Parameters:

• context object
  • context.start - ID of an item, from which drag process was started
  • context.source - array of dragged IDs (drag source)
  • context.target - ID of target item (drop target)
  • context.from - view object, to which dragged item belongs
  • context.to - view object, to which drop target belongs
  • context.index - index in the target view, at which new item will be added
  • context.new_id - id which will be used for the moved item ( actual only while moving between different views)
• native html event

  view.attachEvent("onBeforeDrop", 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