canvas动画，并可以用上下左右箭头控制方向，空格键是停止，代码如下
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<!DOCTYPE html >
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<html>
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<head>
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<title>cars</title>
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<script src=""></script>
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</head>
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<body>
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<canvas id ="canvas" width="200" height="200"></canvas>
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</body>
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<script>
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var canvas = document.getElementById("canvas");
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var ctx = canvas.getContext("2d");
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var width = canvas.width;
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var height = canvas.height;
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var circle = function(x,y,radius,fillCircle){
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ctx.beginPath();
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ctx.arc(x,y,radius,0,Math.PI*2,false);
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if(fillCircle){
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ctx.fill();
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}else{
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ctx.stroke();
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}
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}
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//the ball constructor
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var Ball = function(){
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this.x = width/2;
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this.y = height/2;
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this.xSpeed = 5;
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this.ySpeed = 0;
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}
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//update the ball's position based on its speed
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Ball.prototype.move = function(){
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this.x += this.xSpeed;
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this.y += this.ySpeed;
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if(this.x<0){
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this.x = width;
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}else if(this.x>width){
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this.x =0;
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}else if(this.y <0){
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this.y = height;
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}else if(this.y >height){
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this.y =0;
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}
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}
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//draw the ball at its current position
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Ball.prototype.draw = function(){
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circle(this.x,this.y,10,true);
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}
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//set the ball's direction based on a string
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Ball.prototype.setDirection = function(direction){
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if(direction === "up"){
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this.xSpeed = 0;
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this.ySpeed = -5;
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}else if(direction === "down"){
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this.xSpeed = 0;
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this.ySpeed = 5;
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}else if(direction === "left"){
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this.xSpeed = -5;
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this.ySpeed = 0;
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}else if(direction === "right"){
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this.xSpeed = 5;
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this.ySpeed = 0;
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}else if(direction === "stop"){
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this.xSpeed = 0;
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this.ySpeed = 0;
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}
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}
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//create the ball object
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var ball = new Ball();
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//an object to convert keycodes into action names
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var keyActions = {
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32:"stop",
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37:"left",
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38:"up",
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39:"right",
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40:"down"
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};
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//the keydown handler that will be called for every keypress
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$("body").keydown(function(event){
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var direction = keyActions[event.keyCode];
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ball.setDirection(direction);
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});
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//the animation function,called every 30ms
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setInterval(function(){
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ctx.clearRect(0,0,width,height);
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ball.draw();
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ball.move();
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ctx.strokeRect(0,0,width,height);
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},30);
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</script>
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</html>
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