Viewstate contradicts controller variable values with ajax refresh of conditionally rendered fields

Question

In my project I have custom object called MyObject__c, it contains two field:

• Field1__c - picklist (with 2 values 'Yes'/'No')

• Field2__c - text

And I have this code (2 classes and one VF-page):

MyWrapper class

public class MyWrapper {

    public MyObject__c myObject {get;set;}

    public void addObject(MyObject__c newObject) {
        myObject = newObject;
    }
}

MyController class

public  class MyController {

    public MyWrapper wrapper {get;set;}

    public MyController() {
        wrapper = new MyWrapper();
        addMyObject();
    }

    public void addMyObject() {
        wrapper.addObject(new MyObject__c());
    }

    public void refreshMyObject() {
        if (wrapper.myObject.Field1__c != 'Yes') {
            wrapper.myObject.Field2__c = null;
        }
    }
}

and MyPage

<apex:page showHeader="true" sidebar="true" controller="MyController">

    <apex:form>
        <apex:outputPanel layout="Block" id="myPanel">
            <apex:inputField value="{!wrapper.myObject.Field1__c}">
                <apex:actionSupport event="onchange" action="{!refreshMyObject}" reRender="myPanel"/>
            </apex:inputField>
            <apex:outputPanel layout="none" rendered="{!wrapper.myObject.Field1__C == 'Yes'}">
                <apex:inputField value="{!wrapper.myObject.Field2__c}"/>
            </apex:outputPanel>
        </apex:outputPanel>
    </apex:form>
</apex:page>

And on MyPage I perform next steps:

1. Change Field1__c value to 'Yes', after this step I can see input for Field2__c
2. Input text in Field2__c, for example 'aaaa'
3. Change Field1__c value to 'No', after this step I can't see input for Field2__c and in myWrapper.myObject.Field2__c in viewstate I see null(!) value.
4. Change Field1__c value to 'Yes', after this step I can see input for Field2__c with value 'aaaa'

Can you explain this behaviour.

Thanks for all, and sorry for my english.

Answer 1

Has to do with timing of rendering VF into HTML, and, browser memory. That value persists in browser memory so that when you re-render you still see the old value.

The fix is simple, add this line to your visualforce page so that the browser can "see" (and thus reset) Field2__c. Add this line, somewhere inside your form tag should be fine:

<apex:inputHidden value="{!wrapper.myObject.Field2__c}" id="theHiddenInput"/>

This is somewhat analogous to how a URL-hack button won't set field values if the fields are not present on the page layout.