Parameters:

• context object

* context.start - ID of an item, from which drag process was started

  * context.source - array of dragged IDs (drag source)
  * context.html - this property can be set to redefine the content of drag-marker
  * context.from - object to which dragged item belongs
* native event object.

  view.attachEvent("onBeforeDrag", function (context, ev){
       //any custom logic here
       return true;
  });

Event is blockable, if non-true value returned from custom method, default reaction will be blocked