Callbacks.
What to test here?
See if all callbacks are correct (in console):

  1. onDragStart: fired when the user clicks in the area. This callback gets the event object.
  2. onDragMove: fired when the user drags. This callback gets the event object.
  3. onElementSelect: fired every time an element is selected. This callback gets the selected node
  4. onElementUnselect: fired every time an element is de-selected. This callback gets the de-selected node
  5. callback: a callback function that gets fired when the element is dropped. This callback gets an array holding all selected nodes. The second property passed is the event object.