微信小程序MUI导航栏透明渐变功能示例(通过改变rgba的a值实现)

<view style="height:100%;position:fixed;width:100%;">
 <scroll-view scroll-y="false" bindscroll="scroll" style="height:100%;">
 <!-- 改变位置，去掉多余的view，直接改变背景色的透明度实现透明渐变 -->
  <view class="page-group" style="background-color: rgba(138, 43, 226,{{scrollTop / 400 > 0.9 ">
   <view class="page-nav-list"><text>首页</text></view>
   <view class="page-nav-list"><text>活动</text></view>
   <view class="page-nav-list"><text>菜单</text></view>
   <view class="page-nav-list"><text>我的</text></view>
  </view>
  <view class="page-banner">
   banner
  </view>
  <view class="goods-list">
   goods-list1
  </view>
  <view class="goods-list list2">
   goods-list2
  </view>
  <view class="goods-list list3">
   goods-list3
  </view>
  <view class="goods-list list4">
   goods-list4
  </view>
 </scroll-view>
</view>

.page-banner{height: 500rpx;background-color: greenyellow;padding: 20rpx;color:#fff;padding-top: 100rpx;}
/*去掉多余的class，直接设置背景色为rgba格式*/
.page-group{
 display: table;
 width: 100%;
 table-layout: fixed;
 background-color: rgba(138, 43, 226,0);
 position: fixed;
 top: 0;
 left: 0;
 z-index: 10;
}
.page-nav-list{
 padding:30rpx 0 ;
 display: table-cell;
 text-align: center;
 width: 100%;
 color: #fff;
}
.goods-list{
 height: 500rpx;
 background-color: green;
 padding: 20rpx;
 color:#fff;
}
.list2{background-color: blue;}
.list3{background-color: yellow;}
.list4{background-color: red;}

Page({
 data: {
  scrollTop: null
 },
 //滚动条监听
 scroll: function (e) {
  this.setData({ scrollTop: e.detail.scrollTop })
 },
})