ColdFusion mixins within singleton objects

Sometimes it's nice to be able to use ColdFusion mixins to include dynamic data at runtime inside CFCs. In the following example, I'm using a CFC to generate a database-driven form, with each form potentially containing multiple templates.

<cfcomponent output="false">

 <cffunction name="displayForm" access="public" output="false" returntype="string">
  <cfargument name="formID" required="true" type="numeric" />

  <cfset var local = StructNew() />

  <!--- get the templates associated with the form --->
  <cfquery name="local.getTemplates" datasource="#application.datasource#">
   select file_path
   from form_templates
   where form_id = <cfqueryparam value="#arguments.formID#" cfsqltype="cf_sql_integer" />
   order by sort_order asc
  </cfquery>

  <!--- capture the generated html --->
  <cfsavecontent variable="local.html">

   <!--- loop over the templates and display (include) each template --->
   <cfoutput query="local.getTemplates">

    <cfinclude template="#local.getTemplates.file_path#" />

   </cfoutput>

  </cfsavecontent>

  <cfreturn local.html />

 </cffunction>

</cfcomponent>

Hopefully that's straightforward enough. This works just fine at first, until you store the component in the application scope as a singleton and a developer doesn't properly scope a variable inside one of the included templates, thereby creating a public variable that could bleed into other requests.

The solution is pretty simple: rather than using the cfinclude tag to include the mixin, use a helper function that mimics the functionality but traps the variables.

Doing this requires creating an include helper and an include proxy. The helper is created as a singleton and its purpose is to create instances of the proxy, which is in charge of including the original file.

<cfcomponent displayname="Include Helper" output="false">

 <cffunction name="template" access="public" output="true" returntype="any">
  <cfargument name="template" required="true" type="string" />

  <cfreturn createObject("component","includeProxy").init(arguments.template) />

 </cffunction>

</cfcomponent>

<cfcomponent displayname="Include Proxy" output="false">

 <cffunction name="init" access="public" output="true" returntype="void">
  <cfargument name="template" required="true" type="string" />

  <cfinclude template="#arguments.template#" />

 </cffunction>

</cfcomponent>

With the include components creating, I can now replace <cfinclude template="#local.getTemplates.file_path#" /> with #helpers.include.template(local.getTemplates.file_path)# and the template variables are now properly hidden from my component.