Oracle APEX shuttle item validation

Using Oracle APEX 3.0.1

开发者_如何学运维I have two shuttle items named P1_SHUTTLE and P1_SHUTTLE_2.

I need a means of checking/validating the shuttle so that when a user selects an item from left to right, only one item is ever allowed in the right shuttle.

What can I do to check if the shuttle has already got an item and disallow further items to be added to it when the user attempts to do so?

I think you're going about this the wrong way.

The whole purpose of using a shuttle item, as opposed to a simple item with a select list or list of values, is that the user can choose more than one item.

If they are only allowed to choose one item, don't use a shuttle.