Question

how would you prevent dragging for some items of your List or DataGrid?

Let’s say I had a list with two items: ‘Tom’ and ‘Jerry’. Only ‘Tom’ should be dragable, not ‘Jerry’.

Ideally I had a ‘isDragEnabled(item:Object):Boolean’ function, which is being queried by the drag source.

My difficulties start with the fact that the ‘dragStart’ event handler has a null value for the dragSource, so right from the start I find it hard to find out what the drag-start is about..

Thanks in advance!

PS There have been a few discussions on preventing or canceling a drop, but I haven’t seen much about preventing the drag start, hence this question.

Answer 1

Ok, got it, thanks Robusto, your tip #2 was the inspiration, however I had to use a mouse-down listener — the selection event fires too late.

In the sample all below I am using code from some other question of mine.

This sample lets you drag only the first item in the List or DataGrid:

<?xml version="1.0" encoding="utf-8"?>
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml" layout="horizontal" minWidth="955" minHeight="600">

    <mx:List id="list" dataProvider="{['Tom','Jerry', 'Amy', 'Betty', 'Chris', 'Dean', 'Email', 'Floyd', 'Grant', 'Helen', 'Iris', 'Jack']}" minWidth="200"
        mouseDown="onMouseDown(event)"
        />

    <mx:DataGrid id="dg" dataProvider="{[{title:'Tom'},{title:'Jerry'}]}" minWidth="200"
         mouseDown="onMouseDown(event)" 
    >
        <mx:columns>
            <mx:DataGridColumn dataField="title" />
        </mx:columns>
    </mx:DataGrid>

    <mx:Script>
        <![CDATA[
            import mx.controls.listClasses.ListBase;
            import mx.events.DragEvent;


            protected function onMouseDown(event:MouseEvent):void
            {
                var listBaseComp:ListBase = ListBase(event.currentTarget);
                var clickIndex:int = this.findClickedItemIndex(event.stageX, event.stageY, listBaseComp);
                listBaseComp.dragEnabled = clickIndex == 0;
            }

            /**
             * Returns a dataProvider item that displays at the given coords for the given dataGrid.
             * Code provided by Stackoverflow user https://stackoverflow.com/users/165297/amarghosh,
             * thanks a lot!
             */
            protected function findClickedItemIndex(globalX:Number, globalY:Number, listComp:ListBase):int
            {
                var p1 : Point;
                var p2 : Point;
                var renderer : DisplayObject;

                for(var i:int=0; i<listComp.dataProvider.length; i++) {
                    renderer = DisplayObject(listComp.indexToItemRenderer(i));
                    if (!renderer) //item is not displayed (scroll to view it)
                        continue;
                    p1 = new Point(renderer.x, renderer.y);
                    p2 = new Point(renderer.width, renderer.height);
                    p1 = renderer.parent.localToGlobal(p1);
                    p2 = renderer.localToGlobal(p2);
                    if(globalX >= p1.x && globalX <= p2.x && globalY >= p1.y && globalY <= p2.y)
                        return i;
                }   
                return -1;
            }

        ]]>
    </mx:Script>
</mx:Application>