行stage.addeventlistener給出錯誤;如何實例化階段？

Question

我遇到了stage.addeventlistner問題。在做了一些研究之後，我發現了一些提示，說明它可能沒有實例化。

我該怎麼做？

類型錯誤錯誤＃1009不能訪問空對象引用的屬性或方法

（線是指是stage.addeventlistener）

僅用重要比特的代碼

package 
{ 
import flash.display.MovieClip; 
import flash.events.*; 
import flash.events.KeyboardEvent; 
import flash.ui.Keyboard; 
import flash.geom.Point; 

public class TesTrun extends MovieClip 
{ 
    var leftPressed:Boolean = false; 
    var rightPressed:Boolean = false; 
    var upPressed:Boolean = false; 
    var downPressed:Boolean = false; 
public function TestRun() 
    {  

stage.addEventListener(KeyboardEvent.KEY_DOWN, keyDownHandler); 
stage.addEventListener(KeyboardEvent.KEY_UP, keyUpHandler); 
stage.addEventListener(Event.ENTER_FRAME, loop); 
     //there is more code down here but i don't think that has to be included 
    } 
} 
} 

Answer 1

您可以使用ADDED_TO_STAGE事件來觸發您的初始化代碼：

public function TestRun() 
{  
    addEventListener(Event.ADDED_TO_STAGE, init); 
} 

public function init(e:Event):void 
{ 
    stage.addEventListener(KeyboardEvent.KEY_DOWN, keyDownHandler); 
    stage.addEventListener(KeyboardEvent.KEY_UP, keyUpHandler); 
    stage.addEventListener(Event.ENTER_FRAME, loop); 

}