Have I found a bug in AS3's XML class?

Question

Let's state a few facts upfront:

Livedocs tells us that dynamic classes enable us to add member variables and member functions. We are told that all classes ultimately derive from Object, which is dynamic, and that dynamic classes must be explicitly marked as such -- inheritance doesn't apply to dynamism.

Object is a dynamic class. Date is a final dynamic class. XML is a final dynamic class. You can create your own final dynamic class and it ought to behave (in terms of dynamic capability) exactly as do XML and Date, above. In fact, final shouldn't affect the issue at hand at all but I include it for exactness in my comparison against the "troubled" XML class.

My code:

public static function setup():void//Object
{
    //Uncomment each in turn to get results:
    //var o:Object = {};
    //var o:MyFinalDynamicClass = new MyFinalDynamicClass();
    //var o:Date = new Date();
    //var o:XML = new XML();

    o.foo = function():String
    {
        return "Works as expected.";
    }

    trace(o.foo());
}

The results for enabling each of the lines where o is defined:

Object: Works as expected.

MyFinalDynamicClass: Works as expected.

Date: Works as expected.

XML: TypeError: Error #1006: value is not a function.

I used Date as it is another core class that is final dynamic. Also note that member variables work just fine in all of the above classes. Only member functions have a problem, and only in class XML.

My conclusion is that not every class in AS3 is derived from Object, some are likely mocked up to look that way but are in fact derived in some other way in native C++, which I believe is what Adobe uses to write the AS languages.

QUESTION: Do you see any flaws in my logic? Or is this class actually bugged?

P.S. For those interested as to why I wanted to add functions to XML, I wanted a way to encapsulate complex access to my XML data model.

Answer 1

According to livedocs, XML is indeed derived from Object. Furthermore, I don't think this is a bug at all but rather the expected behavior of the XML class. It's simply rejecting your method, because it's not an XML object. Think of it as a runtime checked typed tree. You can only get/set other XML objects. Try this following example:

var foo:XML = <foo><bar /><bar /></foo>;
trace(foo); // Prints <foo><bar /></foo>

foo.bar = <baz />;
trace(foo); // Prints <foo><baz /></foo>

The XML object is an ActionScript representation of the XML, so what's happening is effectively that I'm taking all of the tags and replacing them with a single tag. If you try to replace the XML with something that isn't XML (i.e. your method), it's not going to understand what it is you want and will convert the value into a string. The result will then be something like:

<foo>
  <bar>function Function() {}</bar>
</foo>

You will need to build a wrapper that takes care of your processing instead.