原生JS实现 MUI导航栏透明渐变效果

body,p,h1{margin: 0;}
.module-layer{
  width: 100%;
  position: fixed;
  top: 0;
  left: 0;
  z-index: 100000;
}
.module-layer-content{
  position: relative;
  min-width: 320px;
  max-width: 750px;
  width: 100%;
  margin: 0 auto;
  background-color: rgba(255, 0, 0, 0.9);
}
.module-layer-bg{
  width: 100%;
  height: 100%;
  background: #000;
  opacity: .7;
  position: absolute;
  top: 0;
  left: 0;
  z-index: -1;
}
.layer-head-name{
  height: 50px;
  line-height: 50px;
  text-align: center;
  padding: 0 50px;
  font-size: 20px;
}
.layer-return,.layer-share{
  width: 50px;
  height: 50px;
  line-height: 50px;
  text-align: center;
  position: absolute;
  top:0;
  z-index: 1;
}
.layer-return{left: 0;}
.layer-share{right: 0;}
.fixed-layer{
  height: 100%;
  display: none;
  z-index: 100001;
}
.relative-layer{height: 100%;}
.layer-content{
  padding:3%;
  position: relative;
  top: 20%;
}
.layer-close{
  width: 2rem;
  height: 2rem;
  position: absolute;
  top:3%;
  right: 3%;
}
.pr {
  position:relative;
}
#shop-input::-webkit-input-placeholder {
  color:#fff;
}
#shop-input:-moz-placeholder {
  color:#fff;
}
#shop-input::-moz-placeholder {
  color:#fff;
}
#shop-input:-ms-input-placeholder {
  color:#fff;
}
#shop-input {
  border:none;
  outline:none;
  background:transparent;
}
.search-box {
  height:30px;
  border-radius:20px;
  top:10px;
  overflow:hidden;
  z-index:10;
}
.search-box:after {
  content:'';
  display:block;
  width:100%;
  height:30px;
  background:#fff;
  opacity:.5;
  position:absolute;
  top:0;
  left:0px;
  z-index:-1;
}
#shop-input {
  height:28px;
  line-height:28px;
  font-size:16px;
  position:absolute;
  top:0;
  left:30px;
}
.shop-search {
  width:16px;
  height:16px;
  position:absolute;
  top:7px;
  left:6px;
}
.layer-return{
  background: url(images/return.png) no-repeat center center/12px 20px;
}
.layer-share{
  background: url(images/share.png) no-repeat center center/20px 30px;
}
a {
 -webkit-tap-highlight-color: transparent;
 -webkit-touch-callout: none;
 -webkit-user-select: none;
}
.module-content{
  min-width: 320px;
  max-width: 750px;
  width: 100%;
  margin: 0 auto;
  background: #fff;
}
.module-content div:first-child img{margin-top: 0;}
.module-content div img{
  display: block;
  width: 100%;
  margin-top: 10px;
}

<header class="module-layer">
  <div class="module-layer-content">
    <p class="layer-return"></p>
    <h1 class="layer-head-name">
      <div class="pr search-box">
        <img class="shop-search" src="images/search.png"/>
        <input id="shop-input" type="text" placeholder="搜索店内商品" value="" />
      </div>
    </h1>
    <p class="layer-share"></p>
  </div>
</header>
<div class="module-content">
  <div><img src="images/banner.png"/></div> 
  <div><img src="images/banner1.png"/></div> 
  <div><img src="images/banner3.png"/></div>
  <div><img src="images/banner4.jpg"/></div>
  <div><img src="images/banner5.png"/></div>
  <div><img src="images/banner6.png"/></div>
  <div><img src="images/banner7.jpg"/></div>
  <div><img src="images/banner8.jpg"/></div>
</div>

(function(){
  //获取滚动条当前位置
  function getScrollTop(){ 
  　　var scrollTop = 0, bodyScrollTop = 0, documentScrollTop = 0; 
  　　if(document.body){ 
  　　　　bodyScrollTop = document.body.scrollTop; 
  　　} 
  　　if(document.documentElement){ 
  　　　　documentScrollTop = document.documentElement.scrollTop; 
  　　} 
  　　scrollTop = (bodyScrollTop - documentScrollTop > 0) ? bodyScrollTop : documentScrollTop; 
  　　return scrollTop; 
  }
  //获取CSS样式
  function getStyle(element, attr){
    if(element.currentStyle){
      return element.currentStyle[attr];
    }else{
      return window.getComputedStyle(element,null)[attr];
    }
  }
  //获取原始backgroundColor值
  var color = getStyle(document.getElementsByClassName('module-layer-content')[0],'backgroundColor');
  //取到RGB
  var colorRgb = color.substring(0,color.lastIndexOf(',') + 1);
  //取到A
  var colorA = color.substring(color.lastIndexOf(',') + 1,color.length - 1);
  //对A判断，如果最终值小于0.9，直接设置为1
  if(colorA < 0.9){colorA = 1;}
  //设置背景色的A的函数
  var setCoverOpacity = function() {
    document.getElementsByClassName('module-layer-content')[0].style.background = colorRgb + (((getScrollTop() / 550) > colorA) ? colorA : (getScrollTop() / 550)) + ')';
  }
  //初始化函数
  setCoverOpacity();
  //绑定滚动监听事件
  window.addEventListener('scroll',setCoverOpacity,false);
}())