[学习交流] 【上海校区】基于原生Javascript的无缝轮播

一、前言

轮播图的种类

说起轮播图，种类那可是五花八门，什么淡入淡出呀、旋转木马呀、3D轮播呀、无缝轮播呀等等，简直是秀的头皮发麻~
本文要讲的就是其中的无缝轮播，小伙伴们知道是怎么实现的吗，如果不知道的话，嘿嘿，你就接着往下看吧。如果你知道的话，就当捧个场，也往下看吧~

基本要求

html和css完成静态布局、js获取并且操纵元素，定时器的应用，节流函数，日期对象等等。

二、无缝轮播的原理

首先，我们来看看什么是无缝轮播，就像下图所示：

其大概过程就是，上一张图片从左往右（或从右往左离开），下一张图片从左往右（或从右往左）进入。哎呀，貌似很简单的样子，有些小伙伴立马就想到了一个大容器里面有一个ul，ul里面有若干个li并且存放图片，把每个li的宽度设置成和容器同宽，然后每次点击的时候改变ul的left值就行了。嗯，很想很有道理的样子，于是三下五除二就做出来了，但是却发现有点问题，就像下面这样：

从第一张到倒数第二张图片还好，但是从最后一张图片到第一张图片就有问题了，并没有从最后一张图片直接切换到第一张图片，而是经过了中间的图片慢慢过渡到第一张图片（第一张图片到最后一张图片也同理），这显然不是预期的效果。问题既然已经发现了，就要去解决它，那么如何才能达到从最后一张图片直接切换到第一张图片（或第一张到最后一张），也就是无缝的效果，答案很简单，我们需要多一张图片（这张图片和第一张图片是相同的）放在图片列表的最后！为什么需要这样一张图片，请往下看。
我们在上面已经清楚要解决的就是首尾图片间的切换问题，所以在引入多一张图片之后，可以让轮播从倒数第二张图片（在没有引入图片之前的最后一张图片）切换到最后一张图片（新引入的图片）的动画完成之后，再瞬间跳转到第一张图片，请看下图（我把外面容器的overflow:hidden去掉了以便大家理解）：

布局

动图演示轮播过程

把overflow:hidden加上，利用视觉差之后

嘿嘿，是不是很神奇，多引入了一张图片我们就把刚才的问题解决了，至于从第一张到最后一张图片的切换，把刚才的过程倒过来就行了。

三、无缝轮播演示

这里我用到了节流函数，如果你还不知道什么是节流函数的话，可以点击下方链接去我另外一篇文章查看。

点我跳转Javascript函数式编程之节流函数

代码部分

/* css代码 */body{ background-color: #333;}ul{ position: absolute; left: 0; list-style: none; padding: 0;}.wrap{ overflow: hidden; position: relative; width: 700px; height: 450px; margin: 100px auto 0;}.wrap .btn{ position: absolute; top: 50%; z-index: 1; width: 50px; height: 80px; margin-top: -40px; background-color: rgba(0,0,0,.5); color: #fff; text-align: center; line-height: 80px; cursor: pointer;}.wrap .left{ left: 0;}.wrap .right{ right: 0;}.img-list{ top: 0; margin: 0; width: 500%; height: 100%;}.img-list li{ float: left; width: 700px; height: 100%;}.img-list li:nth-of-type(1){ background: url("images/01.jpg") no-repeat center/cover;}.img-list li:nth-of-type(2){ background: url("images/02.png") no-repeat center/cover;}.img-list li:nth-of-type(3){ background: url("images/03.png") no-repeat center/cover;}.img-list li:nth-of-type(4){ background: url("images/04.png") no-repeat center/cover;}.img-list li:nth-of-type(5){ background: url("images/01.jpg") no-repeat center/cover;}.tab-list{ right: 0; bottom: 10px; width: 100px; margin: auto;}.tab-list:after{ content: ""; display: block; clear: both;}.tab-list li{ float: left; transition: 1s; width: 15px; height: 15px; margin-left: 5px; background-color: #bbb; border-radius: 50%;}.tab-list li:hover{ cursor: pointer;}.tab-list li:first-child{ margin-left: 0;}.tab-list .on{ width: 40px; border-radius: 8px;}

