requestAnimationFrame移动端实现回到顶部效果

$('#backToTop').click(function(){
var dist = $(document).height() - $(window).height();
timer = requestAnimationFrame(function fn(){
if($(window).scrollTop() > 0){
//console.log($(window).scrollTop() + ':dist:'+dist);
$(window).scrollTop($(window).scrollTop()-20);
timer = requestAnimationFrame(fn);
}else{
cancelAnimationFrame(timer);
}
});
});