I wrote a program to move a ball when a button is clicked. It is a part of experiment for a bigegr project:
Here is the screen shot:
And here is the code:
<!DOCTYPE html>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>HTML5 Random Bouncing Ball</title>
<style type="text/css">
<!--
body { background-color:#ededed; font:normal 12px/18px Arial, Helvetica, sans-serif; }
h1 { display:block; width:600px; margin:20px auto; padding-bottom:20px; font:normal 24px/30px Georgia, "Times New Roman", Times, serif; color:#333; text-shadow: 1px 2px 3px #ccc; border-bottom:1px solid #cbcbcb; }
#myCanvas { background:#fff; border:1px solid #cbcbcb; }
#nav { display:block; width:100%; text-align:center; }
#nav li { display:block; font-weight:bold; line-height:21px; text-shadow:1px 1px 1px #fff; width:100px; height:21px; padding:5px; margin:0 10px; background:#e0e0e0; border:1px solid #ccc; -moz-border-radius:4px;-webkit-border-radius:4px; border-radius:4px; float:left; }
#nav li a { color:#000; display:block; text-decoration:none; width:100%; height:100%; }
-->
</style>
<script type="text/javascript">
function myFunction () {
if(document.getElementById('button').clicked == true)
{
var context;
var dx= 4;
var dy=4;
var y=150;
var x=10;
function draw(){
context= myCanvas.getContext('2d');
context.clearRect(0,0,400,400);
context.beginPath();
context.fillStyle="#000000";
context.arc(x,y,10,0,Math.PI*2,true);
context.closePath();
context.fill();
if( x<0 || x>400)
dx=-dx;
if( y<0 || y>300)
dy=-dy;
x+=dx;
y+=dy;
}
setInterval(draw,10);
}
}
</script>
</head>
<body>
<div id="container">
<canvas id="myCanvas" width="400" height="300"></canvas>
</div>
<input id="button" type="submit" name="button" value="enter" onclick="myFunction();"/>
</body>
</html>
问题是：当我单击该按钮时，球不会动画或移动。我哪里错了？
第 19 行的代码返回 undefined
document.getElementById('button').clicked == true
删除 if 语句，它就可以工作了
我做了一个jsbin：http://jsbin.com/sozicuruja/1/ http://jsbin.com/sozicuruja/1/
function myFunction () {
var context;
var dx= 4;
var dy=4;
var y=150;
var x=10;
function draw(){
context= myCanvas.getContext('2d');
context.clearRect(0,0,400,400);
context.beginPath();
context.fillStyle="#000000";
context.arc(x,y,10,0,Math.PI*2,true);
context.closePath();
context.fill();
if( x<0 || x>400)
dx=-dx;
if( y<0 || y>300)
dy=-dy;
x+=dx;
y+=dy;
}
setInterval(draw,10);
}body { background-color:#ededed; font:normal 12px/18px Arial, Helvetica, sans-serif; }
h1 { display:block; width:600px; margin:20px auto; padding-bottom:20px; font:normal 24px/30px Georgia, "Times New Roman", Times, serif; color:#333; text-shadow: 1px 2px 3px #ccc; border-bottom:1px solid #cbcbcb; }
#myCanvas { background:#fff; border:1px solid #cbcbcb; }
#nav { display:block; width:100%; text-align:center; }
#nav li { display:block; font-weight:bold; line-height:21px; text-shadow:1px 1px 1px #fff; width:100px; height:21px; padding:5px; margin:0 10px; background:#e0e0e0; border:1px solid #ccc; -moz-border-radius:4px;-webkit-border-radius:4px; border-radius:4px; float:left; }
#nav li a { color:#000; display:block; text-decoration:none; width:100%; height:100%; }<div id="container">
<canvas id="myCanvas" width="400" height="300"></canvas>
</div>
<input id="button" type="submit" name="button" value="enter" onclick="myFunction();"/>