onLaunch: function () {},
// 设置监听器
watch: function (ctx, obj) {
 Object.keys(obj).forEach(key => {
  this.observer(ctx.data, key, ctx.data[key], function (value) {
   obj[key].call(ctx, value)
  })
 })
},
// 监听属性，并执行监听函数
observer: function (data, key, val, fn) {
 Object.defineProperty(data, key, {
  configurable: true,
  enumerable: true,
  get: function () {
   return val
  },
  set: function (newVal) {
   if (newVal === val) return
   fn && fn(newVal)
   val = newVal
  },
 })
}

const app = getApp()
Page({
 data: {
  test: 0
 },
 onLoad: function () {
  // 调用监听器，监听数据变化
  app.watch(this, {
   test: function (newVal) {
    console.log(newVal)
   }
  })
 }