我在这里是新手（并且在AS3中也相对较新），所以请多多包涵。我只是在两周前才发现OOP，在那之前，我只了解AS3的最基本知识。所以我确实做了很多改进，但是有件事困扰着我。
我似乎永远无法从父类中调用函数和方法。即使使用setter和getter，子类也总是给我输出错误。看起来像这样。
TypeError: Error #1009: Cannot access a property or method of a null object reference.
at Axiom/clicked()
这是我现在正在处理的一个AS3项目，这给了我这个问题。
这是项目的一些基本背景。
我有一个叫做Main的主类，还有一些叫做Axiom和Textbox的其他类。Main将Axiom创建到舞台上已经存在的动画片段（背景）中。单击时，Axiom将创建文本框。Axiom从Main调用一种称为mouseClick的方法（播放声音），而Textbox从Axiom调用一些属性（该文本框的文本）。
我尝试使用
MovieClip(this.parent).mouseClick();
并在子类中声明一个新变量，如下所示。
private var main:Main;
...
main.mouseClick();
这就引出我一个问题-我做错了什么，应该如何正确做？
这是供参考的类。
Main.as
package {
import flash.ui.Keyboard;
import flash.events.Event;
import flash.events.EventDispatcher;
import flash.display.MovieClip;
import flash.events.KeyboardEvent;
import flash.ui.Mouse;
import flash.events.MouseEvent;
public class Main extends MovieClip {
// sound
private var music:Music = new Music();
private var clickSound:Click = new Click();
// instructions
private var instructions:Instructions = new Instructions();
// mouse
private var cursor:Cursor = new Cursor();
// player
private var player:Player = new Player();
private var animationState:String = "standing";
private var directionState:String = "right";
// Axiom
private var axiom:Axiom = new Axiom();
// movement
private var rightPressed:Boolean = false;
private var leftPressed:Boolean = false;
private var upPressed:Boolean = false;
private var downPressed:Boolean = false;
private var xMovement:Number = 0;
private var yMovement:Number = 0;
private var speed:Number = 22;
private var friction:Number = 0.9;
private var rDoubleTapCounter:int = 0;
private var lDoubleTapCounter:int = 0;
private var dDoubleTapCounter:int = 0;
private var uDoubleTapCounter:int = 0;
private var doubleTapCounterMax:int = 5;
private var running:Boolean = false;
public function Main() {
// constructor code
// mouse
stage.addChild(cursor);
cursor.mouseEnabled = false;
Mouse.hide();
// instructions
instructions.x = 640;
instructions.y = 120;
stage.addChild(instructions);
// add player
player.x = 642;
player.y = 448.95;
player.gotoAndStop(directionState);
player.right.gotoAndStop(animationState);
addChild(player);
// add Axiom
axiom.x = 300;
axiom.y = -150;
back.addChild(axiom);
// keyboard events
stage.addEventListener(KeyboardEvent.KEY_DOWN, keyPressed);
stage.addEventListener(KeyboardEvent.KEY_UP, keyReleased);
// music
music.play(0, int.MAX_VALUE);
// loop
stage.addEventListener(Event.ENTER_FRAME, loop);
}
private function loop(e:Event):void {
// set mouse
cursor.x = stage.mouseX;
cursor.y = stage.mouseY;
// set Movement to speed
if (rightPressed) {
if (upPressed) {
if (running || (rDoubleTapCounter <= doubleTapCounterMax && uDoubleTapCounter <= doubleTapCounterMax)) {
xMovement = speed * 2;
yMovement = speed * -2;
} else {
xMovement = speed;
yMovement = speed * -1;
}
} else if (downPressed) {
if (running || (rDoubleTapCounter <= doubleTapCounterMax && dDoubleTapCounter <= doubleTapCounterMax)) {
xMovement = speed * 2;
yMovement = speed * 2;
} else {
xMovement = speed;
yMovement = speed;
}
} else if (running || rDoubleTapCounter <= doubleTapCounterMax) {
xMovement = speed * 2;
} else {
xMovement = speed;
}
} else if (leftPressed) {
if (upPressed) {
if (running || (lDoubleTapCounter <= doubleTapCounterMax && uDoubleTapCounter <= doubleTapCounterMax)) {
xMovement = speed * -2;
yMovement = speed * -2;
} else {
xMovement = speed * -1;
yMovement = speed * -1;
}
} else if (downPressed) {
if (running || (lDoubleTapCounter <= doubleTapCounterMax && dDoubleTapCounter <= doubleTapCounterMax)) {
xMovement = speed * -2;
yMovement = speed * 2;
} else {
xMovement = speed * -1;
yMovement = speed;
}
} else if (running || lDoubleTapCounter <= doubleTapCounterMax) {
xMovement = speed * -2;
} else {
xMovement = speed * -1;
}
} else if (downPressed) {
if (dDoubleTapCounter <= doubleTapCounterMax || running) {
yMovement = speed * -2;
} else {
yMovement = speed * -1;
}
} else if (upPressed) {
if (uDoubleTapCounter <= doubleTapCounterMax || running) {
yMovement = speed * -2;
} else {
yMovement = speed * -1;
}
}
// double tap counter
if (rightPressed == false) {
rDoubleTapCounter++;
}
if (leftPressed == false) {
lDoubleTapCounter++;
}
if (downPressed == false) {
dDoubleTapCounter++;
}
if (upPressed == false) {
uDoubleTapCounter++;
}
// change labels
if (player.currentLabel != animationState) {
player.right.gotoAndStop(animationState);
}
// friction
