Vue组件的生命周期

vue组件生命周期3步走：
挂载： beforeCreate => created => beforeMount => mounted
更新： beforeUpdate => updated
销毁： beforeDestroy => destroyed

父子组件：
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
总结就是：
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
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