如果你想做逐帧动画的时候，你应该用这个方法。这就要求你的动画函数执行会先于浏览器重绘动作。通常来说，被调用的频率是每秒60次，但是一般会遵循W3C标准规定的频率。如果是后台标签页面，重绘频率则会大大降低。

var canvas=document.getElementById("canvas");var ctx=canvas.getContext('2d');var running=false;
//    get requestAnimationFramefunction AFrame(callback){return window.requestAnimationFrame = window.requestAnimationFrame || window.mozRequestAnimationFrame || window.webkitRequestAnimationFrame || window.msRequestAnimationFrame;}var raFrame=AFrame();var raf;var ball={x:100,y:100,vx:5,vy:1,radius:25,color:'blue',draw:function(){ctx.beginPath();ctx.arc(this.x,this.y,this.radius,0,Math.PI*2,true);ctx.closePath();ctx.fillStyle=this.color;ctx.fill();}};function  clear(){ctx.fillStyle=ctx.fillStyle = 'rgba(255,255,255,0.3)';ctx.fillRect(0,0,canvas.width,canvas.height);}function draw(){
//        ctx.fillStyle = 'rgba(255,255,255,0.3)';
//        ctx.fillRect(0,0,canvas.width,canvas.height);
//      ctx.clearRect(0,0,canvas.width,canvas.height);//每次画之前清除上一次的clear();ball.draw();ball.x += ball.vx;ball.y += ball.vy;if (ball.y + ball.vy > canvas.height || ball.y + ball.vy < 0) {ball.vy = -ball.vy;}if (ball.x + ball.vx > canvas.width || ball.x + ball.vx < 0) {ball.vx = -ball.vx;}raf= raFrame(draw);console.log("运行")}canvas.addEventListener('mousemove',function(e){if(!running){clear();ball.x = e.clientX;ball.y = e.clientY;ball.draw();}
//        raf=raFrame(draw);});canvas.addEventListener("click",function(e){if (!running) {raf=raFrame(draw);running = true;}});canvas.addEventListener('mouseout',function(e){window.cancelAnimationFrame(raf);running = false;});ball.draw();