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<div class="wrap"> <div class="btn left"><</div> <div class="btn right">></div> <ul class="img-list"> <li></li> <li></li> <li></li> <li></li> <li></li> </ul> <ul class="tab-list"> <li class="on"></li> <li></li> <li></li> <li></li> </ul></div>

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//js代码(function(){       var oImgList = document.getElementsByClassName("img-list")[0],          aButton = document.getElementsByClassName("btn"),          aImgLi = document.querySelectorAll(".img-list li"),          oWidth = parseFloat(getComputedStyle(aImgLi[0]).width),          oWrap = document.getElementsByClassName("wrap")[0],          aTab = document.querySelectorAll(".tab-list li");          len = aImgLi.length,          index = 0;       function throttle(fn,time){          var startTime = new Date();          return function(){             var time_ = (new Date() - startTime) >= time;             if(time_){                   fn.apply(this);                   startTime = new Date();             }          }       }       function btnTab(){          var t = new Date();          for(var i = 0,tabLen = aTab.length;i < tabLen;i++){             (function(i){                   aTab.onclick = function(){                      if(new Date() - t >= 1000){                         aTab[index].className = "";                         if((i - index) === (tabLen - 1)){                               oImgList.style.transition = 0 + "s";                               oImgList.style.left = -oWidth*(len-1) + "px";                               index = len - 2;                               setTimeout(function(){                                  oImgList.style.transition = 1 + "s";                                  oImgList.style.left = -oWidth*(index) + "px";                               },1000/60);                         }                         else if((i - index) === (1 - tabLen)){                               oImgList.style.left = -oWidth*(len - 1) + "px";                               index = 0;                               setTimeout(function(){                                  oImgList.style.transition = 0 + "s";                                  oImgList.style.left = index + "px";                               },1000);                         }                         else{                               oImgList.style.left = -oWidth*(i) + "px";                               oImgList.style.transition = 1 + "s";                         }                         index = i;                         this.className = "on";                         t = new Date();                      }                   }             })(i);          }       }       function btnPre(){          index--;          if(index < 0){             oImgList.style.transition = 0 + "s";             oImgList.style.left = -oWidth*(len-1) + "px";             aTab[0].className = "";             index = len - 2;             aTab[index].className = "on";             setTimeout(function(){                   oImgList.style.transition = 1 + "s";                   oImgList.style.left = -oWidth*(index) + "px";             },1000/60);          }          else{             oImgList.style.transition = 1 + "s";             oImgList.style.left = -oWidth*(index) + "px";             aTab[index + 1].className = "";             aTab[index].className = "on";          }       }       function btnNext(){          index++;          oImgList.style.transition = 1 + "s";          if(index === len-1){             oImgList.style.left = -oWidth*index + "px";             aTab[len - 2].className = "";             index = 0;             aTab[index].className = "on";             setTimeout(function(){                   oImgList.style.transition = 0 + "s";                   oImgList.style.left = index + "px";             },1000);          }          else{             oImgList.style.left = -oWidth*index + "px";             aTab[index - 1].className = "";             aTab[index].className = "on";          }       }       aButton[0].onclick = throttle(btnPre,1000);       aButton[1].onclick = throttle(btnNext,1000);       btnTab();       var timer = setInterval(btnNext,5000);       oWrap.onmouseover = function(){          clearInterval(timer);       }       oWrap.onmouseout = function(){          timer = setInterval(btnNext,5000);       } })();
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demo演示

[url=https://htmlpreview.github.io/?https://raw.githubusercontent.co ... BD%AE%E6%92%AD.html]无缝轮播演示demo[/url]
四、无缝轮播的改进
在上面的无缝轮播中，可以发现对于不是相邻的两张图片的切换也并不是直接切换的，而是会过渡中间的图片再切换到目标图片，如下图：

