fires before the mouse button is pressed and the cursor is moved over a draggable item
| context | object | the drag-n-drop context | 
| native_event | Event | an HTML event object | 
| boolean | returning false will prevent dragging of the element | 
grida.attachEvent("onBeforeDrag", function(context, native_event){
    context.html = "<div style='padding:8px;'>";
    for (var i=0; i< context.source.length; i++){
        context.html += context.from.getItem(context.source[i]).title + "<br>" ;
    }
    context.html += "</div>";
    return true;
});
If you drag multiple items, the event will be called once for the entire batch of items.
The drag-and-drop context can have the following properties: