# Vue 的双向绑定原理
```!
概述:Vue是采用数据劫持结合发布者-订阅者模式的方式, 通过ES5(vue2.x)提供的Object.defineProperty()方法来劫持(监视)各个属性的setter， getter，在数据变动时发布消息给订阅者, 触发响应的监听回调. 并且，由于是在不同的数据上触发同步，可以精确的变更发送给绑定数据的视图，而不是对所有的数据都执行一次检测
```
## 具体的步骤
1. 需要observer的数据对象进行递归遍历， 包括子属性对象的属性， 都加上getter和setter，这样的话， 给这个对象的某个值赋值，就会触发setter， 那么就能监听到了数据变化
2. compile解析模板指令， 将模板中的变量替换成数据,然后初始化渲染页面视图,，并将每个指令对应的节点绑定更新函数， 添加监听数据的订阅者，一旦数据有变动，收到通知， 更新视图
3. Watcher订阅者是Observer和compile之间通信桥梁, 主要做的事情是：
- 在自身实例化是往属性订阅器(dep)里面添加自己
- 自身必须有一个update()的方法
- 待属性变动 dep.notice()通知的时候,能调用自身的update()方法,并触发cpmpile中绑定的回调
4. MVVM作为数据绑定的入口, 整合observer、compile和watcher三者，通过observer来监听自己的model的变化，通过compile来编译模板指令，最终利用watcher搭起的observer和compile之间的通信桥梁，达到数据变化--->视图更新。例如：视图交互变化（input）--->数据model变更的双向绑定效果
## 版本比较
vue是基于依赖收集的双向绑定；
### 1.基于数据劫持、依赖收集的双向绑定的特点
-不需要显示的调用，Vue利用数据劫持+发布订阅，可以直接通知变化并且驱动视图
-直接得到精确的变化数据，劫持了属性setter，当属性值改变，我们可以精确的获取变化的内容newValue，不需要额外的diff操作
### 2.Object.defineProperty缺点
-不能监听数组：因为数组没有getter和setter，因为数组长度不确定，太长性能负担太大
-只能监听属性，而不是整个对象，需要遍历循环整个属性
-只能监听属性变化，不能监听属性的删减
### 3.proxy 的好处
-可以监听数组
-监听整个对象而不是属性
-返回新对象而不是直接修改原对象，更符合immutable
```!
immutable对象是不可直接赋值的对象，它可以有效的避免错误赋值的问题
```
-功能强大，多种拦截方法
```javascript
//数组
let arr = [7, 8, 9];
arr = new Proxy(arr, {
get(target, prop) {
return prop in target ? target[prop] : 'error'
}
})
console.log(arr[3]) //error
console.log(arr[1]) //8
//apply 拦截函数的调用，call 和 apply 操作
let sum = function(...args) {
let count = 0;
args.forEach((item) => {
count += item
})
return count;
};
let p = new Proxy(sum, {
apply(target, ctx, args) {
return target(...args) * 2
}
})
console.log(p(1, 2, 3, 4)) //20
console.log(p.call(null, 1, 2, 3, 4)) //20
console.log(p.apply(null, [1, 2, 3, 4])) //20
```
### 4.proxy缺点
-兼容性不好，无法用polyfill磨平
> polyfill 在英文中有垫片的意思，意为兜底的东西。在计算机科学中，指的是对未能实现的客户端上进行的"兜底"操作。
## 详细解释
### Observer
```js
function defineReactive(data, key, val) {
observe(val)
Object.defineProperty(data, key, {
enumerable: true,
configurable: true,
get: () => {
return val;
},
set: (newVal) => {
val = newVal;
console.log('属性' + key + '已经被监听，现在值为：'+ newVal.toString())
}
})
}
function observe(data) {
if (!data || typeof data !== 'object') {
return;
}
Object.keys(data).forEach(key => {
defineReactive(data,key,data[key])
})
}
let library = {
book1: {
name: "",
},
book2: {
name: "",
},
};
observe(library);
library.book1.name = "编译原理";//属性name已经被监听，现在值为：编译原理
library.book2.name = "高中数学";//属性name已经被监听，现在值为：高中数学
library.book1.name = "编译原理2";//属性name已经被监听，现在值为：编译原理2
//待续
```
需要创建一个可以容纳订阅者的消息订阅器Dep，订阅器Dep主要负责收集订阅者，然后在属性变化的时候执行对应订阅者的更新函数。所以订阅器需要一个容器。
```javascript
function Dep() {
this.subs = []
}
Dep.prototype = {
addSub: (sub) => {
this.subs.push(sub)
},
notify: () => {
this.subs.forEach(sub => {
sub.update()
})
}
}
```
### Watcher
get 函数时执行添加订阅者操作（初始化时），set时执行notify
```js
function Watcher(vm, exp, cb) {
this.cb = cb;
this.vm = vm;
this.exp = exp;
this.value = this.get();
}
Watcher.prototype = {
update: function () {
this.run();
},
run: function () {
const value = this.vm.data[this.exp];
const oldVal = this.value;
if (value !== oldVal) {
this.value = value;
this.cb.call(this.vm, value, oldVal);
}
},
get: function () {
Dep.target = this;
const value = this.vm.data[this.exp];
Dep.target = null;
return value;
},
};
```
测试代码：过2s界面上会显示无名二字
```html
