<!DOCTYPE html>
<html>
<head>
<title>躁动的小球</title>
<meta charset="utf-8">
</head>
<body>
<canvas id="canvas" style="display: block;margin:50px auto">
<p>you browser not support canvas!</p>
</canvas>
<script type="text/javascript">
var WIDTH = 900;
var HEIGHT = 500;
var balls = [];
window.onload = function(){
var canvas = document.getElementById('canvas'),
context = canvas.getContext('2d');
canvas.width = WIDTH;
canvas.height = HEIGHT;
for(var i=0;i<20;i++){
var radius = Math.random()*25+25;
aBall = {
color:"rgb("+Math.floor(Math.random()*255)+","+Math.floor(Math.random()*255)+","+Math.floor(Math.random()*255)+")",
radius:radius,
x:Math.random()*(canvas.width-2*radius)+radius,
y:Math.random()*(canvas.height-2*radius)+radius,
vx:(Math.random()*5+5)*Math.pow(-1,Math.floor(Math.random()*100)),
vy:(Math.random()*5+5)*Math.pow(-1,Math.floor(Math.random()*100))
}
balls[i] = aBall;
}
setInterval(function(){
draw(context);
update(canvas.width,canvas.height);
},50);
}
function draw(cxt){
var canvas = cxt.canvas;
cxt.clearRect(0,0,canvas.width,canvas.height);
for(var i=0;i<balls.length;i++){
cxt.fillStyle = balls[i].color;
cxt.beginPath();
cxt.arc(balls[i].x,balls[i].y,balls[i].radius,0,Math.PI*2);
cxt.closePath();
cxt.fill();
}
}
function update(canvasWidth,canvasHeight){
for (var i = 0; i < balls.length; i++) {
balls[i].x += balls[i].vx;
balls[i].y += balls[i].vy;
if(balls[i].x - balls[i].radius <= 0){
balls[i].vx = -balls[i].vx;
balls[i].x = balls[i].radius;
}
if(balls[i].x + balls[i].radius >= canvasWidth){
balls[i].vx = -balls[i].vx;
balls[i].x = canvasWidth - balls[i].radius;
}
if(balls[i].y - balls[i].radius <= 0){
balls[i].vy = -balls[i].vy;
balls[i].y = balls[i].radius;
}
if(balls[i].y + balls[i].radius >= canvasHeight){
balls[i].vy = -balls[i].vy;
balls[i].y = canvasHeight - balls[i].radius;
}
}
}
</script>
</body>
</html>29行，写错了，vx应该是vy
代码已没问题