生命周期
beforeCreate-created-beforeMount-mounted-beforeUpdate-updated-beforeDestroy-destroyed
父子组件的钩子函数执行顺序
加载时
父beforeCreate->父created->父beforeMount->子beforeCreate->子created->子beforeMount->子mounted->父mounted
更新时
父beforeUpdate->子beforeUpdate->子updated->父updated
销毁时
父beforeDestroy->子beforeDestroy->子destroyed->父destroyed