Extracting class assets from an external SWF at runtime.

I find one of the single most powerful (and unbelievably cool!) new features in AS3 to be the ability to extract classes at runtime from an externally loaded SWF movie. I was really excited to take advantage of the feature while implementing the Lassie engine’s new library system, which works by dynamically pulling classes out of an externally loaded SWF library and then composing them into a game layout on the fly. Now this process can be a bit cumbersome without helper methods; and in the case of a large dynamic asset management framework like Lassie, I’ve found it most convenient to just to standardize the SWF library publishing method for quick, easy, and reliable class accessibility.

So, the goal here is to create standardized SWF packaging with easily extractable class assets. That packaging should allows us to quickly read the class contents of the SWF and then to grab them out without hassle. As such, the logical place to start here is with an interface. So, here it is:

*Save as: com/lassie/lib/IMediaLibrary.as

package com.lassie.lib
{
    public interface IMediaLibrary
    {
        function get contents():Array;
        function getAsset(className:String):*;
    }
}

That interface gives us the ability to get “contents” which will return an array of class names that are contained within the library. We can then feed those class names (listed in the contents array) into the “getAsset()” method to retrieve an instance of that class. Now for the actual MediaLibrary class, which gets assigned as a document class to the library FLA document.

*Save as: com/lassie/lib/MediaLibrary.as

package com.lassie.lib
{
    import flash.display.MovieClip;

    public dynamic class MediaLibrary extends MovieClip implements IMediaLibrary
    {
        public function MediaLibrary():void
        {
            super();
        }

    // --------------------------------------------------
    //  Include a library media class
    // --------------------------------------------------

        public function addClass($class:Class, $path:String=""):void
        {
            if (($path.split(".")).length > 1)
            {
                this[$path] = $class;
            }
            else
            {
                var $cname:String = $class.toString();
                this[$cname.substr(7, $cname.length-8)] = $class;
            }
        }

    // --------------------------------------------------
    //  Get library contents (lists class names)
    // --------------------------------------------------

        public function get contents():Array
        {
            var classList:Array = new Array();
            for (var j:String in this)
            {
                classList.push(j);
            }
            classList.sort();
            return classList;
        }

    // --------------------------------------------------
    //  Extract library class asset
    // --------------------------------------------------

        public function getAsset(className:String):*
        {
            try
            {
                var classObj:Class = this[className];
                return new classObj();
            }
            catch(e:Error)
            {
                trace("error extracting requested class.");
            }
            return null;
        }
    }
}

That document class implements the two interface methods, plus gives us one additional feature: to add classes to the library. Classes are added with the “addClass()” method, which takes a reference to the class object and an optional string of the class’ fully qualified path. You can add classes from the first-frame timeline actions of the library FLA’s root timeline. To add a class:

addClass(com.project.MyMovieClip, "com.project.MyMovieClip");

Or if the class is unpackaged and has no class path, you can add it by direct reference (the class’ “toString()” value is logged as the path):

addClass(MyMovieClip);

Assuming that your MediaLibrary document publishes without errors, you can load it into another movie, get the MediaLibrary’s list of contents, and then start requesting those assets out of it with ease. The load/retrieval script goes something like this:

var libLoader:Loader = new Loader();
// ... load your library SWF into the current movie ...
var lib:IMediaLibrary = libLoader.content as IMediaLibrary;
var mc:MovieClip = lib.getAsset(lib.contents[0]) as MovieClip;

Fun!