jquery如何实现随滚动条滚动效果？方法示例

//侧栏随动
var rollstart = $('.feed-mail'), //滚动到此区块的时候开始随动
rollout = $('.cookie-list'); //隐藏rollstart之后的区块

rollstart.before('<div class="da_rollbox"></div>');

var offset = rollstart.offset(),objwindow = $(window),rollbox = rollstart.prev(),
rollset = $('.search,.weibo,.group'); //添加rollstart之前的随动区块

objwindow.scroll(function() {
if (objwindow.scrolltop() > offset.top){
if(rollbox.html(null)){
rollset.clone().prependto('.da_rollbox');
}
rollout.fadeout();
rollbox.show().stop().animate({margintop: objwindow.scrolltop() - offset.top + 20},400);
} else {
rollout.fadein();
rollbox.hide().stop().animate({margintop:0},400);
}
});