在添加和删除两个类之间添加动画

Question

我正在处理下面的演示。为什么在添加和删除两个类fixed-top和fixed-bottom之间，我无法生成平滑的过渡（类似平滑滚动），而我已经添加了CSS角色？

  -webkit-transition: all 3s ease;
  -moz-transition: all 3s ease;
  -o-transition: all 3s ease;
  transition: all 3s ease;

var lastScrollTop = 0;
$(window).scroll(function(event) {
  var st = $(this).scrollTop();
  if (st > lastScrollTop) {
    if (st > 500) {
      $('.box').removeClass("fixed-bottom").addClass("fixed-top");
    }
  } else {
    if (st < 500) {
      $('.box').removeClass("fixed-top").addClass("fixed-bottom");
    }
  }
  lastScrollTop = st;
});

html,
body {
  height: 100%;
}
.container {
  height: 2000px;
}
.box {
  width: 100%;
  height: 50px;
  background: #777;
}
.fixed-top {
  position: fixed;
  top: 0;
  -webkit-transition: all 3s ease;
  -moz-transition: all 3s ease;
  -o-transition: all 3s ease;
  transition: all 3s ease;
}
.fixed-bottom {
  position: fixed;
  bottom: 0;
  -webkit-transition: all 3s ease;
  -moz-transition: all 3s ease;
  -o-transition: all 3s ease;
  transition: all 3s ease;
}

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<div class="container">
  <div class="box fixed-bottom"></div>
</div>

做到这一点的最佳方法是什么（上下移动）？

Answer 1

一个条带上下跳跃，因为您没有在 .fixed-top中设置底部值，而在 .fixed-bottom中没有设置底部值，然后过渡专业人士无法意识到transite的属性。您需要获取window.height（）才能正确穿透。您可以使用jQuery做到这一点，使您的CSS较短看摘要

var lastScrollTop = 0;
var y = $( window ).height() - $('.box').height() + "px";
$('.box').css("top", y);
$(window).scroll(function(event) {
  var st = $(this).scrollTop();
  if (st > lastScrollTop) {
    if (st > 500) {
      $('.box').css("bottom", y);
      $('.box').css("top","0");
    }
  } else {
    if (st < 500) {
      $('.box').css("top", y);
      $('.box').css("bottom","0");
    }
  }
  lastScrollTop = st;
});

html,
body {
  height: 100%;
}
.container {
  height: 2000px;
}
.box {
  width: 100%;
  height: 50px;
  position: fixed;
  bottom: 0;
  background: #777;
  -webkit-transition: all 3s ease;
  -moz-transition: all 3s ease;
  -o-transition: all 3s ease;
  transition: all 3s ease;
}

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<div class="container">
  <div class="box fixed-bottom"></div>
</div>