Question

I’m having some problems with trying to remove Event.ADD_TO_STAGE basically in this game ive developed once the lives is less than or equal to zero the game will end and switch to the game over screen however when I do that it kicks back with this error.

ArgumentError: Error #2025: The supplied DisplayObject must be a child of the caller.   
at flash.display::DisplayObjectContainer/removeChild()
at States/changeState()
at AvoiderGame/onTick()
at flash.utils::Timer/_timerDispatch()
at flash.utils::Timer/tick()

This then turns into..

TypeError: Error #1009: Cannot access a property or method of a null object reference.
at Enemy/StayOnScreen()
at AvoiderGame/onTick()
at flash.utils::Timer/_timerDispatch()
at flash.utils::Timer/tick()

I’m not entirely sure why – could anyone clarify why it does and how I could fix it?

package
{
import flash.display.MovieClip;
import flash.utils.Timer;
import flash.events.Event;
import flash.events.TimerEvent;
import com.freeactionscript.CollisionTest;
import flash.display.Stage;

public class AvoiderGame extends MovieClip
{
    var theCallBackFunction:Function;

    public static var enemyArray:Array;
    public var enemy:Enemy
    public var Background:gameBackground;
    public var Lives:Number = 3;

    public var avatar:Avatar;
    public var gameTimer:Timer;

    private var _collisionTest:CollisionTest;
    private var numStars:int = 80;

    private var fireTimer:Timer; //causes delay between fires
    private var canFire:Boolean = true; //can you fire a laser

    public function AvoiderGame(callBack)
    {
        this.addEventListener(Event.ADDED_TO_STAGE, init);
        theCallBackFunction = callBack;
    }

    private function init(e:Event):void
    {
        Background = new gameBackground();
        addChild(Background);

        enemyArray = new Array();

        avatar = new Avatar(stage);
        addChild(avatar);

        avatar.x = stage.stageWidth / 2;
        avatar.y = stage.stageHeight / 2;

        _collisionTest = new CollisionTest();

        gameTimer = new Timer(25);
        gameTimer.addEventListener(TimerEvent.TIMER, onTick);
        gameTimer.start();

        for (var i:int = 0; i < numStars; i++)
        {
            stage.addChildAt(new Star(stage), 1);
        }

        fireTimer = new Timer(1000, 1);
        fireTimer.addEventListener(TimerEvent.TIMER, fireTimerHandler, false, 0, true);
        fireTimer.start();
    }

    public function onTick(timerEvent:TimerEvent):void 
    {

        if (Math.random() < 0.1)
        {
            trace('array length: ', AvoiderGame.enemyArray.length);
            enemy = new Enemy(Math.round(1 + (500 - 1) * Math.random()), - 28, stage);
            enemyArray.push(enemy);
            addChild(enemy);
            enemy.gotoAndStop("Enemy" + Math.round(1 + (5 - 1) * Math.random()))
        }

        canFire = avatar.UpdateAvatar(canFire);
        if (canFire == false)
        {
            fireTimer.start();
        }
        avatar.StayOnScreen();

        for each (var enemy:Enemy in enemyArray)
        {
            enemy.moveDown();
            enemy.StayOnScreen();
            if (_collisionTest.complex(enemy, avatar)) 
            {
                gameTimer.stop();
                for each (var enemy:Enemy in enemyArray)
                {
                    for(var i:int = 0; i < enemyArray.length; i++)
                    {
                        removeChild(enemyArray[i]);
                        enemyArray.splice(i, 1); //remove the i'th element as i'th element is the enemy containing the ID of hit enemy
                    }
                }
                Lives--;
                trace('lives: ', Lives);
                gameTimer.start();
            }
        }

        if (Lives == 0)
        {
            removeEventListener(Event.ADDED_TO_STAGE, init)
            theCallBackFunction(this, "over");
        }
    }
    private function fireTimerHandler(e:TimerEvent) : void
    {
        //timer ran, we can fire again.
        canFire = true;
    }
}
}

Answer 1

//use weak references so the listener can be disposed of if necessary

this.addEventListener(Event.ADDED_TO_STAGE, init, false, 0, true);

private function init(e:Event):void {

//remove this now as we have added the object to the stage
this.removeEventListener(Event.ADDED_TO_STAGE, init);

//add a new listener that will fire when the object is removed from stage
this.addEventListener(Event.REMOVED_FROM_STAGE, reset, false, 0, true);

…

private function reset(e:Event):void {

//this function is called when the object has been removed from the stage
this.removeEventListener(Event.REMOVED_FROM_STAGE, reset);

//now make sure all timers and stopped and nulled if required
//as well as all objects removed from stage and nulled if required. e.g.

fireTimer.stop();
fireTimer = null;

enemyArray = [];
enemyArray = null;

enemy

removeChild(Background);
Background = null;

removeChild(avatar);
avatar = null;

gameTimer.stop();
gameTimer = null;

_collisionTest = null;

}

//NB. You are adding stars to the stage and have no way of referencing them to remove them.

for (var i:int = 0; i < numStars; i++)
{
   stage.addChildAt(new Star(stage), 1);
}

//you might want to add them to an array so you can remove them.

var starArray = [];

for (var i:int = 0; i < numStars; i++)
{
   var newStar:Star = new Star(stage);
   stage.addChildAt(newStar, 1);
   starArray.push(newStar);
}

Then to remove them:

for (var i:int = starArray.length; i > 0; i--)
{

   stage.removeChild(starArray[starArray.length-1]);

}
starArray = [];
starArray = null;