代码如下：

app.js:

  //如果担心openid的安全，就用这个函数
  getCloudOpenid: async function () {
    return this.openid = this.openid || (await wx.cloud.callFunction({name: 'login'})).result.OPENID
  },
  //最佳方案。
  getOpenid: async function () {
    (this.openid = this.openid || wx.getStorageSync('openid')) || wx.setStorageSync('openid', await this.getCloudOpenid())
    return this.openid
  },

任何page:

onLoad: async function () {
  console.log(this.openid = await getApp().getOpenid())
},

  //在本page的其他函数里获得openid。
yourFunc: function(){
  console.log(this.openid)
}

云函数login:

const cloud = require('wx-server-sdk')
cloud.init()
exports.main=async()=>{return cloud.getWXContext()}