Javascript对象的方法赋值

八哥

发布于 2018-01-18 17:22:27

2.6K0

发布于 2018-01-18 17:22:27

Javascript对象编程学习中，一直不能很好的掌握对象的属性(property)和方法(method)。今天在写代码过程中，又犯了一个低级错误。

<!DOCTYPE html>
<html>
    <head>
        <meta charset="utf-8"/>
        <title>Javascript对象的方法</title>
    </head>
    <body>
    <p>
    通过自定义一个球的对象。在页面加载完成时，在canvas上绘制一个球。
    </p>
    <canvas id="mycanvas" width="400" height="300">
    您的浏览器不支持HTML5 Canvas标签。
    </canvas>
    <script type="text/javascript">
    var ctx;
    function Ball(sx,sy,rad,styleString){
        this.sx=sx;
        this.sy=sy;
        this.rad=rad;
        this.fillStyle=styleString;
        this.draw=drawball();
        this.moveit=moveball();
    }
    function drawball(){
        ctx.fillStyle=this.fillStyle;
        ctx.beginPath();
        ctx.arc(this.sx,this.sy,this.rad,0,Math.PI*2,true);
        ctx.fill();
    }
    function moveball(){
        //todo
    }
    var aball=new Ball(100,100,10,"rgb(234,20,200)");
    function init(){
        var canvas=document.getElementById("mycanvas");
        ctx=canvas.getContext("2d");
        aball.draw();
    }
    window.addEventListener("load",init,false);
    </script>
    </body>
</html>

 .csharpcode, .csharpcode pre { 	font-size: small; 	color: black; 	font-family: consolas, "Courier New", courier, monospace; 	background-color: #ffffff; 	/*white-space: pre;*/ } .csharpcode pre { margin: 0em; } .csharpcode .rem { color: #008000; } .csharpcode .kwrd { color: #0000ff; } .csharpcode .str { color: #006080; } .csharpcode .op { color: #0000c0; } .csharpcode .preproc { color: #cc6633; } .csharpcode .asp { background-color: #ffff00; } .csharpcode .html { color: #800000; } .csharpcode .attr { color: #ff0000; } .csharpcode .alt  { 	background-color: #f4f4f4; 	width: 100%; 	margin: 0em; } .csharpcode .lnum { color: #606060; } <!-- .csharpcode, .csharpcode pre { 	font-size: small; 	color: black; 	font-family: consolas, "Courier New", courier, monospace; 	background-color: #ffffff; 	/*white-space: pre;*/ } .csharpcode pre { margin: 0em; } .csharpcode .rem { color: #008000; } .csharpcode .kwrd { color: #0000ff; } .csharpcode .str { color: #006080; } .csharpcode .op { color: #0000c0; } .csharpcode .preproc { color: #cc6633; } .csharpcode .asp { background-color: #ffff00; } .csharpcode .html { color: #800000; } .csharpcode .attr { color: #ff0000; } .csharpcode .alt  { 	background-color: #f4f4f4; 	width: 100%; 	margin: 0em; } .csharpcode .lnum { color: #606060; } --> <!-- .csharpcode, .csharpcode pre { 	font-size: small; 	color: black; 	font-family: consolas, "Courier New", courier, monospace; 	background-color: #ffffff; 	/*white-space: pre;*/ } .csharpcode pre { margin: 0em; } .csharpcode .rem { color: #008000; } .csharpcode .kwrd { color: #0000ff; } .csharpcode .str { color: #006080; } .csharpcode .op { color: #0000c0; } .csharpcode .preproc { color: #cc6633; } .csharpcode .asp { background-color: #ffff00; } .csharpcode .html { color: #800000; } .csharpcode .attr { color: #ff0000; } .csharpcode .alt  { 	background-color: #f4f4f4; 	width: 100%; 	margin: 0em; } .csharpcode .lnum { color: #606060; } -->

自己想这个方法在init()函数里面调用，ctx怎么会为”undefined”，不可能。监听事件不会出错呀。那问题说明这个函数在init()之前就运行了。断点跟踪就证明自己猜想是对的，但是这是一个对象，我只是new一下，不可能回去调用它的方法呀！并且drawball()方法也是在init()函数里面调用的。仔细一行一行的看代码，看到底哪里出了问题。最后发现下面两行很低级错误的代码，它们是：

this.draw=drawball(); this.moveit=moveball();
 <!-- .csharpcode, .csharpcode pre { 	font-size: small; 	color: black; 	font-family: consolas, "Courier New", courier, monospace; 	background-color: #ffffff; 	/*white-space: pre;*/ } .csharpcode pre { margin: 0em; } .csharpcode .rem { color: #008000; } .csharpcode .kwrd { color: #0000ff; } .csharpcode .str { color: #006080; } .csharpcode .op { color: #0000c0; } .csharpcode .preproc { color: #cc6633; } .csharpcode .asp { background-color: #ffff00; } .csharpcode .html { color: #800000; } .csharpcode .attr { color: #ff0000; } .csharpcode .alt  { 	background-color: #f4f4f4; 	width: 100%; 	margin: 0em; } .csharpcode .lnum { color: #606060; } -->

本意是在自定义对象里面，给对象一个方法。我给了它方法，但是后面的()会马上调用这个方法。这是不应该的。this.draw指向一个函数的地址，但是在这里不需要马上调用它。所以在函数init()还没有进行时，这个drawball()函数就开始运行了。所以此时的ctx确实为undefined。

正确的做法是把方法的名字赋值给对象的方法。代码改为下面就正常运行:

this.draw=drawball; 
this.moveit=moveball;

 .csharpcode, .csharpcode pre { 	font-size: small; 	color: black; 	font-family: consolas, "Courier New", courier, monospace; 	background-color: #ffffff; 	/*white-space: pre;*/ } .csharpcode pre { margin: 0em; } .csharpcode .rem { color: #008000; } .csharpcode .kwrd { color: #0000ff; } .csharpcode .str { color: #006080; } .csharpcode .op { color: #0000c0; } .csharpcode .preproc { color: #cc6633; } .csharpcode .asp { background-color: #ffff00; } .csharpcode .html { color: #800000; } .csharpcode .attr { color: #ff0000; } .csharpcode .alt  { 	background-color: #f4f4f4; 	width: 100%; 	margin: 0em; } .csharpcode .lnum { color: #606060; } <!-- .csharpcode, .csharpcode pre { 	font-size: small; 	color: black; 	font-family: consolas, "Courier New", courier, monospace; 	background-color: #ffffff; 	/*white-space: pre;*/ } .csharpcode pre { margin: 0em; } .csharpcode .rem { color: #008000; } .csharpcode .kwrd { color: #0000ff; } .csharpcode .str { color: #006080; } .csharpcode .op { color: #0000c0; } .csharpcode .preproc { color: #cc6633; } .csharpcode .asp { background-color: #ffff00; } .csharpcode .html { color: #800000; } .csharpcode .attr { color: #ff0000; } .csharpcode .alt  { 	background-color: #f4f4f4; 	width: 100%; 	margin: 0em; } .csharpcode .lnum { color: #606060; } -->

运行结果如下：