vue組件生命周期3步走：
掛載： beforeCreate => created => beforeMount => mounted
更新： beforeUpdate => updated
銷毀： beforeDestroy => destroyed
父子組件：
掛載： 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
銷毀： parent beforeDestroy => child beforeDestroy => child destroyed => parent destroyed
總結就是：
掛載時蛙吏，子組件是在父組件before mount后開始掛載土陪，并且子組件先mounted
更新時，子組件是在父組件before update后開始更新，子組件先于父組件更新
銷毀時纳鼎，子組件是在父組件before destroy后開始銷毀，并且是子組件先銷毀
