js中的requestAnimationFrame

requestAnimationFrame的作用就是重绘
一个简单的demo如下

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <style>
        #box{
            background-color: rebeccapurple;
             100px;
            height: 100px;    
        }
    </style>
</head>
<body>
    <div id="box"></div>
    <script>
        var duration = 4000.0;
        var startTime = -1.0;

        function render(currentTime) {
            var h1 = document.getElementById("box");
            var opacity = currentTime/duration;
            h1.style.opacity = opacity;
        }

        function eachFrame() {
            var timeElapse = (new Date()).getTime()-startTime;
            if(startTime < 0) {
                startTime = (new Date()).getTime();
                render(0.0);
            }else if(timeElapse < duration) {
                render(timeElapse);
            }else {
                return;
            }

            window.requestAnimationFrame(eachFrame);
        }

        window.requestAnimationFrame(eachFrame);
    </script>
</body>
</html>