Here is a sample program to get the number of arguments that a function takes.
<?xml version="1.0" encoding="utf-8"?>
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml" layout="vertical"
creationComplete="initAPP()">
<mx:Script>
<![CDATA[
private function initAPP(one:int=0, obj:Object=null):void {
}
private function showFunctionArgumentsLength():void {
trace("Length: "+ (initAPP as Function).length)
}
]]>
</mx:Script>
<mx:Button label="Check" click="showFunctionArgumentsLength()" />
</mx:Application>
Note: This will give you all the arguments length, not only required params.