R – How to instantiate class from a swf

actionscript-3apache-flexflashloader

I have an FLA file with objects in the library which I have set to be "classes" (In CS3, right click an item in the library select properties, make sure it's set to export for action-script, and has a class name)

For this exercise, let's call the class "MyClass"

If I publish that FLA to an SWC and SWF:

I can load the SWC statically, and instantiate "MyClass" by simply doing:

var inst:MyClass = new MyClasS();

Now, the problem: I'd like to be able to do this at runtime by loading the SWF file using a loader object.

I understand how to access instances which have been created by hand in the FLA before publishing, but what I want to be able to do, is create new instances of the class "MyClass".

I can get a "MovieClip" representing the swf file, I can add it to my displaylist, but I can't seem to get at the classes contained therein. (I hope this makes sense)

Any suggestions for how to attack this would be much appreciated.

Edit : Format code

Best Answer

To complete Christian's answer:

var cls : Class = loader.contentLoaderInfo.applicationDomain.getDefinition("ClassName");

var instance : Object = new cls();

Additionally, it's worth noting that you won't get strong typing (ie. it must be declared as Object) unless the class implements interface which is also defined in your main application. You will then be able to declare the instance variable as the interface and have compile-time access to it's members.

Tracking it down

At first I thought this was a coercion bug where null was getting coerced to "null" and a test of "null" == null was passing. It's not. I was close, but so very, very wrong. Sorry about that!

I've since done lots of fiddling on wonderfl.net and tracing through the code in mx.rpc.xml.*. At line 1795 of XMLEncoder (in the 3.5 source), in setValue, all of the XMLEncoding boils down to

currentChild.appendChild(xmlSpecialCharsFilter(Object(value)));

which is essentially the same as:

currentChild.appendChild("null");

This code, according to my original fiddle, returns an empty XML element. But why?

Cause

According to commenter Justin Mclean on bug report FLEX-33664, the following is the culprit (see last two tests in my fiddle which verify this):

var thisIsNotNull:XML = <root>null</root>;
if(thisIsNotNull == null){
    // always branches here, as (thisIsNotNull == null) strangely returns true
    // despite the fact that thisIsNotNull is a valid instance of type XML
}

When currentChild.appendChild is passed the string "null", it first converts it to a root XML element with text null, and then tests that element against the null literal. This is a weak equality test, so either the XML containing null is coerced to the null type, or the null type is coerced to a root xml element containing the string "null", and the test passes where it arguably should fail. One fix might be to always use strict equality tests when checking XML (or anything, really) for "nullness."

Solution

The only reasonable workaround I can think of, short of fixing this bug in every damn version of ActionScript, is to test fields for "null" and escape them as CDATA values.

CDATA values are the most appropriate way to mutate an entire text value that would otherwise cause encoding/decoding problems. Hex encoding, for instance, is meant for individual characters. CDATA values are preferred when you're escaping the entire text of an element. The biggest reason for this is that it maintains human readability.

Best Answer

Related Solutions

Actionscript – How to pass “Null” (a real surname!) to a SOAP web service in ActionScript 3

Tracking it down

Cause

Solution

Related Topic