单组件声明周期图

       挂载： beforeCreate => created => beforeMount => mounted

       更新： beforeUpdate => updated

       销毁： beforeDestroy => destroyed

       父子组件生命周期图

       挂载： parent beforeCreate => parent created => parent beforeMount => child beforeCreate => child created => child beforeMount => child mounted => parent mounted

       更新： parent beforeUpdate => child beforeUpdate => child updated => parent updated

       销毁： parent beforeDestroy => child beforeDestroy => child destroyed => parent destroyed

       从以上能够看出：

       挂载时，子组件是在父组件before mount后开始挂载，并且子组件先mounted，父组件随后

       更新时，子组件是在父组件before update后开始更新，子组件先于父组件更新

       销毁时，子组件是在父组件before destroy后开始销毁，并且是子组件先销毁，父组件随后。

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