Visualforce passing static resource URL to Apex - salesforce

Question

I am trying to pass a static resource URL, via the apex:param tag. The code I have so far is:
VisualForce:
<apex:selectList value="{!SelectedFamily}" onchange="renderGallery();" size="1" label="Product Family">
<apex:actionFunction name="renderGallery" rerender="gallery" oncomplete="renderScripts();">
<apex:param value="{!URLFOR($Resource.NoImage)}" assignTo="{!noImage}"/>
<apex:param value="{!URLFOR($Resource.NoImageThumb)}" assignTo="{!noImageThumb}"/>
</apex:actionFunction>
<apex:actionFunction name="renderScripts" rerender="scriptPanel">
</apex:actionFunction>
<apex:selectOptions value="{!Family}" />
</apex:selectList>
Controller:
public string noImage{ get; set; };
public string noImageThumb { get; set; }
My understanding of apex:param was that I would now be able to call the controller variables after the re-render had occurred and they would be populated with the static resource URL. But unfortunately I keep getting null.
Anyone have any idea why it isn't working?

Answer 1

It works if you provide names for your params --- Visualforce won't process params in this context unless they're named.
<apex:actionFunction name="renderGallery" rerender="gallery" oncomplete="renderScripts();">
<apex:param name="noImg" value="{!URLFOR($Resource.NoImage)}" assignTo="{!noImage}"/>
<apex:param name="noImgUrl" value="{!URLFOR($Resource.NoImageThumb)}" assignTo="{!noImageThumb}"/>
</apex:actionFunction>
If this doesn't work for you right away, please post the rest of your Visualforce code so that we can see where "gallery" and "scriptPanel" are in relation to your apex form tag --- getting rerender to work correctly is notoriously tricky, and entirely dependent on the hierarchical position of the DOM elements getting rerendered. To guarantee that gallery and scriptPanel successfully rerender, put them in separate outputPanels outside the apex form tag, like this:
<apex:outputPanel id="scriptPanel">
Selected Family: {!SelectedFamily}<br/>
</apex:outputPanel><br/><br/>
<apex:outputPanel id="gallery">
No Image: {!noImage}<br/>
No Image Thumb: {!noImageThumb}<br/>
</apex:outputPanel><br/><br/>
#MatthewKeefe, there's absolutely no reason why {!URLFOR()} can't be used as an extension variable --- it compiles to text (e.g. '/resource/123718923'), so Jim's solution here is actually pretty interesting, as it saves him from having to do a SOQL query on the StaticResource object in his controller.

Answer 2

{!URLFOR($Resource.NoImage)} is not meant to be used as a controller/extension variable. It is a direct reference to the static resource (no controller/extension required).
I would recommend you use output panels with a rendered property bound to an Apex variable that is controlled by the action function. That way you could show or hide each version of the image by showing or hiding the output panel.
Alternatively, you may want to look into using JavaScript Remoting for something like this.