rendered attribute does not work for me

Question

Let's say I want to conditional rendering of outputPanel markup

<apex:page controller="SOQLBC">  

<apex:outputPanel id="sobj" rendered="{!LEN(sObj)>0}">
    <span>  --- {!sObj} --- </span>
</apex:outputPanel> 
<apex:form>
         -- {!LEN(sObj)>0} --
    <apex:selectList value="{!sObj}" size="1">
        <apex:selectOptions value="{!sObjList}"/>
        <apex:actionSupport event="onchange" reRender="sobj"/>
    </apex:selectList>
</apex:form>

controller

public with sharing class SOQLBC {

public SOQLBC() {
    sObj = '';
}
public string sObj {get; set;}
public List<System.SelectOption> getsObjList() {
    List<System.SelectOption> options = new List<System.SelectOption>();
    options.add(new SelectOption('', '-- Select SObject --'));
     for(Schema.EntityDefinition entity : SchAccess.getsObjectsLite()) {
        options.add(new SelectOption(entity.qualifiedapiname, entity.label));
     }
     return options;
}

}

I want to show updatePanel only if sObj does not equal '' but I can't reach my goal - updatePanel is always hided

   <apex:outputPanel id="sobj">
    <apex:outputPanel rendered="{!LEN(sObj)>0}">
        <span >  --- {!sObj} --- </span>
    </apex:outputPanel>         
</apex:outputPanel> 

helped me

Answer 1

By Default value of sObj is blank hence outputPanel with sobj id does not exist on page load. So actionSupport was not working.

Update your code like below and it should work

  <apex:page controller="SOQLBC">  
       <apex:form>
<!-- Add parent outputPanel which will always render-->
           <apex:outputPanel id="parentPanel">
               <apex:outputPanel id="sobj" rendered="{!LEN(sObj)>0}">
                   <span>  --- {!sObj} --- </span>
               </apex:outputPanel> 
           </apex:outputPanel>
            <apex:selectList value="{!sObj}" size="1">
                <apex:selectOptions value="{!sObjList}"/>
                <apex:actionSupport event="onchange" reRender="parentPanel"/>
            </apex:selectList>
        </apex:form>
    </apex:page>