how to make the popup performed according to some specified condition?

[ym feng]

          <af:popup id="popupAmend"
                    popupFetchListener="#{someBean.popupAmendFetchListener}">
                   //...
          </af:popup>


            <af:commandButton text="#{res['rs.cmd.amend']}" id="cmdAmend">
              <af:showPopupBehavior popupId="popupAmend" triggerType="action"/>
            </af:commandButton>

if i click the commandButton, popup div appears immediately, but how to add some condition logic, according to which popup div appears or not.