Question

我刚刚尝试实现一个菜单系统我在一个正在研究的游戏上看到了一个教程，直到我现在遇到了一个问题，我把舞台设置为null并且我不知道我不知道如何阻止它。不断破坏它的行在AvoiderGame.as文件中，stage.addEventListener(Event.ADDED_TO_STAGE, init);它会一直返回以下错误

TypeError: Error #1009: Cannot access a property or method of a null object reference.
at AvoiderGame()[M:\Users\Andy\Documents\Programming For Fun\Flash\Tutorial - Avoider Game\Classes\AvoiderGame.as:29]

at States/changeState()[M:\Users\Andy\Documents\Programming For Fun\Flash\Tutorial - Avoider Game\States.as:26]

at mainMenu/brnP_Button()[M:\Users\Andy\Documents\Programming For Fun\Flash\Tutorial - Avoider Game\Classes\mainMenu.as:34]

目前我编写游戏的方式是在States.as中启动，其中包含以下代码。

package
{
import flash.display.*;
import flash.system.fscommand;

public class States extends MovieClip
{
    public function States()
    {
        changeState(null, "menu");
    }
    public function changeState (currentState, nextState)
    {
        if (currentState != null)
        {
            removeChild(currentState);
        }
        switch(nextState)
        {
            case "menu": var mm:mainMenu = new mainMenu(changeState); 
                         addChild(mm);
            break;
            case "game": var g:AvoiderGame = new AvoiderGame(changeState);
                         addChild(g);
            break;
            case "exit": fscommand("quit");
            break;
        }
    }

}

}

从那里进入mainMenu.as直到用户点击播放 - 在mainMenu.as中是以下代码

package
{
import flash.display.*;
import flash.events.*;

public class mainMenu extends MovieClip
{
var theCallBackFunction:Function;

public function mainMenu(callBack)
{
    var Background:gameBackground;
    Background = new gameBackground();
    addChild(Background);

    var btnPlay:mmPlay = new mmPlay();
    btnPlay.addEventListener(MouseEvent.MOUSE_DOWN, brnP_Button);
    btnPlay.x = width/2;
    btnPlay.y = height/2 - btnPlay.height/2;
    addChild(btnPlay);

    var btnExit:mmExit = new mmExit();
    btnExit.addEventListener(MouseEvent.MOUSE_DOWN, brnE_Button);
    btnExit.x = width/2;
    btnExit.y = height/2 - btnExit.height/2;
    btnExit.y += btnExit.height + 4;
    addChild(btnExit);

    theCallBackFunction = callBack;
}
public function brnP_Button(e:MouseEvent)
{
    theCallBackFunction(this, "game");
    return;
}

public function brnE_Button(e:MouseEvent)
{
    theCallBackFunction(this, "exit");
    return;
}

}
}

现在这就是它出错的地方 - 它进入了AvoiderGame.as然后它将我带回一个我不知道如何解决的错误 - 有人可以告诉我如何修复它吗？

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 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)
    {
        stage.addEventListener(Event.ADDED_TO_STAGE, init);
        theCallBackFunction = callBack;
    }

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

        enemyArray = new Array();
        var enemy = new Enemy(Math.round(1 + (500 - 1) * Math.random()), - 2, stage);
        enemyArray.push(enemy);
        addChild(enemy);

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

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

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

        _collisionTest = new CollisionTest();

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

        fireTimer = new Timer(300, 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);
        }

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

        for each (var enemy:Enemy in enemyArray)
        {
            enemy.moveDown();
            enemy.StayOnScreen();
            if (_collisionTest.complex(enemy, avatar)) 
            {
                gameTimer.stop();
            }
        }
    }
    private function fireTimerHandler(e:TimerEvent) : void
    {
        //timer ran, we can fire again.
        canFire = true;
    }
}
}

Answer 1

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

应该像

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

调用init函数时，不要忘记删除事件监听器。

Answer 2

如果未将显示对象添加到舞台或显示列表中已有的其他显示对象，则舞台为空。当stage已设置且可用时，将调度Event.ADDED_TO_STAGE。

Answer 3

当您创建AvoiderGame对象时，它将触发构造函数。此时，对象不会添加到舞台或显示列表中的任何对象，因此它不会对舞台有任何引用。

您可以在ADDED_TO_STAGE事件中添加侦听器，也可以在手动将其添加到舞台之前调用自定义init方法。

AS3阶段= null？

3 个答案: