October 11, 2011

Using Rerender to Render — One Solution for Headaches

It's a regular occurrence to have an <apex:outputPanel> or similar element in a Visualforce page, where you want to toggle visibility of said element according to some variable. Something that's caught both myself and fellow teammates out on a few such occasions is finding that it just won't appear again after being removed from the screen, when the page code looks analogous like the following.

<apex:outputPanel id="thePanel" rendered="{!bRenderThePanel}"> 
    <!-- Content --> 
</apex:outputPanel> 

<apex:commandButton action="{!DoSomeCalcs}" rerender="thePanel" value="FIRE!"/>

In this example, the action DoSomeCalcs is implemented such that it will toggle the value of the variable bRenderThePanel and so the result should be that the panel is alternately displayed and hidden with each click of the button. Woe is the developer, as this is not the case. Chances are you'll find yourself confused as to why it just won't reappear (or appear in the first place).

Essentially re-rendering a specified panel (or other component) that is not currently displayed will fail, and the solution is to wrap it with a component that is always present, using that as the target for the re-render. Thus our example becomes:

<apex:outputPanel id="thePanelWrapper"> 
    <apex:outputPanel rendered="{!bRenderThePanel}"> 
        <!-- Content --> 
    </apex:outputPanel> 
</apex:outputPanel> 

<apex:commandButton action="{!DoSomeCalcs}" rerender="thePanelWrapper" value="FIRE!"/>

What does seem strange to me is that Salesforce have specifically ensured that you can access DOM elements within an <apex:outputPanel> when it's not displayed, as indicated by the documentation for the layout attribute, it seems odd that this functionality does not carry over for the element itself when it's being used as a re-render target.

The layout style for the panel. Possible values include "block" (which generates an HTML div tag), "inline" (which generates an HTML span tag), and "none" (which does not generate an HTML tag). If not specified, this value defaults to "none". However, if layout is set to "none", for each child element with the rendered attribute set to "false", the outputPanel generates a span tag, with the ID of each child, and a style attribute set to "display:none". Thus, while the content is not visible, JavaScript can still access the elements through the DOM ID.

Happy coding!