What is the reactive mechanism in Vue3

What is responsiveness

Responsiveness has always been one of the special features of Vue; in contrast, variables in JavaScript do not have the concept of responsiveness; you The first concept that is instilled when learning JavaScript is that the code is executed from top to bottom;

Let’s look at the following code. After the code is executed, the two double results printed out are also 2 ;Even after we modify the value of count in the code, the value of double will not change.

let count = 1
let double = count * 2
count = 2

The value of double is calculated based on the value of count multiplied by two. If we now want double can change with the change of count, then we need to recalculate double after each change of count value

For example, in the following code, we first encapsulate the logic of calculating double into a function, and then in After modifying count, execute it again, and you will get the latest double value

let count = 1
// 计算过程封装成函数
let getDouble = n=>n*2 //箭头函数
let double = getDouble(count)
count = 2
// 重新计算double ，这里我们不能自动执行对double的计算
double = getDouble(count)

The calculation logic in actual development will be much more complicated than calculating double, but it can be encapsulated into a function for execution; next step , what we have to consider is how to make the value of double automatically calculated

If we can make the getDouble function execute automatically, that is, as shown in the figure below, we use some mechanism of JavaScript to wrap count in a layer , whenever count is modified, the value of double is updated synchronously, then there is a feeling that double automatically changes with the change of count. This is the prototype of responsiveness

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Responsive principle

What is the responsive principle? Three responsive solutions have been used in Vue, namely defineProperty, Proxy and value setter. Let’s first look at Vue 2’s defineProperty API

Here I will illustrate with an example. In the following code, we define Each object obj uses defineProperty to proxy the count attribute; in this way, we intercept the value attribute of the obj object. When reading the count attribute, execute the get function, when modifying the count attribute, execute the set function, and reset it inside the set function. Calculating double

let getDouble = n=>n*2
let obj = {}
let count = 1
let double = getDouble(count)
Object.defineProperty(obj,'count',{
    get(){
        return count
    },
    set(val){
        count = val
        double = getDouble(val)
    }
})
console.log(double) // 打印2
obj.count = 2
console.log(double) // 打印4 有种自动变化的感觉

so that we can achieve a simple responsive function. In the fourth part of the course, I will also take you to write a more complete responsive system

But defineProperty As the principle of implementing responsiveness in Vue 2, API also has some flaws in its syntax; for example, in the following code, if we delete the obj.count attribute, the set function will not be executed, and the double will still be the previous value; this is why in Vue 2 , we need $delete a special function to delete data

delete obj.count
console.log(double) // doube还是4

The responsive mechanism of Vue 3 is implemented based on Proxy; as far as the name Proxy is concerned, you can also see it This is the meaning of proxy. The important significance of Proxy is that it solves the shortcomings of Vue 2's responsiveness

Let's look at the following code, in which we proxy the obj object through new Proxy, and then use get, set and The deleteProperty function proxies the reading, modifying and deleting operations of the object, thus realizing the responsive function

let proxy = new Proxy(obj,{
    get : function (target,prop) {
        return target[prop]
    },
    set : function (target,prop,value) {
        target[prop] = value;
        if(prop==='count'){
            double = getDouble(value)
    }
},
deleteProperty(target,prop){
    delete target[prop]
    if(prop==='count'){
        double = NaN
    }
   }
})
console.log(obj.count,double)
proxy.count = 2
console.log(obj.count,double)
delete proxy.count
// 删除属性后，我们打印log时，输出的结果就会是 undefined NaN
console.log(obj.count,double)

We can see from here that the functions implemented by Proxy are similar to Vue 2's definePropery, and they can both be used by the user. The set function is triggered when the data is modified, thereby realizing the function of automatically updating the double. Moreover, Proxy has also improved several deficiencies of definePropery. For example, it can monitor the deletion of properties.

Proxy monitors objects, not specific properties, so it can not only proxy those that do not exist when they are defined. Properties can also proxy for richer data structures, such as Map, Set, etc., and we can also implement the proxy for deletion operations through deleteProperty

Of course, in order to help you understand Proxy, we can also use double-related The code is written in the set and deleteProperty functions for implementation. In the second half of the course, I will lead you to make a more complete encapsulation; for example, in the following code, Vue 3's reactive function can turn an object into responsive data, and Reactive is implemented based on Proxy; we can also use watchEffect to print data after modifying obj.count

import {reactive,computed,watchEffect} from 'vue'
let obj = reactive({
    count:1
})
let double = computed(()=>obj.count*2)
obj.count = 2
watchEffect(()=>{
    console.log('数据被修改了',obj.count,double.value)
})

With Proxy, the reactive mechanism is relatively complete; but in Vue 3 there is still There is another responsive implementation logic, which is to use the get and set functions of the object to monitor. This responsive implementation can only intercept the modification of a certain attribute. This is also the implementation of the ref API in Vue 3

In the following code, we intercept the value attribute of count and intercept the set operation, which can also achieve similar functions.

let getDouble = n => n * 2
let _value = 1

double = getDouble(_value)
let count = {
    get value() {
        return _value
    },
    set value(val) {
        _value = val
        double = getDouble(_value)
    }
}

console.log(count.value,double)
count.value = 2
console.log(count.value,double)

The comparison table of the three implementation principles is as follows to help you understand the three The difference between responsiveness

