下面是完整代码，可以实现发送以后自动把屏幕的焦点放在最后一条信息

class='tab'>
  class='lan'>{{tabdata.title}}
  class='tent'>
    {{tabdata.attribute_attribute}}
    class='fl_r '>{{tabdata.num}}
  
  class='xiahuaxian1'>
  
    class='fabu'>发布时间: {{tabdata.time_agree}}
  

class='news'>
  class='xiahuaxian1 xiahuaxia'>
  class='new_top_txt'>您正在与{{tabdata.nickname}}进行沟通
  class="historycon">
    "true" scroll-top="{{scrollTop}}" class="history" wx:for="{{centendata}}" wx:key=''>
      
        class='time'>{{item.time}}
      
      if="{{item.is_show_right ==1}}">
        class='my_right'>
          class='page_row'>
            if='{{!item.is_img}}' class='new_txt'>{{item.content}}
            if='{{item.is_img}}' src='{{item.content}}' class='new_imgtent'>
            if='{{!item.is_img}}' class='sanjiao my'>
            class='new_img' wx:if='{{item.show_rignt == "是自己的内容，显示在右边，右边渲染 nickname_owner，head_owner"}}' src='{{item.head_owner}}'>
             class='new_img' wx:if='{{item.show_rignt == "是自己的内容，显示在右边，右边渲染 nickname_open，head_open"}}' src='{{item.head_open}}'>
          
        
      
      else>
        class='you_left'>
          class='page_row'>
            class='new_img' wx:if='{{item.show_rignt == "不是自己的内容，显示在左边，左边渲染 nickname_owner，head_owner"}}' src='{{item.head_owner}}'>
             class='new_img' wx:if='{{item.show_rignt == "不是自己的内容，显示在左边，左边渲染 nickname_open，head_open"}}' src='{{item.head_open}}'>
            if='{{!item.is_img}}' class='sanjiao you'>
            if='{{!item.is_img}}' class='new_txt'>{{item.content}}
            if='{{item.is_img}}' src='{{item.content}}' class='new_imgtent'>
          
        
      
    
  

class='hei' id="hei">
class="sendmessage">
  "emoji" bindinput="bindChange" confirm-type="done" value='{{news_input_val}}' placeholder="" />"add">发送
  'display:none' type="" bindinput="bindChange" confirm-type="done" placeholder="" />
  "upimg1" class='jia_img' src='../../../images/jia_img.png'>

// pages/index/tonews/tonews.js

var app = getApp();
var util = require("../../../utils/util.js")
var message = '';
var text = '';
var user = {};
var length;
var zx_info_id;
var openid_talk;
Page({
  data: {
    news: '',
    scrollTop: 0,
    message: '',
    text: text,
    centendata: '',
    nickName: '',
    avatarUrl: '',
    news_input_val:'',
    tabdata: ''
  },
  bindChange: function (e) {
    message = e.detail.value
  },
  //事件处理函数
  add: function (e) {
    var that = this
    var data = {
      program_id: app.jtappid,
      openid: app._openid,
      zx_info_id: zx_info_id,
      content: message,
      openid_talk:openid_talk
    }
    util.request('/session_submit', 'post', data, '正在加载数据', function (res) {
      if (res.data.state == 1) {
        var a = true;
        that.loaddata(a);
        that.setData({
          news_input_val:''
        })
        message = ''
      } else {
        wx.showToast({
          title: '网络错误,请稍后',
        })
      }
    })
  },
  onLoad: function (options) {
    openid_talk = options.openid_talk;
    zx_info_id = options.zx_info_id;
    console.log(openid_talk)
    //调用应用实例的方法获取全局数据
    this.setData({
      zx_info_id: zx_info_id,
      nickName: app.nickName,
      avatarUrl: app.avatarUrl,
    });
    this.loaddata()
  },
  // 页面加载
  loaddata: function (a) {
    var that = this;
    var is_img = true;
    var data = {
      program_id: app.jtappid,
      openid: app._openid,
      zx_info_id: zx_info_id,
      openid_talk: openid_talk
    }
    util.request('/session_page', 'post', data, '', function (res) {
      if (res.data.k1) {
        res.data.k1.time_agree = util.js_date_time(res.data.k1.time_agree)
      }
      for (var i = 0; i < res.data.k2.length; i++) {
        res.data.k2[i].time = util.js_date_time(res.data.k2[i].time)
        var n = res.data.k2[i].content.charAt(res.data.k2[i].content.length - 1);
        switch (n) {
          case 'g':
            res.data.k2[i].is_img = is_img
            break;
          default:
        }
      }
      that.setData({
        tabdata: res.data.k1,
        centendata: res.data.k2.reverse()
      })
      wx.setNavigationBarTitle({ title: that.data.tabdata.nickname });
      if (a) {
        setTimeout(function () {
            that.bottom()
        }, 500);
      }
    })
    setTimeout(function () {
      if (that.data.centendata.length != length) {
        length = that.data.centendata.length
      }
      that.loaddata()
    }, 3000);
  },
  // 获取hei的id节点然后屏幕焦点调转到这个节点
  bottom: function () {
    var query = wx.createSelectorQuery()
    query.select('#hei').boundingClientRect()
    query.selectViewport().scrollOffset()
    query.exec(function (res) {
      wx.pageScrollTo({
        scrollTop: res[0].bottom  // #the-id节点的下边界坐标
      })
      res[1].scrollTop // 显示区域的竖直滚动位置
    })
  },
  // 选择图片并把图片保存  
  upimg1: function () {
    var that = this;
    wx.chooseImage({
      success: function (res) {
        var data = {
          program_id: app.jtappid,
          openid: app._openid,
          zx_info_id: zx_info_id,
        }
        var tempFilePaths = res.tempFilePaths
        wx.uploadFile({
          url: '/session_submit', //提交信息至后台
          filePath: tempFilePaths[0],
          name: 'content', //文件对应的参数名字(key)
          formData: data,  //其它的表单信息
          success: function (res) {
            var a = true;
            that.loaddata(a);
            message = ''
          }
        })
      }
    })
  }
})

/* pages/index/tonews/tonews.wxss */

page {
  background-color: #f7f7f7;
}
.tab {
  padding: 20rpx 20rpx 40rpx 50rpx;
  height: 20%;
  background-color: white;
}
.tab .tent {
  font-size: 33rpx;
  margin-bottom: 30rpx;
}
.jia_img{
  height: 80rpx;
  width: 90rpx;
}
.new_imgtent{
    height: 180rpx;
  width: 190rpx;
}
.tab .fabu {
  font-size: 33rpx;
  margin-top: 30rpx;
  margin-bottom: 30rpx;
}
.xiahuaxia {
  width: 80%;
  text-align: center;
  margin: 0 auto;
  position: relative;
  top: 60rpx;
}
.time {
  text-align: center;
  padding: 5rpx 20rpx 5rpx 20rpx;
  width: 200rpx;
  font-size: 26rpx;
  background-color: #E8E8E8;
}
.new_top_txt {
  width: 50%;
  position: relative;
  top: 38rpx;
  text-align: center;
  margin: 0 auto;
  font-size: 30rpx;
  color: #787878;
  background-color: #f7f7f7;
}
/* 聊天内容 */
.news {
  margin-top: 30rpx;
  text-align: center;
  margin-bottom: 150rpx;
}
.img_null {
  height: