6，组合式Api-setup

解决： 提取逻辑关注点

setup 的调用发生在 data property、computed property 或 methods 被解析之前

setup(){}

入参： props, contentxt(attrs, slots, emit, expose)

内容：

生命周期钩子

onBeforeMount
onMounted
...
onDeactivated

watch

computed

provide/Ieject

eg:  provide('location', 'North Pole')
     inject('location', 'The Universe')
     

模板引用

    <div ref="root">This is a root element</div>
    const root = ref(null)
    onMounted(() => {
      // DOM 元素将在初始渲染后分配给 ref
      console.log(root.value) // <div>This is a root element</div>
    })

return