code: function invokeCallback(expression, event, dropEffect, itemType, index, item) {
          return $parse(expression)(scope, {
            callback: dndState.callback,
            dropEffect: dropEffect,
            event: event,
            external: !dndState.isDragging,
            index: index !== undefined ? index : getPlaceholderIndex(),
            item: item || undefined,
            type: itemType
          });
        }
docs: Invokes a callback with some interesting parameters and returns the callbacks return value.