to get anything to appear on the data grid with either XML 
or XMLListCollection I have to do the column layout (like this)?

<mx:DataGrid id="dg3" x="42" y="300" width="950" height="100"
                 dataProvider="{bookStock2}" >
 <mx:columns>
  <mx:DataGridColumn dataField="name" headerText="name"/>
  <mx:DataGridColumn dataField="author" headerText="author"/>
  <mx:DataGridColumn dataField="category"headerText="category"/>
  <mx:DataGridColumn dataField="description" headerText="description"/>
 </mx:columns>
</mx:DataGrid>

Thanks in advance,
brad