Question

When I start a drag operation I would like to be able to highlight the possible valid Drop objects. For this I need to know when the drag operation starts and which items are being dragged. I am trying to do this using the dragStart, but the event.dragSource is null on this event.

I have this list:

<s:List
    width="100%"
    height="100%"
    id="productsListing" 
    dragEnabled="true"
    dataProvider="{products}"
    dragStart="dragStartHandler(event);"
    dragComplete="dragCompleteHandler(event);"
    itemRenderer="views.productListed" />

And I have the listener as:

public function dragStartHandler(event:DragEvent):void {
    var itemsVector:Vector.<Object> = event.dragSource.dataForFormat('itemsByIndex') as Vector.<Object>;
    //Verify Items
    //Highlight appropriated dropZones
}

Anyone have a good sugestion how to overcome this?

Answer 1

The problem here is that your dragStartHandler is taking higher precedence than the List components internal dragStartHandler – which is where the drag operation is started and the dragSource property created.

Suggestion, manually add your dragStartHandler with a lower precedence than the List components dragStartHandler method – looking at the code this needs to be less than -50.

MXML Code:

<s:List width="100%" height="100%"
        id="productsListing"  
        dragEnabled="true"   
        dataProvider="{products}"
        initialize="productsListing_initializeHandler(event)"
        dragComplete="productsListing_dragCompleteHandler(event)"
        itemRenderer="views.productListed"
        />

AS Code:

protected function productsListing_initializeHandler(event:FlexEvent):void
{
    // Needs to be handled AFTER the List component has handled the event and attached the dragSource data, hence priority is -51
    this.productsListing.addEventListener(DragEvent.DRAG_START, productsListing_dragStartHandler, false, -51, true);
}

protected function productsListing_dragStartHandler(event:DragEvent):void
{
    // Your code here...
}

I hope you find that useful.