实现原理	defineProperty	Proxy	value setter
实际场景	Vue 2 响应式	Vue 3 reactive	Vue 3 ref
优势	兼容性	基于proxy实现真正的拦截	实现简单
劣势	数组和属性删除等拦截不了	兼容不了 IE11	只拦截了 value 属性
实际应用	Vue 2	Vue 3 复杂数据结构	Vue 3 简单数据结构

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定制响应式数据

简单入门响应式的原理后，接下来我们学习一下响应式数据在使用的时候的进阶方式；我们看下使用 <script setup></script> 重构之后的 todolist 的代码；这段代码使用 watchEffect，数据变化之后会把数据同步到 localStorage 之上，这样我们就实现了 todolist 和本地存储的同步

import { ref, watchEffect, computed } from "vue";
let title = ref("");
let todos = ref(JSON.parse(localStorage.getItem('todos')||'[]'));
watchEffect(()=>{
    localStorage.setItem('todos',JSON.stringify(todos.value))
})
function addTodo() {
    todos.value.push({
        title: title.value,
        done: false,
    });
    title.value = "";
}

更进一步，我们可以直接抽离一个 useStorage 函数，在响应式的基础之上，把任意数据响应式的变化同步到本地存储；我们先看下面的这段代码，ref 从本地存储中获取数据，封装成响应式并且返回，watchEffect 中做本地存储的同步，useStorage 这个函数可以抽离成一个文件，放在工具函数文件夹中

function useStorage(name, value=[]){
    let data = ref(JSON.parse(localStorage.getItem(name)||'[]'))
    watchEffect(()=>{
        localStorage.setItem(name,JSON.stringify(data.value))
    })
    return data
}

在项目中我们使用下面代码的写法，把 ref 变成 useStorage，这也是 Composition API 最大的优点，也就是可以任意拆分出独立的功能

let todos = useStorage('todos',[])
function addTodo() {
    ...code
}

现在，你应该已经学会了在 Vue 内部进阶地使用响应式机制，去封装独立的函数;在后续的实战应用中，我们也会经常对通用功能进行封装;如下图所示，我们可以把日常开发中用到的数据，无论是浏览器的本地存储，还是网络数据，都封装成响应式数据，统一使用响应式数据开发的模式;这样，我们开发项目的时候，只需要修改对应的数据就可以了

基于响应式的开发模式，我们还可以按照类似的原理，把我们需要修改的数据，都变成响应式;比如，我们可以在 loading 状态下，去修改浏览器的小图标 favicon;和本地存储类似，修改 favicon 时，我们需要找到 head 中有 icon 属性的标签

在下面的代码中，我们把对图标的对应修改的操作封装成了 useFavicon 函数，并且通过 ref 和 watch 的包裹，我们还把小图标变成了响应式数据

import {ref,watch} from 'vue'
export default function useFavicon( newIcon ) {
    const favicon = ref(newIcon)
    const updateIcon = (icon) => {
        document.head
        .querySelectorAll(`link[rel*="icon"]`)
        .forEach(el => el.href = `${icon}`)
    }
watch( favicon,
    (i) => {
        updateIcon(i)
    }
)
    return {favicon,reset}
}

这样在组件中，我们就可以通过响应式的方式去修改和使用小图标，通过对 faivcon.value 的修改就可以随时更换网站小图标；下面的代码，就实现了在点击按钮之后，修改了网页的图标为 geek.png 的操作

<script setup>
    import useFavicon from &#39;./utils/favicon&#39;
    let {favicon} = useFavicon()
    function loading(){
        favicon.value = &#39;/geek.png&#39;
    }
</script>
<template>
    <button @click="loading">123</button>
</template>

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Vueuse 工具包

我们自己封装的 useStorage，算是把 localStorage 简单地变成了响应式对象，实现数据的更新和localStorage 的同步；同理，我们还可以封装更多的类似 useStorage 函数的其他 use 类型的函数，把实际开发中你用到的任何数据或者浏览器属性，都封装成响应式数据，这样就可以极大地提高我们的开发效率

Vue 社区中其实已经有一个类似的工具集合，也就是 VueUse，它把开发中常见的属性都封装成为响应式函数

VueUse 趁着这一波 Vue 3 的更新，跟上了响应式 API 的潮流；VueUse 的官方的介绍说这是一个 Composition API 的工具集合，适用于 Vue 2.x 或者 Vue 3.x，用起来和 React Hooks 还挺像的

在项目目录下打开命令行里，我们输入如下命令，来进行 VueUse 插件的安装：

npm install @vueuse/core

然后，我们就先来使用一下 VueUse；在下面这段代码中，我们使用 useFullscreen 来返回全屏的状态和切换全屏的函数；这样，我们就不需要考虑浏览器全屏的 API，而是直接使用 VueUse 响应式数据和函数就可以很轻松地在项目中实现全屏功能

<template>
    <h2 @click="toggle">click</h2>
</template>
<script setup>
import { useFullscreen } from &#39;@vueuse/core&#39;
const { isFullscreen, enter, exit, toggle } = useFullscreen()
</script>

useFullscreen 的封装逻辑和 useStorage 类似，都是屏蔽了浏览器的操作，把所有我们需要用到的状态和数据都用响应式的方式统一管理，VueUse 中包含了很多我们常用的工具函数，我们可以把网络状态、异步请求的数据、动画和事件等功能，都看成是响应式的数据去管理。

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