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
        });
      }
Invokes a callback with some interesting parameters and returns the callbacks return value.