javascript - 如何在滚动特定的 div 时切换显示/隐藏固定元素？

Question

我想弄清楚如何在滚动特定的 div 时在显示和隐藏固定位置的元素之间切换。

这是我想要完成的图表:

到目前为止，当另一个 div (#div3) 在窗口中可见时，我已经能够隐藏 #div2，(如提供的代码片段所示) ，但我希望它在到达 #div1 的滚动顶部位置之前也被隐藏。

或者，我想一个更理想的解决方案是将 div 指定为仅在 #div1 的顶部和底部滚动点参数内切换显示。

有什么建议吗？

$(document).ready(function() {
  var $window = $(window);
  var div2 = $('#div2');
  var div1 = $('#div1');
  var div1_top = div1.offset().top;
  var div1_height = div1.height();
  var div1_bottom = div1_top + div1_height;
  console.log(div1_bottom);
  $window.on('scroll', function() {
    var scrollTop = $window.scrollTop();
    var viewport_height = $(window).height();
    var scrollTop_bottom = scrollTop + viewport_height;
    div2.toggleClass('hide', scrollTop_bottom > div1_bottom);
  });
});

body {
  background: #ccffcc;
  padding: 0;
  margin: 0;
  border: 0;
  text-align: center;
}

#div1 {
  background: #0099ff;
  height: 1500px;
  color: #fff;
}

#div2 {
  width: 100px;
  height: 100px;
  text-align: center;
  position: fixed;
  margin: auto;
  top: 0;
  left: 0;
  bottom: 0;
  right: 0;
  background: #ffff00;
  color: #000;
}

#div2.hide {
  display: none;
}

#div3 {
  height: 100px;
  color: #000;
}

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>

<br>
<br>
<br>
<br>
<br>
Scroll area <b>before</b> arriving to the top of <b>div1</b>
<br>
<i>(Scrolling through this area <u>should not</u> display <b>div2</b>)</i>
<br>
<br>
<br>
<br>
<br>
<br>
<div id="div1">
  <div id="div2">This is <b>div2</b></div>
  This is <b>div1</b>
  <br>
  <i>(Toggle show/hide <b>div2</b> when scrolling past the top of this element)</i>
</div>
<div id="div3">
  This is <b>div3</b>
  <br>
  <i>(Toggle show/hide <b>div2</b> every time this div becomes visible in the window)</i>
</div>

Answer 1

我修改了您的 scroll 处理程序。您只需要第二个条件，将视口(viewport)高度添加到当前 scrollTop 以进行底部检查。

此外，使用 document.documentElement.scrollTop 效果更好相反。

最后，我将您的.hide 更改为.show 并以display: none 开始#div2 .您的切换现在使用此类。

$(document).ready(function() {
  var $window = $(window);
  var div2 = $('#div2');
  var div1 = $('#div1');
  var div1_top = div1.offset().top;
  var div1_height = div1.height();
  var div1_bottom = div1_top + div1_height;
  console.log(div1_bottom);
  $window.on('scroll', function() {
    var scrollTop = document.documentElement.scrollTop;
    var viewport_height = $window.height();
    var scrollTop_bottom = scrollTop + viewport_height;
    div2.toggleClass('show', scrollTop > div1_top && (scrollTop + window.innerHeight) < div1_bottom);
  });
});

body {
  background: #ccffcc;
  padding: 0;
  margin: 0;
  border: 0;
  text-align: center;
}

#div1 {
  background: #0099ff;
  height: 1500px;
  color: #fff;
}

#div2 {
  width: 100px;
  height: 100px;
  text-align: center;
  position: fixed;
  margin: auto;
  top: 0;
  left: 0;
  bottom: 0;
  right: 0;
  background: #ffff00;
  color: #000;
display: none;
}

#div2.show {
  display: block;
}

#div3 {
  height: 100px;
  color: #000;
}

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>

<br>
<br>
<br>
<br>
<br>
Scroll area <b>before</b> arriving to the top of <b>div1</b>
<br>
<i>(Scrolling through this area <u>should not</u> display <b>div2</b>)</i>
<br>
<br>
<br>
<br>
<br>
<br>
<div id="div1">
  <div id="div2">This is <b>div2</b></div>
  This is <b>div1</b>
  <br>
  <i>(Toggle show/hide <b>div2</b> when scrolling past the top of this element)</i>
</div>
<div id="div3">
  This is <b>div3</b>
  <br>
  <i>(Toggle show/hide <b>div2</b> every time this div becomes visible in the window)</i>
</div>