前言：在对比Vue和微信小程序具体用法 相同点和不同时，发现，Vue的钩子函数是被遗忘的知识点。
钩子函数是什么呢？
钩子就好像 把 人 出生到死亡分成一个个阶段，每个阶段做这个阶段的事，比如人在出生时候取名字，不会在死的时候取名字。组件也是一样，每个阶段的内部构造是不同的。弄清楚每个阶段Vue做了什么也是需要的。一般特定的钩子做特定的事。
ajax获取数据就是在 Mounted阶段。
把如下代码执行
<!DOCTYPE html>
<html>
<head>
<title>钩子函数</title>
<script type="text/javascript" src="https://cdn.jsdelivr.net/vue/2.1.3/vue.js"></script>
<body>
<div id="app">
<p>{{ message }}</p>
<input type="button" @click="change" value="更新数据" />
<input type="button" @click="destroy" value="销毁" />
</div>
<script type="text/javascript">
var vm = new Vue({
el: '#app',
data: {
message : "Welcome Vue"
},
methods:{
change() {
this.message = 'Datura is me';
},
destroy() {
vm.$destroy();
}
},
beforeCreate: function () {
console.group('beforeCreate 创建前状态===============》');
console.log("%c%s", "color:red","el : " + this.$el); //undefined
console.log("%c%s", "color:red","data : " + this.$data); //undefined
console.log("%c%s", "color:red","message: " + this.message);//undefined
},
created: function () {
console.group('created 创建完毕状态===============》');
console.log("%c%s", "color:red","el : " + this.$el); //undefined
console.log("%c%s", "color:green","data : " + this.$data); //[object Object] => 已被初始化
console.log("%c%s", "color:green","message: " + this.message); //Welcome Vue => 已被初始化
},
beforeMount: function () {
console.group('beforeMount 挂载前状态===============》');
console.log("%c%s", "color:green","el : " + (this.$el)); //已被初始化
console.log(this.$el); // 当前挂在的元素
console.log("%c%s", "color:green","data : " + this.$data); //已被初始化
console.log("%c%s", "color:green","message: " + this.message); //已被初始化
},
mounted: function () {
console.group('mounted 挂载结束状态===============》');
console.log("%c%s", "color:green","el : " + this.$el); //已被初始化
console.log(this.$el);
console.log("%c%s", "color:green","data : " + this.$data); //已被初始化
console.log("%c%s", "color:green","message: " + this.message); //已被初始化
},
beforeUpdate: function () {
alert("更新前状态");
console.group('beforeUpdate 更新前状态===============》'); //这里指的是页面渲染新数据之前
console.log("%c%s", "color:green","el : " + this.$el);
console.log(this.$el);
console.log("%c%s", "color:green","data : " + this.$data);
console.log("%c%s", "color:green","message: " + this.message);
alert("更新前状态2");
},
updated: function () {
console.group('updated 更新完成状态===============》');
console.log("%c%s", "color:green","el : " + this.$el);
console.log(this.$el);
console.log("%c%s", "color:green","data : " + this.$data);
console.log("%c%s", "color:green","message: " + this.message);
},
beforeDestroy: function () {
console.group('beforeDestroy 销毁前状态===============》');
console.log("%c%s", "color:red","el : " + this.$el);
console.log(this.$el);
console.log("%c%s", "color:red","data : " + this.$data);
console.log("%c%s", "color:red","message: " + this.message);
},
destroyed: function () {
console.group('destroyed 销毁完成状态===============》');
console.log("%c%s", "color:red","el : " + this.$el);
console.log(this.$el);
console.log("%c%s", "color:red","data : " + this.$data);
console.log("%c%s", "color:red","message: " + this.message)
}
})
</script>
</body>
</html>
看到vue组件在
beforeCreate
created
beforeMount
mounted 不同的表现（注释部分即是区别）。
另外，从下面截图可看出
在挂载前阶段，el还是 {{message}}，这里就是应用的 Virtual DOM（虚拟Dom）技术，先把坑占住了。
