vue生命周期的理解

如下图为Vue官网（https://cn.vuejs.org/v2/guide/instance.html#实例生命周期）给出的生命周期图示

光看图或许不够直观，接下来就是一段代码来加强理解。

mounted方法及其之前

dom部分：

<div id="app">
    <p>{{message}}</p>
</div>

js部分：

<script>
        var app = new Vue({
            el: '#app',
            data: {
              message : "some data" 
            },
            beforeCreate: function () {
                console.log('beforeCreate 创建前状态===============》');
                console.log("%c%s", "color:red" , "el     : " + this.$el); 
                console.log("%c%s", "color:red","data   : " + this.$data);  
                console.log("%c%s", "color:red","message: " + this.message)  
            },
            created: function () {
                console.log('created 创建完毕状态===============》');
                console.log("%c%s", "color:red","el     : " + this.$el); 
                console.log("%c%s", "color:red","data   : " + this.$data);  
                console.log("%c%s", "color:red","message: " + this.message); 
            },
            beforeMount: function () {
                console.log('beforeMount 挂载前状态===============》');
                console.log("%c%s", "color:red","el     : " + (this.$el)); 
                console.log(this.$el);
                console.log("%c%s", "color:red","data   : " + this.$data);   
                console.log("%c%s", "color:red","message: " + this.message);   
            },
            mounted: function () {
                console.log('mounted 挂载结束状态===============》');
                console.log("%c%s", "color:red","el     : " + this.$el); 
                console.log(this.$el);    
                console.log("%c%s", "color:red","data   : " + this.$data); 
                console.log("%c%s", "color:red","message: " + this.message);  
            },
            beforeUpdate: function () {
                console.log('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.$data); 
                console.log("%c%s", "color:red","message: " + this.message); 
            },
            updated: function () {
                console.log('updated 更新完成状态===============》');
                console.log("%c%s", "color:red","el     : " + this.$el);
                console.log(this.$el); 
                console.log("%c%s", "color:red","data   : " + this.$data); 
                console.log("%c%s", "color:red","message: " + this.message); 
            },
            beforeDestroy: function () {
                console.log('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.$data); 
                console.log("%c%s", "color:red","message: " + this.message); 
            },
            destroyed: function () {
                console.log('destroyed 销毁完成状态===============》');
                console.log("%c%s", "color:red","el     : " + this.$el);
                console.log(this.$el);  
                console.log("%c%s", "color:red","data   : " + this.$data); 
                console.log("%c%s", "color:red","message: " + this.message)
            }
        })
        </script>

以上代码的运行结果为：

从运行结果可以看出来：

　　在beforeCreated阶段，vue实例的挂载元素$el和数据对象data都为undefined，还未初始化，

　　在created阶段，vue实例的数据对象data有了，$el还没有，

　　在beforeMount阶段，vue实例的$el和data都初始化了，但还是挂载之前为虚拟的dom节点，data.message还未替换

　　在mounted阶段，vue实例挂载完成，data.message成功渲染

update部分

在浏览器控制台中输入 app.message = "new data"，会出现如下变化

可以发现，当data变化时，会触发beforeUpdate和updated方法

destroy部分

在浏览器控制台中输入app.$destroy()

此时再继续修改message的值，在浏览器控制台输入app.message = "change after destory"，你会发现：

因此，在执行destroy方法后，对data的改变不会再触发周期函数，说明此时vue实例已经解除了事件监听以及和dom的绑定，但是dom结构依然存在。