Specify conditional rendering of element inside <ui:repeat>? The <c:if> does not seem to work

问题

I am trying to conditionally build a custom list using <ui:repeat>. On every occurrence of -1 as item-value in list, I need to add a line break.

I tried to use <c:if> inside <ui:repeat> for that, but it does not seem to work. It always evaluates false.

<ul>      
    <ui:repeat value=\"#{topics.list}\" var=\"topicId\" >
        <li>#{topicId}</li>
        <c:if test=\"#{topicId eq -1}\">  <br/>  </c:if>
    </ui:repeat>
</ul>

Is this possible?

回答1:

Not with JSTL tags, no. They runs during view build time, not during view render time. You can visualize it as follows, when JSF builds the view, JSTL tags runs from to bottom first and the result is a pure JSF component tree. Then when JSF renders the view, JSF components runs from top to bottom and the result is a bunch of HTML. So, JSTL and JSF doesn't run in sync as you'd expect from the coding. At the moment your <c:if> runs, the #{topicId} isn't available in the scope.

Instead of using <c:if>, you need to specify the condition in the rendered attribute of the JSF component of interest. As you've actually none, you could wrap it in a <ui:fragment>.

<ul>      
    <ui:repeat value="#{topics.list}" var="topicId" >
        <li>#{topicId}</li>
        <ui:fragment rendered="#{topicId eq -1}"><br/></ui:fragment>
    </ui:repeat>
</ul>

Alternatives are <h:panelGroup>

<h:panelGroup rendered="#{topicId eq -1}"><br/></h:panelGroup>

or in your specific case <h:outputText escape="false">

<h:outputText value="&lt;br/&gt;" escape="false" rendered="#{topicId eq -1}" />

as both also emits nothing else to the HTML output when no client side attributes are specified.

See also:

• JSTL in JSF2 Facelets... makes sense?

---

Unrelated to the concrete problem, that's the wrong place for a <br/>. It would be ignored by any webbrowser respecting the HTML specification. Don't you mean it to be inside the <li>? Or better, give it a class and let CSS give it a margin-bottom.

来源：https://stackoverflow.com/questions/7248439/specify-conditional-rendering-of-element-inside-uirepeat-the-cif-does-not