解决zepto 回到顶部动画

    var $this = $('.container-scroll'),
        $goup = $('.goup'),
        //元素可视区的高度
        offtop = $(window).height() - $this.offset().top,
        isscroll =false;
    //监听滚动事件 控制 回到顶部按钮出现
    $this.on('scroll',function () {
        var rolltop = $this.scrollTop();
        if ( rolltop > offtop) {
            $goup.css({'opacity':.5,'bottom':40+'px'});
        }else if(rolltop < offtop){
            $goup.css({'opacity':.5,'bottom':-40+'px'});
        }
    });
    
    $goup.on('click',function () {
        isscroll =true;
        gotop();
    });
    
    $this.on('touchstart',function () {
        isscroll = false;
    });


    function gotop(){
        var y = Math.max(0,$this.scrollTop());
        var speed = 1.2;
        $this.scrollTop( Math.floor(y / speed));
        if (isscroll && $this.scrollTop() > 0) {
            var timer= setTimeout(gotop, 10);
        }else if(!isscroll){
            clearTimeout(timer);
        }
    }