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8" />
<meta http-equiv="X-UA-Compatible" content="IE=edge" />
<meta name="viewport" content="width=device-width, initial-scale=1.0" />
<title>Document</title>
</head>
<body>
<h1 id="name">{{name}}</h1>
</body>
<script>
function Dep() {
this.subs = [];
}
Dep.prototype = {
addSub: function (sub) {
this.subs.push(sub);
},
notify: function () {
this.subs.forEach((sub) => {
sub.update();
});
},
};
function Watcher(vm, exp, cb) {
this.cb = cb;
this.vm = vm;
this.exp = exp;
this.value = this.get();
}
Watcher.prototype = {
update: function () {
this.run();
},
run: function () {
const value = this.vm.data[this.exp];
const oldVal = this.value;
if (value !== oldVal) {
this.value = value;
this.cb.call(this.vm, value, oldVal);
}
},
get: function () {
Dep.target = this;
const value = this.vm.data[this.exp];
Dep.target = null;
return value;
},
};
function defineReactive(data, key, val) {
observe(val);
const dep = new Dep();
Object.defineProperty(data, key, {
enumerable: true,
configurable: true,
get: () => {
if (Dep.target) {
dep.addSub(Dep.target);
}
return val;
},
set: (newVal) => {
if (val === newVal) {
return;
}
val = newVal;
console.log(
"属性" + key + "已经被监听，现在值为：" + newVal.toString()
);
dep.notify();
},
});
}
function observe(data) {
if (!data || typeof data !== "object") {
return;
}
Object.keys(data).forEach((key) => {
defineReactive(data, key, data[key]);
});
}
</script>
<script>
function Vue1(data, el, exp) {
const self = this;
this.data = data;
Object.keys(data).forEach(key=>{
self.proxyKeys(key)
})
observe(data);
el.innerHTML = this.data[exp];
new Watcher(this, exp, (value) => {
el.innerHTML = value;
});
return this;
}
Vue1.prototype = {
proxyKeys:function(key){
const self = this
Object.defineProperty(this,key,{
enumerable:false,
configurable:true,
get:()=>{
return self.data[key]
},
set:(newVal)=>{
self.data[key] = newVal
}
})
}
}
const ele = document.querySelector("#name");
const vue1 = new Vue1(
{
name: "hellow world",
},
ele,
"name"
);
window.setTimeout(() => {
console.log("name值改变了");
vue1.name = "无名";
}, 2000);
</script>
</html>
```
### Compile
实现解析器去解析dom节点
1. 解析模板指令，并替换模板数据，初始化视图
2. 将模板指令对应的及诶按绑定对应的更新函数，初始化响应的订阅器
```js
function Compile(el, vm) {
this.vm = vm;
this.el = document.querySelector(el);
this.fragment = null;
this.init();
}
Compile.prototype = {
init: function () {
this.fragment = this.nodeToFragment(this.el);
this.compileElement(this.fragment);
this.el.appendChild(this.fragment);
},
nodeToFragment: function (el) {
const fragment = document.createDocumentFragment();
let child = el.firstChild;
while (child) {
//将dom元素移入fragment中
fragment.appendChild(child);
child = el.firstChild;
}
return fragment;
},
compileElement: function (el) {
const childNodes = el.childNodes;
const self = this;
[].slice.call(childNodes).forEach(function (node) {
const reg = /{{(.*)}}/;
const text = node.textContent;
if (self.isElementNode(node)) {
self.compile(node);
} else if (self.isTextNode(node) && reg.test(text)) {
self.compileText(node, reg.exec(text)[1]);
}
if (node.childNodes && node.childNodes.length) {
self.compileElement(node);
}
})
},
compile: function (node) {
const nodeAttrs = node.attributes;
const self = this;
if(nodeAttrs){
Array.prototype.forEach.call(nodeAttrs, function (attr) {
