Evalute Expression

Currently I am calling single View state which is having one evalute expression. This viewe state is called from two places.

1. During first call evaluate expression should not execute depending upon boolean getting from method.

2. For 2nd call evalute expression should excecute.

Can we define any condition in the evalute expression so that i can achive my target?

<view-state id="warningDeleteUserFromUserDetails" view="../../user/deleteUser/warningDeleteUser.xhtml" model="userFormBean">
<on-render>
<evaluate expression="deleteUserAction.getUserDetails(flowRe questContext)" />
</on-render>
<transition on="systemErrorOverlay" to="errorPage" validate="false" />
<transition on="globalErrorOverlay" to="globalError" validate="false" />
</view-state>

Regards,
Swapnil