javascript - 返回顶部链接在消失前闪烁两次

Question

根据下面的 fiddle ，我有一个滚动回到顶部的链接，但是在单击链接滚动回到顶部时，它会闪烁两次。

我已经尝试了常见的可疑对象，例如 $(body,html)，但即使我使用其中一个，它仍然会出现。

在 fadeIn 和 fadeOut 之前停止()确实有助于防止这种情况，但会导致 fadeIn 非常慢。

因此，我想知道是否有人可以解释为什么会发生这种元素闪烁，以及我如何才能阻止它发生？

jQuery(function() {
  jQuery(window).scroll(function() {
    if (jQuery(this).scrollTop() > 50) {
      jQuery('#back_top').fadeIn();
    } else {
      jQuery('#back_top').fadeOut();
    }
  });
  jQuery('#back_top').click(function() {
    jQuery('body,html').animate({
      scrollTop: 0
    }, 500);
  });
});

#back_top {
  background-color: #447282;
  /* button color */
  color: white;
  /* text/arrow color */
  display: none;
  z-index: 999;
  /* float in bottom right corner */
  /* 20 pixels from edge */
  position: fixed;
  right: 10px;
  bottom: 10px;
  /* size of button is 50 pixels*/
  width: 54px;
  height: 54px;
  -webkit-box-sizing: border-box;
  -moz-box-sizing: border-box;
  box-sizing: border-box;
  opacity: 0.9;
}

a#back_top {
  font-weight: 600;
  letter-spacing: 2px;
  font-size: 12px;
  text-transform: uppercase;
  text-align: center;
  line-height: 1.6;
  padding-left: 2px;
  padding-top: 8px;
}

a#back_top .arrow:before {
  content: "\e902";
  font-family: 'vishIcon';
  font-style: normal;
  font-weight: 600;
  font-size: 18px;
  line-height: 1;
  speak: none;
  -webkit-font-smoothing: antialiased;
  content: "\e902";
  text-align: center;
  display: block;
  vertical-align: middle;
  cursor: pointer;
  /*     margin-left: -4px; */
}

<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.11.1/jquery.min.js"></script>
What is Lorem Ipsum? Lorem Ipsum is simply dummy text of the printing and typesetting industry. Lorem Ipsum has been the industry's standard dummy text ever since the 1500s, when an unknown printer took a galley of type and scrambled it to make a type
specimen book. It has survived not only five centuries, but also the leap into electronic typesetting, remaining essentially unchanged. It was popularised in the 1960s with the release of Letraset sheets containing Lorem Ipsum passages, and more recently
with desktop publishing software like Aldus PageMaker including versions of Lorem Ipsum. Why do we use it? It is a long established fact that a reader will be distracted by the readable content of a page when looking at its layout. The point of using
Lorem Ipsum is that it has a more-or-less normal distribution of letters, as opposed to using 'Content here, content here', making it look like readable English. Many desktop publishing packages and web page editors now use Lorem Ipsum as their default
model text, and a search for 'lorem ipsum' will uncover many web sites still in their infancy. Various versions have evolved over the years, sometimes by accident, sometimes on purpose (injected humour and the like). Where does it come from? Contrary
to popular belief, Lorem Ipsum is not simply random text. It has roots in a piece of classical Latin literature from 45 BC, making it over 2000 years old. Richard McClintock, a Latin professor at Hampden-Sydney College in Virginia, looked up one of the
more obscure Latin words, consectetur, from a Lorem Ipsum passage, and going through the cites of the word in classical literature, discovered the undoubtable source. Lorem Ipsum comes from sections 1.10.32 and 1.10.33 of "de Finibus Bonorum et Malorum"
(The Extremes of Good and Evil) by Cicero, written in 45 BC. This book is a treatise on the theory of ethics, very popular during the Renaissance. The first line of Lorem Ipsum, "Lorem ipsum dolor sit amet..", comes from a line in section 1.10.32. The
standard chunk of Lorem Ipsum used since the 1500s is reproduced below for those interested. Sections 1.10.32 and 1.10.33 from "de Finibus Bonorum et Malorum" by Cicero are also reproduced in their exact original form, accompanied by English versions
from the 1914 translation by H. Rackham. Where can I get some? There are many variations of passages of Lorem Ipsum available, but the majority have suffered alteration in some form, by injected humour, or randomised words which don't look even slightly
believable. If you are going to use a passage of Lorem Ipsum, you need to be sure there isn't anything embarrassing hidden in the middle of text. All the Lorem Ipsum generators on the Internet tend to repeat predefined chunks as necessary, making this
the first true generator on the Internet. It uses a dictionary of over 200 Latin words, combined with a handful of model sentence structures, to generate Lorem Ipsum which looks reasonable. The generated Lorem Ipsum is therefore always free from repetition,
injected humour, or non-characteristic words etc.
<a href="#" id="back_top" style="display: none;"><span class="arrow"></span>Top</a>

Answer 1

可以在

中添加 console.log来测试原因

  jQuery(window).scroll(function() {
      console.log(jQuery(this).scrollTop());
    ...
  });
  jQuery('#back_top').click(function() {
    console.log("start");
      ...
  });

