添加一个按键侦听器的动作脚本3 [英] Adding a key listener in Action Script 3

问题描述

我是比较新的动作脚本，我试图做一个游戏蛇。很显然，我需要实现一个全球性的按键侦听器，但我有奇怪的问题。我尝试添加监听器应用程序标记，但它似乎没有任何效果（电影依然能编译）。每当我称之为

I am relatively new to Action Script, and I am trying to make a game of Snake. Obviously I need to implement a global key listener, but I am having odd problems. I tried adding the listener to the application tag but it didn't seem to have any effect (the movie was still able to compile). Whenever I call

this.stage.addEventListener（KeyboardEvent.KEY_DOWN，钥匙，真正的）;

我的程序崩溃。下面是我的main.mxml文件的内容：

my program crashes. Below is the content of my main.mxml file:

<?xml version="1.0" encoding="utf-8"?>
<mx:Application 
 xmlns:mx="http://www.adobe.com/2006/mxml" 
 layout="absolute"
 width="800" 
 height="600"
 frameRate="15"
 creationComplete="creationComplete();"
 enterFrame="enterFrame(event);"
 currentState="MainMenu">

 <mx:states>
  <mx:State 
   name="Game"   
   enterState="enterGame(event)"
         exitState="exitGame(event)">
  </mx:State>
  <mx:State 
   name="LevelEnd">
   <mx:AddChild relativeTo="{myCanvas}" position="lastChild">
    <mx:Button x="380" y="344" label="Continue" id="btnContinue" click="btnContinueClicked(event)" width="90" height="30"/>
   </mx:AddChild>
   <mx:AddChild relativeTo="{myCanvas}" position="lastChild">
    <mx:Label x="10" y="10" text="Congratulations, you finished the level."/>
   </mx:AddChild>
  </mx:State>
  <mx:State name="MainMenu">
   <mx:AddChild relativeTo="{myCanvas}" position="lastChild">
    <mx:Button x="381" y="344" label="Start" id="btnStart" click="startGameClicked(event)" width="90" height="30"/>
   </mx:AddChild>
   <mx:AddChild relativeTo="{myCanvas}" position="lastChild">
    <mx:Image x="10" y="49" source="@Embed('../media/mainmenu.png')"/>
   </mx:AddChild>
   <mx:AddChild relativeTo="{myCanvas}" position="lastChild">
    <mx:Label x="10" y="10" text="Snake Pro" fontSize="20" fontWeight="bold"/>
   </mx:AddChild>
  </mx:State>
 </mx:states>

 <mx:Canvas x="0" y="0" width="100%" height="100%" id="myCanvas"/>

 <mx:Script>
 <![CDATA[   

  protected var inGame:Boolean = false;
  protected var currentLevel:int = 1;
  import flash.events.KeyboardEvent; 

  public function creationComplete():void
     {
   LevelDefinitions.Instance.startup();
   addKeyEvent();
   //stage.focus = stage;
     }

  private function addKeyEvent():void
  {
   this.stage.addEventListener(KeyboardEvent.KEY_DOWN, key, true);
  }

     public function enterFrame(event:Event):void
     {
      if (inGame)
      {
       GameObjectManager.Instance.enterFrame();

       myCanvas.graphics.clear();
       myCanvas.graphics.beginBitmapFill(GameObjectManager.Instance.backBuffer, null, false, false);
       myCanvas.graphics.drawRect(0, 0, this.width, this.height);
       myCanvas.graphics.endFill();
      } 
     }

        private function key(event:KeyboardEvent):void {
            //t1.text = event.keyCode + "/" + event.charCode;

   GameObjectManager.Instance.setDirection(0, 1);
   currentState = "MainMenu";
   inGame = false;
        }

     protected function startGameClicked(event:Event):void
  {
   currentState = "Game"
  }     

     protected function enterGame(event:Event):void
  {
   Mouse.hide();
   GameObjectManager.Instance.startup();
   Level.Instance.startup(currentLevel);
   inGame = true;
     }

     protected function exitGame(event:Event):void
  {
   Mouse.show();
   Level.Instance.shutdown();
   GameObjectManager.Instance.shutdown();
   inGame = false;
  }  

  protected function btnContinueClicked(event:Event):void
  {
   currentLevel = LevelDefinitions.Instance.getNextLevelID(currentLevel);
   if (currentLevel == 0)
   {
    currentLevel = 1;
    currentState = "MainMenu";
   }
   else
   {
    currentState = "Game"
   }
     }
 ]]>
    </mx:Script>

</mx:Application>

此外，看来我得到这个堆栈跟踪：

Also, it seems I am getting this stack trace:

TypeError: Error #1009: Cannot access a property or method of a null object reference.
    at main/addKeyEvent()[C:\Users\Me\Desktop\Flash\Snake\src\main.mxml:58]
    at main/creationComplete()[C:\Users\Me\Desktop\Flash\Snake\src\main.mxml:52]
    at main/___main_Application1_creationComplete()[C:\Users\Me\Desktop\Flash\Snake\src\main.mxml:10]
    at flash.events::EventDispatcher/dispatchEventFunction()
    at flash.events::EventDispatcher/dispatchEvent()
    at mx.core::UIComponent/dispatchEvent()[E:\dev\4.x\frameworks\projects\framework\src\mx\core\UIComponent.as:12528]
    at mx.core::UIComponent/set initialized()[E:\dev\4.x\frameworks\projects\framework\src\mx\core\UIComponent.as:1627]
    at mx.managers::LayoutManager/doPhasedInstantiation()[E:\dev\4.x\frameworks\projects\framework\src\mx\managers\LayoutManager.as:759]
    at mx.managers::LayoutManager/doPhasedInstantiationCallback()[E:\dev\4.x\frameworks\projects\framework\src\mx\managers\LayoutManager.as:1072]

我在我束手无策这里，我AP preciate你的时间和努力。谢谢！

I am at my wit's end here, and I appreciate your time and efforts. Thanks!

推荐答案

因为你附加的事件侦听器的舞台属性，它是在你尝试了一次空你得到一个运行时错误。相反，对的creationComplete事件这样做的，尽量要做到这一点就applicationComplete事件。该阶段的目标将可再

You are getting a runtime error because you attach an event listener to the "stage" property, which is null at the time you try it. Instead of doing this on the "creationComplete" event, try to do this on the "applicationComplete" event. The stage object will be available then.

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