微信小程序滚动居中导航选项卡

微信小程序头部导航经常会用到选项卡，一个会自动居中的选项卡的体验会比普通的选项卡要好很多。

效果示意图：

被选中的tab会始终居中显示。

利用微信小程序自带的组件scroll-view的scroll-left就很容易实现这个效果。

wxml代码：

    
    
      
        
          {{navItem.cat_name}}
          
        
      
      
    

遮罩层是右边提示可以滑动的一个遮罩图。

wxss代码：

/* 顶部导航条 */

.nav {
  height: 100rpx;
  width: 630rpx;
  position: relative;
  box-sizing: border-box;
  overflow: hidden;
  font-size: 0;
  line-height: 100rpx;
  white-space: nowrap;
  opacity: 0;
  transition: all 300ms cubic-bezier(0.23, 1, 0.32, 1) 0ms;
}

.nav.showScroll {
  opacity: 1;
}

.slideLine {
  height: 8rpx;
  /* width: 24px; */
  position: absolute;
  top: 94rpx;
  /* top: 0; */
  background: #e64340;
  transition: all 300ms cubic-bezier(0.23, 1, 0.32, 1) 0ms;
}

.nav-item {
  /* padding: 0 5px; */
  margin: 0 5px;
  position: relative;
  display: inline;
  /* height: 100rpx; */
  text-align: center;
  font-size: 14px;
  color: #666;
  border-bottom: none;
  /* background: green; */
  transition: all 500ms cubic-bezier(0.23, 1, 0.32, 1) 0ms;
}

.nav-item .hot{
  width: 19rpx;
  height: 19rpx;
  position: absolute;
  right: -5rpx;
  top: -5rpx;
}

.nav-item.active {
  color: #e64340;
  /* height: 94rpx; *//* border-bottom: 8rpx solid #e64340; */
}

.page-top {
  width: 100%;
  position: relative;
  background: white;
}

.page-top.top {
  position: fixed;
  top: 0;
  left: 0;
  z-index: 99999;
}

.search {
  position: absolute;
  right: 30rpx;
  top: 20rpx;
  width: 60rpx;
  height: 60rpx;
  z-index: 999999;
}

.mask-scroll {
  position: absolute;
  right: 120rpx;
  top: 0;
  width: 45rpx;
  height: 100rpx;
  z-index: 999999;
}

js代码：

data: {
    slideWidth: 38, // 导航滑块长度
    sliedeLeft: 5,  // 导航滑块距离
    showMaskScroll: true,  // 是否显示顶部遮罩
  },
switchNav(event) {
      let that = this;
      let listLength = that.data.navData.length;
      let cur = home.getDataSet(event, 'current');
      let id = home.getDataSet(event, 'id');
      let name = home.getDataSet(event, 'name');
      let onheadactive = event.currentTarget.offsetLeft; // 元素距离左侧的位置
      let winweight = this.data.windowWidth; // 设备宽度
      let domWidth = ''; // 元素的宽度
      let query = wx.createSelectorQuery();
      query.select('.navItem' + cur).boundingClientRect(function (rect) {
        domWidth = rect.width;
        let distance = onheadactive - winweight / 2 + domWidth / 2;
        let sliedeLeft = onheadactive;

        if (sliedeLeft <= 0) {
          sliedeLeft = 0;
        }

        console.log('navScrollLeft', distance)
        //tab选项居中                            
        that.setData({
          navScrollLeft: distance,
          slideWidth: domWidth,
          sliedeLeft,
          currentIndex: 0
        })

      }).exec();

      // 最后一个元素隐藏遮罩层
      if (cur + 1 >= listLength) {
        that.setData({
          showMaskScroll: false
        })
      } else {
        that.setData({
          showMaskScroll: true
        })
      }
  },