xMovement *= friction;
yMovement *= friction;
// animationState and stop
if (Math.abs(xMovement) > 1) {
if (Math.abs(xMovement) > 22) {
animationState = "running";
running = true;
} else {
animationState = "trotting";
running = false;
}
} else {
animationState = "standing";
xMovement = 0;
}
// right or left facing
if (xMovement > 0) {
player.scaleX = 1;
} else if (xMovement < 0) {
player.scaleX = -1;
}
//movement
if (back.x >= back.width / 2 - 50) {
if (player.x >= 642 && xMovement > 0) {
player.x = 642;
back.x -= xMovement;
} else {
if (player.x <= player.width / 2 && xMovement < 0) {
xMovement = 0;
} else {
player.x += xMovement;
}
}
} else if (back.x <= 1280 - back.width / 2 + 50) {
if (player.x <= 642 - 30 && xMovement < 0) {
player.x = 642;
back.x -= xMovement;
} else {
if (player.x >= 1280 + 30 - player.width / 2 && xMovement > 0) {
xMovement = 0;
} else {
player.x += xMovement;
}
}
} else {
back.x -= xMovement;
}
}
private function keyPressed(e:KeyboardEvent):void {
if (e.keyCode == Keyboard.RIGHT) {
rightPressed = true;
} else if (e.keyCode == Keyboard.LEFT) {
leftPressed = true;
} else if (e.keyCode == Keyboard.DOWN) {
downPressed = true;
} else if (e.keyCode == Keyboard.UP) {
upPressed = true;
}
}
private function keyReleased(e:KeyboardEvent):void {
if (e.keyCode == Keyboard.RIGHT) {
rightPressed = false;
rDoubleTapCounter = 0;
} else if (e.keyCode == Keyboard.LEFT) {
leftPressed = false;
lDoubleTapCounter = 0;
} else if (e.keyCode == Keyboard.DOWN) {
downPressed = false;
dDoubleTapCounter = 0;
} else if (e.keyCode == Keyboard.UP) {
upPressed = false;
uDoubleTapCounter = 0;
}
}
public function mouseClick():void {
clickSound.play();
}
}
}
公理
package {
import flash.events.MouseEvent;
import flash.events.EventDispatcher;
import flash.display.MovieClip;
public class Axiom extends MovieClip {
private var speechBox:Textbox = new Textbox();
private var speech:String = "Something came out of that pop.";
private var main:Main;
public function Axiom() {
// constructor code
this.addEventListener(MouseEvent.CLICK, onClickStage);
this.addEventListener(MouseEvent.CLICK, clicked);
}
private function onClickStage(e:MouseEvent):void {
trace(e.target,e.target.name);
}
private function clicked(e:MouseEvent):void {
main.mouseClick();
stage.addChild(speechBox);
this.removeEventListener(MouseEvent.CLICK, clicked);
}
public function get words():String {
return speech;
}
public function removeThis():void {
this.addEventListener(MouseEvent.CLICK, clicked);
removeChild(speechBox);
}
}
}
文本框
package {
import flash.events.MouseEvent;
import flash.display.MovieClip;
import com.greensock.TweenLite;
public class Textbox extends MovieClip{
private var axiom:Axiom;
private var main:Main;
public function Textbox() {
// constructor code
this.x = 40;
this.y = 360;
this.textBox.text = axiom.words;
TweenLite.from(this, 0.3, {x: "10", alpha: 0});
this.addEventListener(MouseEvent.CLICK, nextPage);
}
private function nextPage(e:MouseEvent):void{
main.mouseClick();
TweenLite.to(this, 0.3, {x: "-10", alpha: 0});
MovieClip(this.parent).removeThis();
}
}
}
与其尝试调用父函数（很少是个好主意），不如使用事件。
在您的公理课程中：
package {
...import statements
public class Axiom extends MovieClip {
private var speechBox:Textbox = new Textbox();
private var speech:String = "Something came out of that pop.";
private var main:Main; //this shouldn't be here, ideally, Axiom knows NOTHING about the main class.
public function Axiom() {
// constructor code
this.addEventListener(MouseEvent.CLICK, onClickStage);
this.addEventListener(MouseEvent.CLICK, clicked);
}
private function onClickStage(e:MouseEvent):void {
trace(e.target,e.target.name);
}
private function clicked(e:MouseEvent):void {
//main.mouseClick(); // unneccesary
dispatchEvent(e); //the event you caught by performing a click will be dispatched again, so that the parent can react to it
stage.addChild(speechBox); //Axiom might not have access to the stage, addChild would suffice
this.removeEventListener(MouseEvent.CLICK, clicked);
}
public function get words():String {
return speech;
}
public function removeThis():void {
this.addEventListener(MouseEvent.CLICK, clicked);
removeChild(speechBox);
}
}
}
和你的主要班级
axiom.addEventListener(MouseEvent.CLICK, onAximClicked);
private function onAxiomClicked(e:MouseEvent):void{
//now you can use the parents (in this case an Object of the Main class) functions
mouseClick();
}
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