解决办法就是把除了当前显示的图片全都设置成display:none，这样做的好处就是display:none的元素不占据位置！比如说从第一张切换到第三张，因为第二张是display:none，第三章需要显示所以display:block，由于第二张图片不占据位置的原因第三张图片会在第一张图片之后，因此就达到了从第一张直接切换到第三张的效果。
动图演示（没有overflow:hidden）

（有overflow:hidden）

改进后的代码

/* css代码 */body{ background-color: #333;}ul{ position: absolute; left: 0; list-style: none; padding: 0;}.wrap{ overflow: hidden; position: relative; width: 700px; height: 450px; margin: 100px auto 0;}.wrap .btn{ position: absolute; top: 50%; z-index: 1; width: 50px; height: 80px; margin-top: -40px; background-color: rgba(0,0,0,.5); color: #fff; text-align: center; line-height: 80px; cursor: pointer;}.wrap .left{ left: 0;}.wrap .right{ right: 0;}.img-list{ top: 0; margin: 0; width: 500%; height: 100%;}.img-list li{ display: none; float: left; width: 700px; height: 100%;}.img-list .active{ display: block;}.img-list li:nth-of-type(1){ background: url("images/01.jpg") no-repeat center/cover;}.img-list li:nth-of-type(2){ background: url("images/02.png") no-repeat center/cover;}.img-list li:nth-of-type(3){ background: url("images/03.png") no-repeat center/cover;}.img-list li:nth-of-type(4){ background: url("images/04.png") no-repeat center/cover;}.img-list li:nth-of-type(5){ background: url("images/01.jpg") no-repeat center/cover;}.tab-list{ right: 0; bottom: 10px; width: 100px; margin: auto;}.tab-list:after{ content: ""; display: block; clear: both;}.tab-list li{ float: left; transition: 1s; width: 15px; height: 15px; margin-left: 5px; background-color: #bbb; border-radius: 50%;}.tab-list li:hover{ cursor: pointer;}.tab-list li:first-child{ margin-left: 0;}.tab-list .on{ width: 40px; border-radius: 8px;}
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<div class="wrap"> <div class="btn left"><</div> <div class="btn right">></div> <ul class="img-list">       <li class="active"></li>       <li></li>       <li></li>       <li></li>       <li></li> </ul> <ul class="tab-list">       <li class="on"></li>       <li></li>       <li></li>       <li></li> </ul></div>
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//js代码(function(){       var oImgList = document.getElementsByClassName("img-list")[0],          aButton = document.getElementsByClassName("btn"),          aImgLi = document.querySelectorAll(".img-list li"),          oWidth = parseFloat(getComputedStyle(aImgLi[0]).width),          oWrap = document.getElementsByClassName("wrap")[0],          aTab = document.querySelectorAll(".tab-list li");          len = aImgLi.length,          index = 0,          index_ = 0;       function throttle(fn,time){          var startTime = new Date();          return function(){             var time_ = (new Date() - startTime) >= time;             if(time_){                   fn.apply(this);                   startTime = new Date();             }          }       }       function btnTab(){          var t = new Date(),             direction;          for(var i = 0,tabLen = aTab.length;i < tabLen;i++){             (function(i){                   aTab.onclick = function(){                      if(new Date() - t >= 1000){                         index_ = index;                         i - index > 0 ? direction = true : direction = false;                         if(this.className !== "on"){                               aTab[index].className = "";                               if((i - index) === (tabLen - 1)){                                  aImgLi[len - 1].className = "active";                                  aImgLi[0].className = "";                                  oImgList.style.transition = 0 + "s";                                  oImgList.style.left = -oWidth + "px";                                  aTab[0].className = "";                                  index = len - 2;                                  aImgLi[index].className = "active";                                  setTimeout(function(){                                     oImgList.style.transition = 1 + "s";                                     oImgList.style.left = 0 + "px";                                  },1000/60);                                  setTimeout(function(){                                     aImgLi[len - 1].className = "";                                  },1000);                               }                               else if((i - index) === (1 - tabLen)){                                  oImgList.style.transition = 1 + "s";                                  oImgList.style.left = -oWidth + "px";                                  aTab[len - 2].className = "";                                  aImgLi[len - 1].className = "active";                                  index = 0;                                  aTab[index].className = "on";                                  setTimeout(function(){                                     oImgList.style.transition = 0 + "s";                                     oImgList.style.left = index + "px";                                     aImgLi[index].className = "active";                                     aImgLi[len-2].className = "";                                     aImgLi[len-1].className = "";                                  },1000);                               }                               else{                                  if(direction){                                     oImgList.style.left = -oWidth + "px";                                     oImgList.style.transition = 1 + "s";                                     setTimeout(function(){                                           aImgLi[index_].className = "";                                           oImgList.style.left = 0 + "px";                                           oImgList.style.transition = 0 + "s";                                     },1000);                                  }                                  else{                                     oImgList.style.transition = 0 + "s";                                     oImgList.style.left = -oWidth + "px";                                     aImgLi[index].className = "active";                                     setTimeout(function(){                                           oImgList.style.transition = 1 + "s";                                           oImgList.style.left = 0 + "px";                                     },1000/60);                                     setTimeout(function(){                                           aImgLi[index_].className = "";                                     },1000);                                  }                                  index = i;                                  aImgLi[index].className = "active";                               }                               this.className = "on";                               t = new Date();                         }                      }                   }             })(i);          }       }       function btnPre(){          index--;          if(index < 0){             aImgLi[len - 1].className = "active";             aImgLi[0].className = "";             oImgList.style.transition = 0 + "s";             oImgList.style.left = -oWidth + "px";             aTab[0].className = "";             index = len - 2;             aImgLi[index].className = "active";             aTab[index].className = "on";             setTimeout(function(){                   oImgList.style.transition = 1 + "s";                   oImgList.style.left = 0 + "px";             },1000/60);             setTimeout(function(){                   aImgLi[len - 1].className = "";             },1000);          }          else{             oImgList.style.transition = 0 + "s";             oImgList.style.left = -oWidth + "px";             aTab[index + 1].className = "";             aTab[index].className = "on";             aImgLi[index].className = "active";             setTimeout(function(){                   oImgList.style.transition = 1 + "s";                   oImgList.style.left = 0 + "px";             },1000/60);             setTimeout(function(){                   aImgLi[index + 1].className = "";             },1000);          }       }       function btnNext(){          index++;          oImgList.style.transition = 1 + "s";          if(index === len-1){             oImgList.style.left = -oWidth + "px";             aTab[len - 2].className = "";             aImgLi[index].className = "active";             index = 0;             aTab[index].className = "on";             setTimeout(function(){                   oImgList.style.transition = 0 + "s";                   oImgList.style.left = index + "px";                   aImgLi[index].className = "active";                   aImgLi[len-2].className = "";                   aImgLi[len-1].className = "";             },1000);          }          else{             oImgList.style.left = -oWidth + "px";             aTab[index - 1].className = "";             aTab[index].className = "on";             aImgLi[index].className = "active";             setTimeout(function(){                   oImgList.style.transition = 0 + "s";                   aImgLi[index - 1].className = "";                   oImgList.style.left = 0 + "px";             },1000);          }       }       aButton[0].onclick = throttle(btnPre,1000);       aButton[1].onclick = throttle(btnNext,1000);       btnTab();       var timer = setInterval(btnNext,5000);       oWrap.onmouseover = function(){          clearInterval(timer);       }       oWrap.onmouseout = function(){          timer = setInterval(btnNext,5000);       } })();
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改进后的无缝轮播demo演示

[url=https://htmlpreview.github.io/?https://raw.githubusercontent.co ... 9E%81%E7%89%88.html]改进后的无缝轮播demo演示[/url]
五、结束语
本文仅仅提到了无缝轮播，其实还有很多优秀的轮播效果，但是由于目前能力有限，还驾驭不了~所以就到这里啦，总之任重而道远，最后感谢阅读咯~

Port · Port

梦缠绕的时候 · 梦缠绕的时候

不二晨 · 不二晨

奈斯，加油加油

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[学习交流] 【上海校区】基于原生Javascript的无缝轮播

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