结果是这样的

start
0
379.6666564941406
376.3166809082031
369.6166687011719
360.6833190917969
351.75
340.5833435058594
327.1833190917969
311
294.25
277.5
259.6333312988281
239
217.78334045410156
201.03334045410156
179.81666564941406
158.60000610351562
140.73333740234375
120.63333129882812
102.76667022705078
83.78333282470703
69.26667022705078
54.75
41.349998474121094
29.066667556762695
19.016666412353516
11.199999809265137
5.616666793823242
1.149999976158142
0

当您调用 .animate 时，第一次发生 scrollTop = 0 事件，然后从 379.6666564941406 减少到 0。所以 FadeIn 发生了两次。

所以你可以添加一个标志来计算 jQuery(window).scroll 动画。

 var animating = false;
jQuery(function() {
  jQuery(window).scroll(function() {
    if (animating) return;
    if (jQuery(this).scrollTop() > 50) {
      jQuery('#back_top').fadeIn();
    } else {
      jQuery('#back_top').fadeOut();
    }
  });
  jQuery('#back_top').click(function() {
    animating = true;
    setTimeout(function(){
       animating = false;
       jQuery('#back_top').fadeOut();
    },500)
    jQuery('body,html').animate({
      scrollTop: 0
    }, 500);
  });
});

#back_top {
  background-color: #447282;
  /* button color */
  color: white;
  /* text/arrow color */
  display: none;
  z-index: 999;
  /* float in bottom right corner */
  /* 20 pixels from edge */
  position: fixed;
  right: 10px;
  bottom: 10px;
  /* size of button is 50 pixels*/
  width: 54px;
  height: 54px;
  -webkit-box-sizing: border-box;
  -moz-box-sizing: border-box;
  box-sizing: border-box;
  opacity: 0.9;
}

a#back_top {
  font-weight: 600;
  letter-spacing: 2px;
  font-size: 12px;
  text-transform: uppercase;
  text-align: center;
  line-height: 1.6;
  padding-left: 2px;
  padding-top: 8px;
}

a#back_top .arrow:before {
  content: "\e902";
  font-family: 'vishIcon';
  font-style: normal;
  font-weight: 600;
  font-size: 18px;
  line-height: 1;
  speak: none;
  -webkit-font-smoothing: antialiased;
  content: "\e902";
  text-align: center;
  display: block;
  vertical-align: middle;
  cursor: pointer;
  /*     margin-left: -4px; */
}

<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.11.1/jquery.min.js"></script>
What is Lorem Ipsum? Lorem Ipsum is simply dummy text of the printing and typesetting industry. Lorem Ipsum has been the industry's standard dummy text ever since the 1500s, when an unknown printer took a galley of type and scrambled it to make a type
specimen book. It has survived not only five centuries, but also the leap into electronic typesetting, remaining essentially unchanged. It was popularised in the 1960s with the release of Letraset sheets containing Lorem Ipsum passages, and more recently
with desktop publishing software like Aldus PageMaker including versions of Lorem Ipsum. Why do we use it? It is a long established fact that a reader will be distracted by the readable content of a page when looking at its layout. The point of using
Lorem Ipsum is that it has a more-or-less normal distribution of letters, as opposed to using 'Content here, content here', making it look like readable English. Many desktop publishing packages and web page editors now use Lorem Ipsum as their default
model text, and a search for 'lorem ipsum' will uncover many web sites still in their infancy. Various versions have evolved over the years, sometimes by accident, sometimes on purpose (injected humour and the like). Where does it come from? Contrary
to popular belief, Lorem Ipsum is not simply random text. It has roots in a piece of classical Latin literature from 45 BC, making it over 2000 years old. Richard McClintock, a Latin professor at Hampden-Sydney College in Virginia, looked up one of the
more obscure Latin words, consectetur, from a Lorem Ipsum passage, and going through the cites of the word in classical literature, discovered the undoubtable source. Lorem Ipsum comes from sections 1.10.32 and 1.10.33 of "de Finibus Bonorum et Malorum"
(The Extremes of Good and Evil) by Cicero, written in 45 BC. This book is a treatise on the theory of ethics, very popular during the Renaissance. The first line of Lorem Ipsum, "Lorem ipsum dolor sit amet..", comes from a line in section 1.10.32. The
standard chunk of Lorem Ipsum used since the 1500s is reproduced below for those interested. Sections 1.10.32 and 1.10.33 from "de Finibus Bonorum et Malorum" by Cicero are also reproduced in their exact original form, accompanied by English versions
from the 1914 translation by H. Rackham. Where can I get some? There are many variations of passages of Lorem Ipsum available, but the majority have suffered alteration in some form, by injected humour, or randomised words which don't look even slightly
believable. If you are going to use a passage of Lorem Ipsum, you need to be sure there isn't anything embarrassing hidden in the middle of text. All the Lorem Ipsum generators on the Internet tend to repeat predefined chunks as necessary, making this
the first true generator on the Internet. It uses a dictionary of over 200 Latin words, combined with a handful of model sentence structures, to generate Lorem Ipsum which looks reasonable. The generated Lorem Ipsum is therefore always free from repetition,
injected humour, or non-characteristic words etc.
<a href="#" id="back_top" style="display: none;"><span class="arrow"></span>Top</a>