依赖追踪

前三章主要讲解的是响应式系统，vue侦测数据的变化，当数据变化时，会通知视图进行响应的更新。

侦测一个对象的变化 Object.defineProperty(后面会使用ES6的Proxy)

Object.defineProperty(obj,key,{
    // 这里都有固定的含义 javascript中自己查找含义
    // enumerable: false,
    // writable: true,
    // configurable: true,
    // 每当从obj的key中读取数据时，get函数被触发，修改key数据时，set被触发
    get() {
        console.log(`getting key "${key}": ${oldvalue}`)
        return oldvalue
    },
    set(newvalue) {
        console.log(`setting key "${key}" to: ${newvalue}`)
        oldvalue = newvalue
    }
})

先收集依赖，把用到数据的地方收集起来，然后等属性发生变化时，把之前收集好的依赖循环触发一遍，即getter中收集依赖，setter中触发依赖

个人感觉书籍上讲解的很麻烦，还是尤雨溪作者本人讲解的简易版本比较清楚

getter和setter函数重写

dependency-tracking

mini-observer

observe() converts修改 the properties in the received object and make them reactive. For each converted property, it gets assigned a Dep instance which keeps track of a list of subscribing update functions, and triggers them to re-run when its setter is invoked.
autorun() takes an update function and re-runs it when properties that the update function subscribes to have been mutated修改. An update function is said to be “subscribing” to a property if it relies on that property during its evaluation.