给网站加入显示实时帧率的代码

$('body').before('<div id="fps" style="z-index:10000;position:fixed;top:3;left:3;font-weight:bold;"></div>');
var showFPS = (function(){ 
    var requestAnimationFrame =  
        window.requestAnimationFrame || 
        window.webkitRequestAnimationFrame || 
        window.mozRequestAnimationFrame ||
        window.oRequestAnimationFrame ||
        window.msRequestAnimationFrame || 
        function(callback) { 
            window.setTimeout(callback, 1000/60); 
        }; 
    var e,pe,pid,fps,last,offset,step,appendFps; 

    fps = 0; 
    last = Date.now(); 
    step = function(){ 
        offset = Date.now() - last; 
        fps += 1; 
        if( offset >= 1000 ){ 
        last += offset; 
        appendFps(fps); 
        fps = 0; 
        } 
        requestAnimationFrame( step ); 
    }; 
    appendFps = function(fps){ 
        console.log(fps+'FPS');
        $('#fps').html(fps+'FPS');
    };
    step();
})();