ActionScript operator as

Question

does anybody have a good explanation of the as operator?

On one hand it seems to me, that it is often better to use as instead of instantiating a new object.

But then there are situations, when this operator let's me down. For example when loading a text file in XML format through an URLLoader:

private function completeHandler(event:Event):void {
    var loader:URLLoader = URLLoader(event.target);
    trace("completeHandler: " + loader.data);

    var x:XML = new XML(loader.data);
    trace("x=" + x);
}

Why do I have to use a constructor here? Why can't I say var x:XML = loader.data as XML; and save some memory?

Thank you for any insights! Alex

try with var x:XML = new XML(loader.data); – Cay Feb 21 '11 at 15:25 — Cay, Feb 21 '11 at 15:25
Yes thanks, that was a typo in my question. – Alexander Farber Feb 21 '11 at 20:07 — Alexander Farber, Feb 21 '11 at 20:07

score 4 · Accepted Answer · answered Feb 21 '11 at 11:17

as evaluates whether a variable's type is a super class or subclass of another class. It does not create a new object. The difference to is being that while is returns a Boolean value, as returns either an object of the desired type, or null. It is used for type casts.

See the ActionScript documentation.

A typical use case would be using a MovieClip on the stage, which is retrieved by instance name:

// This will not compile:
var d:DisplayObject = stage.getChildByName ("instance1"); 
d.gotoAndPlay (2);

// This will play the MovieClip from frame 2
var m : MovieClip = stage.getChildByName ("instance1") as MovieClip;
m.gotoAndPlay (2);

stage.getChildByName() always returns a DisplayObject, regardless if it's really a MovieClip, Sprite, Button, etc. So if you want to use any of class MovieClip's methods, you need to type cast to MovieClip first. This does not, however, create a new MovieClip but simply ensures that you are using the correct type.

+1 Also @Alexander Farber you can cast by the following notation. var m : MovieClip = MovieClip(stage.getChildByName ("instance1")); The difference here is if the cast is unsuccessful a runtime error will be thrown. In contrast the as operator would return null. — Allan, Feb 21 '11 at 12:02
Thanks. Any explanation on why do I get null in my example: var x:XML = loader.data as XML; ? — Alexander Farber, Feb 21 '11 at 13:02
Naturally, loader.data isn't XML object (even if it contains xml text). If you write XML(loader.data), you are using top-level operator XML(), which gets text and creates XML object from it. — alxx, Feb 21 '11 at 13:31

ActionScript operator as

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