Vue：生命周期

一、什么是vue的生命周期

Vue中的生命周期是指组件从创建到销毁的一系列过程。看下面这张官方文档的图：

从图片中可以看出Vue的整个生命周期包括8个状态，按照先后顺序分别为：

beforeCreate
Created
beforeMount
mounted
beforeUpdate
updated
beforeDestroy
destroyed

Vue组件的生命周期共分为三个阶段，如下图所示：

创建阶段和销毁阶段在组件的生命周期中只会执行一次，而更新阶段会执行多次。

先看一下创建阶段完成的事情：

在看更新阶段完成的事情：

最后在看一下销毁阶段完成的事情：

先看下面的一段代码：

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>生命周期</title>
    <!--引入vue.js-->
    <script src="./js/vue.js" ></script>
    <script>
        window.onload=function(){
            new Vue({
                el:'#app',// 2.0不允许挂载到html,body元素上
                data:{
                    msg:'welcome'
                },
                methods:{
                    update(){
                        this.msg="欢迎";
                    },
                    destroy(){
                        this.$destroy();
                    }
                },
                //创建前状态  el和data并未初始化
                beforeCreate(){
                    console.group('------beforeCreate创建前状态------');
                    console.log("%c%s", "color:red" , "el     : " + this.$el); //undefined
                    console.log("%c%s", "color:red","data   : " + this.$data); //undefined 
                    console.log("%c%s", "color:red","message: " + this.msg) 
                    console.log('组件实例刚刚创建，还未进行数据观测和事件配置');
                },
                created(){//常用  创建完毕状态   完成了data数据的初始化  el没有
                    console.group('------created创建完毕状态------');
                    console.log("%c%s", "color:red","el     : " + this.$el); //undefined
                    console.log("%c%s", "color:red","data   : " + this.$data); //已被初始化 
                    console.log("%c%s", "color:red","message: " + this.msg); //已被初始化
                    console.log("实例已经创建完成，并且已经进行数据观测和事件配置")
                },
                beforeMount(){  //挂载前状态 完成了el和data初始化
                    this.msg="112233";
                    console.group('------beforeMount挂载前状态------');
                    console.log("%c%s", "color:red","el     : " + (this.$el)); //已被初始化
                    console.log(this.$el);
                    console.log("%c%s", "color:red","data   : " + this.$data); //已被初始化  
                    console.log("%c%s", "color:red","message: " + this.msg); //已被初始化
                    console.log("模板编译之前，还没挂载");
                },
                mounted(){//常用  挂载结束状态  完成挂载
                    console.group('------mounted 挂载结束状态------');
                    console.log("%c%s", "color:red","el     : " + this.$el); //已被初始化
                    console.log(this.$el);    
                    console.log("%c%s", "color:red","data   : " + this.$data); //已被初始化
                    console.log("%c%s", "color:red","message: " + this.msg); //已被初始化 
                    console.log("模板编译之后，已经挂载，此时才会有渲染页面，才能看到页面上数据的显示")
                },
                beforeUpdate(){   //更新前状态
                    console.group('------beforeUpdate 更新前状态------');
                    console.log("%c%s", "color:red","el     : " + this.$el);
                    console.log(this.$el);   
                    console.log("%c%s", "color:red","data   : " + this.$data); 
                    console.log("%c%s", "color:red","message: " + this.msg); 
                },
                updated(){   //更新完成状态
                    console.group('------updated 更新完成状态------');
                    console.log("%c%s", "color:red","el     : " + this.$el);
                    console.log(this.$el); 
                    console.log("%c%s", "color:red","data   : " + this.$data); 
                    console.log("%c%s", "color:red","message: " + this.msg); 
                },
                beforeDestroy(){   //销毁前状态
                    console.group('------beforeDestroy 销毁前状态------');
                    console.log("%c%s", "color:red","el     : " + this.$el);
                    console.log(this.$el);    
                    console.log("%c%s", "color:red","data   : " + this.$data); 
                    console.log("%c%s", "color:red","message: " + this.msg); 
                },
                destroyed(){  //销毁完成状态
                    console.group('------destroyed 组件销毁完成状态------');
                    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.msg)
                }
            });
        }
    </script>
</head>
<body>
    <div id="app">
        <input type="text" v-model="msg" />
        <button @click="update">更新数据</button>
        <button @click="destroy">销毁组件</button> 
    </div>
</body>
</html>