# [js高手之路]html5 canvas动画教程 - 边界判断与反弹

 1 <head>
2     <meta charset='utf-8' />
3     <style>
4         #canvas {
5             border: 1px dashed #aaa;
6         }
7     </style>
8     <script src="./ball.js"></script>
9     <script>
10         window.onload = function () {
11             var oCanvas = document.querySelector("#canvas"),
12                 oGc = oCanvas.getContext('2d'),
13                 width = oCanvas.width, height = oCanvas.height,
14                 ball = new Ball(width / 2, height / 2),
15                 vx = Math.random() * 2 + 5,
16                 vy = Math.random() * 2 + 5;
17             ball.fill(oGc);
18             ( function move(){
19                 oGc.clearRect( 0, 0, width, height );
20                 ball.x += vx;
21                 ball.y += vy;
22                 ball.fill( oGc );
23
24                 if ( ball.x < ball.radius ) { //碰到左边的边界
25                     ball.x = ball.radius;
26                     vx = -vx;
27                 }else if ( ball.y < ball.radius ){
28                     ball.y = ball.radius;
29                     vy = -vy;
30                 }else if ( ball.x > width - ball.radius ){
31                     ball.x = width - ball.radius;
32                     vx = -vx;
33                 }else if ( ball.y > height - ball.radius ){
34                     ball.y = height - ball.radius;
35                     vy = -vy;
36                 }
37                 requestAnimationFrame( move );
38             } )();
39         }
40     </script>
42
43 <body>
44     <canvas id="canvas" width="1200" height="600"></canvas>
45 </body>

原理跟之前写的文章[js高手之路]html5 canvas动画教程 - 边界判断与小球粒子模拟喷泉,散弹效果差不多，只是在碰到边界的时候，把速度调成反向的，小球就会反弹.

多个物体的反弹

 1 <head>
2     <meta charset='utf-8' />
3     <style>
4         #canvas {
5             border: 1px dashed #aaa;
6         }
7     </style>
8     <script src="./ball.js"></script>
9     <script>
10         window.onload = function () {
11             var oCanvas = document.querySelector("#canvas"),
12                 oGc = oCanvas.getContext('2d'),
13                 width = oCanvas.width, height = oCanvas.height,
14                 balls = [], n = 50;
15             function getRandColor() {
16                 return '#' + (function (color) {
17                     return (color += '0123456789abcdef'[Math.floor(Math.random() * 16)]) && (color.length == 6) ? color : arguments.callee(color);
18                 })('');
19             }
20             for (var i = 0; i < n; i++) {
21                 var ball = new Ball(width / 2, height / 2, 20, getRandColor());
22                 ball.vx = (Math.random() * 2 - 1) * 5;
23                 ball.vy = (Math.random() * 2 - 1) * 5;
24                 balls.push(ball);
25             }
26             (function move() {
27                 oGc.clearRect(0, 0, width, height);
28                 balls.forEach(function (ball) {
29                     ball.x += ball.vx;
30                     ball.y += ball.vy;
31                     ball.fill(oGc);
32
33                     if (ball.x < ball.radius) { //碰到左边的边界
34                         ball.x = ball.radius;
35                         ball.vx = -ball.vx;
36                     } else if (ball.y < ball.radius) {
37                         ball.y = ball.radius;
38                         ball.vy = -ball.vy;
39                     } else if (ball.x > width - ball.radius) {
40                         ball.x = width - ball.radius;
41                         ball.vx = -ball.vx;
42                     } else if (ball.y > height - ball.radius) {
43                         ball.y = height - ball.radius;
44                         ball.vy = -ball.vy;
45                     }
46                 });
47                 requestAnimationFrame(move);
48             })();
49         }
50     </script>
55 </body>