stage access out of DocumentRoot in AS3

One of my first challenges in understanding AS3 was the idea that Stage and _root are no longer global (and this is a good thing).

DISCLAIMER: In general, accessing root is considered a crap coding practice - I like to call it CCP. There are a few cases that using root may be helpful - but if you are questioning whether to use it or not - then the answer is probably, “NO!”

Essentially, Stage and _root can now be accessed as:

// accessing root is almost always a bad idea
_root = MC.root;
Stage = MC.stage;

If you try and access

root

and

stage

anywhere else than your document root class - you will get this error:

Error 1120: Access of undefined property stage.
Error 1120: Access of undefined property root.

The biggest issue for me was stage. Before, I used to always access the Stage.height and Stage.width dimensions a lot in order to set properties of things that I created outside of the DocumentRoot.

If you want to access stage from anywhere else but the DocumentRoot class - you’ll need to do something like this:

var myMC:MovieClip = new MovieClip();
addChild(myMC);
var _stage:Stage = myMC.stage;
trace(“the AS3 Stage.width: “+_stage.stageWidth);
trace(“returns the width of objects on stage: “+_stage.width);
trace(“the AS3 Stage.height: “+_stage.stageHeight)
trace(“returns the height of objects on stage: “+_stage. height);

Something cool to note is that

stage.width

refers to the width of ALL objects in the stage - flash actually is treating it as its own object so you can differentiate between the width of what’s on the stage and the stage itself.

I heart AS3.

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written:: 09.27.07 / 7pm

category:: AS3, Tutorials

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I heart actionscript is a site devoted to the education and sharing of all things actionscript. These tutorials and articles are based on my experiences and problems I have encountered in my struggle to make the leap from designer to stellar coder. I hope iHeartActionscript finds you well.

Matt 10.19.07 / 2pm

would you care to explain why this new model is a good thing, as you so faithfully pointed out?

JM 01.06.08 / 7pm

Still can’t figure this out.
So if I want to addChild from an AS file other than the document class, I would have to do something like:
_stage.addChild(MC); ?

Rob Gungor 01.06.08 / 8pm

JM - addChild is a method of a display object - if you’re trying to access the Stage - it would be defined as “stage” in your document root. If you added an object to the stage - you can then add objects to that via addChild. So - stage.addChild(myObject). myObject.addChild(whatever);

Does that help?

SRB 05.08.08 / 12pm

In case it’s helpful to anyone:
“.stage” is a member of the DisplayObject object. That means that DisplayObjectContainer, MovieClip, Sprite, etc. all inherit .stage from DisplayObject.

In other words, every sprite and every movieclip and every custom class that extends either of them — everything that can possibly appear on the stage — has a .stage property.

That means you can do this:

public class MyClass extends Sprite {
public function MyClass( ) {
trace( this.stage.width );
}
}

Hope it helps.

Dave 06.22.08 / 8pm

SRB, I’ve done exactly what you say, but keep getting “Cannot access a property or method of a null object reference.”

Jarred 10.14.08 / 3am

Thanks for this, it helped me out.

Cedric 11.19.08 / 8pm

I tried a lot of thing but I’m getting

Error #1009: Cannot access a property or method of a null object reference.

So flex let me access this stage property but the flash player tell me it’s null although I added my Class to the display list…

import mx.core.Container;
import flash.display.StageAlign;
import flash.display.StageScaleMode;

import mx.managers.SystemManager;
import flash.events.Event;

import mx.core.Application;

public class ResolutionController extends Container
{

public function ResolutionController()
{
}
public function register():void{
//access the stage after having added this class to the display list
this.stage.addEventListener(Event.RESIZE, onResize);
}

stage access out of DocumentRoot in AS3

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stage access out of DocumentRoot in AS3

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7 Comments

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