AS3 Access a method inside the Main class from another class . Gives error? - actionscript-3

Question

I have trouble with actionScript , im trying to use a simple one line code to access a method inside the Document Class (Main) , but every time i got error . i tried the same code with a movieClip on stage, it work nicely .
Main Class linked to the fla :
package {
import flash.display.*;
import flash.events.*;
public class Main extends MovieClip {
public function Main() {
if (stage) {
init();
}
else addEventListener(Event.ADDED_TO_STAGE, init);
}
private function init(e:Event = null):void {
removeEventListener(Event.ADDED_TO_STAGE, init);
button.addEventListener(MouseEvent.CLICK,_click);
}
private function _click(e:MouseEvent):void {
var l:Leecher = new Leecher();
l.leech();
}
public function callMe():void {
trace("hey nice work");
}
}
}
Leecher Class :
package {
import flash.display.*;
public class Leecher extends MovieClip {
public function leech():void
{
trace(" leech function ");
Main(parent).callMe(); // output null object
Main(root).callMe(); // output null object
Main(Main).callMe(); // output null object
}
}
}
The Same code , but the class linked to a button on stage
package
{
import flash.display.*;
import flash.events.*;
public class Button extends MovieClip {
public function Button() {
this.addEventListener(MouseEvent.CLICK,r_click);
}
private function r_click(e:MouseEvent):void {
var l:Leecher = new Leecher();
l.leech();
Main(parent).callMe(); // hey nice work
Main(root).callMe(); // hey nice work
Main(Main).callMe(); // output null object
}
}
}

Answer 1

The errors are because when that code runs, the Leecher instance has not yet been added to the display list, and as such does not have a parent or root or stage (so parent is null).
Here is a breakdown of what's happening (explained with code comments):
private function _click(e:MouseEvent):void {
//you instantiate a new Leecher object
var l:Leecher = new Leecher();
//you call leech, but this new object does not have a parent because you haven't added it to the display list (via `addChild(l)`)
l.leech();
}
//your saying parent should be of type Main, then call the callMe method. However, parent is null because this object is not on the display list
Main(parent).callMe();
//same as above, except using root
Main(root).callMe();
//Here you are saying the Main class is of type Main (which since Main is a class and not an instance of Main will error or be null)
Main(Main).callMe();
The root, parent & stage vars of a display object are only populated when said display object is added to to the display list. In the case of root & stage the parent (and any grand parents) must also be added so that the top most parent/grandparent is the stage.
As a result, you need to wait until it's safe to access parent by listening for the Event.ADDED_TO_STAGE event.
private function _click(e:MouseEvent):void {
var l:Leecher = new Leecher();
//call the leech method once the child has been added to the stage and has a parent value
l.addEventListener(Event.ADDED_TO_STAGE, l.leech, false, 0, true);
addChild(l);
}
If you do the above, you'll need to add an optional event parameter to the leech method or you'll get an error:
public function leech(e:Event = null):void
{
To make your Main class easily accessible, you could use a static reference.
Static vars are not tied to an instance of an object, but to the class itself.
public class Main extends MovieClip {
//create a static var that holds a reference to the root/main instance
public static var main:Main;
public function Main() {
//assign the static var to this (the instance of Main)
main = this;
//...rest of code
If you do that, you can asses your root anywhere in your code by doing Main.main so in your example you could then do:
Main.main.callMe();
I'd recommend reading about static vars more before going crazy using them. Doing what I've just shown for an easy reference to your document class / root is safe, but in other contexts there are some memory & performance nuances it's best to be aware of.