ActionScript: TileList, deleting, scrolling backwards

[Another day, another blog cross-post...]

I have a Flex app which shows users a TiledList of chemical structure depictions. Since it can be a large list, it's lazy-loaded from the server as the user scrolls through the list.

Users can delete items from the list, with undo. In order to do this with reasonable performance, once the app has received confirmation from the server that an item has been deleted, it clears out the single deleted item from its local lazy-list.

To undo the deletion locally, the Flex app fills the correct lazy-list entry with an ItemPendingError; that error gets thrown as soon as the TileList tries to retrieve the item.

All of this works okay when the row containing the undeleted item is already visible. On the other hand, if the user has scrolled away from the row where the undeleted item will reappear, then when (s)he scrolls back the TileList simply empties out that item and all of the successive items in the row. Ugly!

Workaround

When the item is undeleted, immediately try to retrieve it via getItemAt(itemIndex). Catch the resulting ItemPendingError and register an ItemResponder. When the ItemResponder's result or fault method is called, tell the TileList to invalidateList(). If the undeleted item actually contains a value, the TileList will repaint correctly -- no more unsightly gaps.

import mx.collections.errors.ItemPendingError;
import mx.collections.ItemResponder;
[...]
try {
    structures.getItemAt(offset);
} catch (e:ItemPendingError) {
    e.addResponder(
    new ItemResponder(
        function(result:Object, token:Object = null):void {
            tilelist.invalidateList();
        },
        function(error:Object, token:Object = null):void {
            tilelist.invalidateList();
        }));
}