到后面mounted挂载的时候再把值渲染进去。
1 <!DOCTYPE html> 2 <html> 3 <head> 4 <title></title> 5 <script type="text/javascript" src="https://cdn.jsdelivr.net/vue/2.1.3/vue.js"></script> 6 </head> 7 <body> 8 9 <div id="app"> 10 <p>{{ message }}</p> 11 </div> 12 13 <script type="text/javascript"> 14 15 var app = new Vue({ 16 el: '#app', 17 data: { 18 message : "xuxiao is boy" 19 }, 20 beforeCreate: function () { 21 console.group('beforeCreate 创建前状态===============》'); 22 console.log("%c%s", "color:red" , "el : " + this.$el); //undefined 23 console.log("%c%s", "color:red","data : " + this.$data); //undefined 24 console.log("%c%s", "color:red","message: " + this.message) 25 }, 26 created: function () { 27 console.group('created 创建完毕状态===============》'); 28 console.log("%c%s", "color:red","el : " + this.$el); //undefined 29 console.log("%c%s", "color:red","data : " + this.$data); //已被初始化 30 console.log("%c%s", "color:red","message: " + this.message); //已被初始化 31 }, 32 beforeMount: function () { 33 console.group('beforeMount 挂载前状态===============》'); 34 console.log("%c%s", "color:red","el : " + (this.$el)); //已被初始化 35 console.log(this.$el); 36 console.log("%c%s", "color:red","data : " + this.$data); //已被初始化 37 console.log("%c%s", "color:red","message: " + this.message); //已被初始化 38 }, 39 mounted: function () { 40 console.group('mounted 挂载结束状态===============》'); 41 console.log("%c%s", "color:red","el : " + this.$el); //已被初始化 42 console.log(this.$el); 43 console.log("%c%s", "color:red","data : " + this.$data); //已被初始化 44 console.log("%c%s", "color:red","message: " + this.message); //已被初始化 45 }, 46 beforeUpdate: function () { 47 console.group('beforeUpdate 更新前状态===============》'); 48 console.log("%c%s", "color:red","el : " + this.$el); 49 console.log(this.$el); 50 console.log("%c%s", "color:red","data : " + this.$data); 51 console.log("%c%s", "color:red","message: " + this.message); 52 }, 53 updated: function () { 54 console.group('updated 更新完成状态===============》'); 55 console.log("%c%s", "color:red","el : " + this.$el); 56 console.log(this.$el); 57 console.log("%c%s", "color:red","data : " + this.$data); 58 console.log("%c%s", "color:red","message: " + this.message); 59 }, 60 beforeDestroy: function () { 61 console.group('beforeDestroy 销毁前状态===============》'); 62 console.log("%c%s", "color:red","el : " + this.$el); 63 console.log(this.$el); 64 console.log("%c%s", "color:red","data : " + this.$data); 65 console.log("%c%s", "color:red","message: " + this.message); 66 }, 67 destroyed: function () { 68 console.group('destroyed 销毁完成状态===============》'); 69 console.log("%c%s", "color:red","el : " + this.$el); 70 console.log(this.$el); 71 console.log("%c%s", "color:red","data : " + this.$data); 72 console.log("%c%s", "color:red","message: " + this.message) 73 } 74 }) 75 </script> 76 </body> 77 </html>
点“更新”按钮时候，发现，更新前函数 alert两次。
然后才执行更新updated函数。
接着点“销毁”按钮，发现
beforeDestroy
destroyed执行。
如果执行顺序换下：先销毁，再点“更新”按钮，是没用的。销毁也就是把组件移除了。
举个栗子：生命周期的四个阶段就是：
beforeCreate （loading事件）
created （结束loading事件，做一些初始化，实现函数的自执行）
beforeMount 此时有了虚拟DOM
mounted el 被新创建的 vm.$el 替换，并挂载到实例上去之后调用该钩子，渲染为真实DOM。 在这发起后端请求，拿回数据，配合路由钩子做些别的事
beforeDestory： 你确认删除XX吗？ destoryed ：当前组件已被删除，清空相关内容