const attrName = attr.name;
if (self.isDirective(attrName)) {
const exp = attr.value;
const dir = attrName.substring(2);
if (self.isEventDirective(dir)) {//事件指令
self.compileEvent(node, self.vm, exp, dir);
} else {//v-model指令
self.compileModel(node, self.vm, exp, dir);
}
node.removeAttribute(attrName);
}
})
}
},
compileEvent: function (node, vm, exp, dir) {
const eventType = dir.split(':')[1];
const cb = vm.methods && vm.methods[exp];
if (eventType && cb) {
node.addEventListener(eventType, cb.bind(vm), false);
}
},
compileModel: function (node, vm, exp, dir) {
const self = this;
const val = this.vm[exp];
this.modelUpdater(node, val);
new Watcher(this.vm, exp, function (value) {
self.modelUpdater(node, value);
})
node.addEventListener('input', function (e) {
const newValue = e.target.value;
if (val === newValue) {
return;
}
self.vm[exp] = newValue;
val = newValue;
})
},
compileText: function (node, exp) {
const self = this;
const initText = this.vm[exp];
this.updateText(node, initText);
new Watcher(this.vm, exp, function (value) {
self.updateText(node, value);
})
},
modelUpdater: function (node, value, oldValue) {
node.value = typeof value == 'undefined' ? '' : value;
},
updateText: function (node, value) {
node.textContent = typeof value == 'undefined' ? '' : value;
},
isDirective: function (attr) {
return attr.indexOf('v-') == 0;
},
isElementNode: function (node) {
return node.nodeType == 1;
},
isTextNode: function (node) {
return node.nodeType == 3;
},
isEventDirective: function (dir) {
return dir.indexOf('on:') === 0;
}
}
```
测试
```html
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8" />
<meta http-equiv="X-UA-Compatible" content="IE=edge" />
<meta name="viewport" content="width=device-width, initial-scale=1.0" />
<title>Document</title>
</head>
<body>
<div id="app">
<h2>{{title}}</h2>
<input v-model="name">
<h1>{{name}}</h1>
<button v-on:click="clickMe">click me!</buttuon>
</div>
</body>
<script>
function Dep() {
this.subs = [];
}
Dep.prototype = {
addSub: function (sub) {
this.subs.push(sub);
},
notify: function () {
this.subs.forEach((sub) => {
sub.update();
});
},
};
function Watcher(vm, exp, cb) {
this.cb = cb;
this.vm = vm;
this.exp = exp;
this.value = this.get();
}
Watcher.prototype = {
update: function () {
this.run();
},
run: function () {
const value = this.vm.data[this.exp];
const oldVal = this.value;
if (value !== oldVal) {
this.value = value;
this.cb.call(this.vm, value, oldVal);
}
},
get: function () {
Dep.target = this;
const value = this.vm.data[this.exp];
Dep.target = null;
return value;
},
};
function defineReactive(data, key, val) {
observe(val);
const dep = new Dep();
Object.defineProperty(data, key, {
enumerable: true,
configurable: true,
get: () => {
if (Dep.target) {
dep.addSub(Dep.target);
}
return val;
},
set: (newVal) => {
if (val === newVal) {
return;
}
val = newVal;
console.log(
"属性" + key + "已经被监听，现在值为：" + newVal.toString()
);
dep.notify();
},
});
}
function observe(data) {
if (!data || typeof data !== "object") {
return;
}
Object.keys(data).forEach((key) => {
defineReactive(data, key, data[key]);
});
}
</script>
<script>
function Compile(el, vm) {
this.vm = vm;
this.el = document.querySelector(el);
this.fragment = null;
this.init();
}
Compile.prototype = {
init: function () {
this.fragment = this.nodeToFragment(this.el);
this.compileElement(this.fragment);
this.el.appendChild(this.fragment);
},
nodeToFragment: function (el) {
const fragment = document.createDocumentFragment();
let child = el.firstChild;
while (child) {
//将dom元素移入fragment中
fragment.appendChild(child);
child = el.firstChild;
}
return fragment;
},
compileElement: function (el) {
const childNodes = el.childNodes;
const self = this;
[].slice.call(childNodes).forEach(function (node) {
const reg = /{{(.*)}}/;
const text = node.textContent;
if (self.isElementNode(node)) {
self.compile(node);
} else if (self.isTextNode(node) && reg.test(text)) {
self.compileText(node, reg.exec(text)[1]);
}
if (node.childNodes && node.childNodes.length) {
self.compileElement(node);
}
})
},
compile: function (node) {
const nodeAttrs = node.attributes;
const self = this;
if(nodeAttrs){
Array.prototype.forEach.call(nodeAttrs, function (attr) {
const attrName = attr.name;
if (self.isDirective(attrName)) {
const exp = attr.value;
const dir = attrName.substring(2);
if (self.isEventDirective(dir)) {//事件指令
self.compileEvent(node, self.vm, exp, dir);
} else {//v-model指令
self.compileModel(node, self.vm, exp, dir);
}
node.removeAttribute(attrName);
}
})
}
},
compileEvent: function (node, vm, exp, dir) {
const eventType = dir.split(':')[1];
const cb = vm.methods && vm.methods[exp];
if (eventType && cb) {
node.addEventListener(eventType, cb.bind(vm), false);
}
},
compileModel: function (node, vm, exp, dir) {
const self = this;
const val = this.vm[exp];
this.modelUpdater(node, val);
new Watcher(this.vm, exp, function (value) {
self.modelUpdater(node, value);
})
node.addEventListener('input', function (e) {
const newValue = e.target.value;
if (val === newValue) {
return;
}
self.vm[exp] = newValue;
val = newValue;
})
},
compileText: function (node, exp) {
const self = this;
const initText = this.vm[exp];
this.updateText(node, initText);
new Watcher(this.vm, exp, function (value) {
self.updateText(node, value);
})
},
modelUpdater: function (node, value, oldValue) {
node.value = typeof value == 'undefined' ? '' : value;
},
updateText: function (node, value) {
node.textContent = typeof value == 'undefined' ? '' : value;
},
isDirective: function (attr) {
return attr.indexOf('v-') == 0;
},
isElementNode: function (node) {
return node.nodeType == 1;
},
isTextNode: function (node) {
return node.nodeType == 3;
},
isEventDirective: function (dir) {
return dir.indexOf('on:') === 0;
}
}
</script>
<script>
function Vue1(options) {
const self = this;
this.data = options.data;
this.methods = options.methods;
Object.keys(this.data).forEach(function (key) {
self.proxyKeys(key);
});
observe(this.data);
new Compile(options.el, this);
// options.mounted.call(this);//所有事情处理好后执行mounted函数
}
Vue1.prototype = {
proxyKeys:function(key){
const self = this
Object.defineProperty(this,key,{
enumerable:false,
configurable:true,
get:()=>{
return self.data[key]
},
set:(newVal)=>{
self.data[key] = newVal
}
})
}
}
const vue1 = new Vue1({
el: '#app',
data: {
title: 'hello world',
name: '123'
},
methods: {
clickMe: function () {
this.title = 'hello world2';
},
modifyName:function(){
this.name='modify'
}
},
mounted(){
this.name = 'modify'
}
})
</script>
</html>
```