面试官问：Vue3 对比 Vue2 有哪些变化？

// vue3
<script setup>      
import { onMounted } from 'vue'
onMounted(() => {
 ...
})
// 可将不同的逻辑拆开成多个onMounted，依然按顺序执行，不被覆盖
onMounted(() => {
 ...
})
</script>
// vue2
<script>      
   export default {          
      mounted() {              
        ...          
      },            
  }
</script>

<template>
 <div>
   <header>...</header>
   <main>...</main>
   <footer>...</footer>
 </div>
</template>

<template>
 <header>...</header>
 <main>...</main>
 <footer>...</footer>
</template>

<tempalte>
  <suspense>
    <template #default>
      <todo-list />
    </template>
    <template #fallback>
      <div>
        Loading...
      </div>
    </template>
  </suspense>
</template>

<button @click="dialogVisible = true">点击</button>
<teleport to="body">
  <div class="dialog" v-if="dialogVisible">
  </div>
</teleport>

// main.vue
<template>
 <span>mouse position {{x}} {{y}}</span>
</template>
<script setup>
import { ref } from 'vue'
import useMousePosition from './useMousePosition'
const {x, y} = useMousePosition()
}
</script>

// useMousePosition.js
import { ref, onMounted, onUnmounted } from 'vue'
function useMousePosition() {
 let x = ref(0)
 let y = ref(0)
 function update(e) {
   x.value = e.pageX
   y.value = e.pageY
 }
 onMounted(() => {
   window.addEventListener('mousemove', update)
 })
 onUnmounted(() => {
   window.removeEventListener('mousemove', update)
 })
 return {
   x,
   y
 }
}
</script>

let obj = {}
let name = '瑾行'
Object.defineProperty(obj, 'name', {
 enumerable: true, // 可枚举（是否可通过for...in 或 Object.keys()进行访问）
 configurable: true, // 可配置（是否可使用delete删除，是否可再次设置属性）
 // value: '', // 任意类型的值，默认undefined
 // writable: true, // 可重写
 get: function() {
   return name
 },
 set: function(value) {
   name = value
 }
})

function defineReactive(obj, key, val) {
 // 一 key 一个 dep
 const dep = new Dep()
 // 获取 key 的属性描述符，发现它是不可配置对象的话直接 return
 const property = Object.getOwnPropertyDescriptor(obj, key)
 if (property && property.configurable === false) { return }
 // 获取 getter 和 setter，并获取 val 值
 const getter = property && property.get
 const setter = property && property.set
 if((!getter || setter) && arguments.length === 2) { val = obj[key] }
 // 递归处理，保证对象中所有 key 被观察
 let childOb = observe(val)
 Object.defineProperty(obj, key, {
   enumerable: true,
   configurable: true,
   // get 劫持 obj[key] 的 进行依赖收集
   get: function reactiveGetter() {
     const value = getter ? getter.call(obj) : val
     if(Dep.target) {
       // 依赖收集
       dep.depend()
       if(childOb) {
         // 针对嵌套对象，依赖收集
         childOb.dep.depend()
         // 触发数组响应式
         if(Array.isArray(value)) {
           dependArray(value)
         }
       }
     }
   }
   return value
 })
 // set 派发更新 obj[key]
 set: function reactiveSetter(newVal) {
   ...
   if(setter) {
     setter.call(obj, newVal)
   } else {
     val = newVal
   }
   // 新值设置响应式
   childOb = observe(val)
   // 依赖通知更新
   dep.notify()
 }
}

const handler = {
 get: function(obj, prop) {
   return prop in obj ? obj[prop] : ''
 },
 set: function() {},
 ...
}

function createReactiveObject(target, isReadOnly, baseHandlers, collectionHandlers, proxyMap) {
 ...
 // collectionHandlers: 处理Map、Set、WeakMap、WeakSet
 // baseHandlers: 处理数组、对象
 const proxy = new Proxy(
   target,
   targetType === TargetType.COLLECTION ? collectionHandlers : baseHandlers
 )
 proxyMap.set(target, proxy)
 return proxy
}

// 依赖收集
function createGetter(isReadonly = false, shallow = false) {
 return function get(target: Target, key: string | symbol, receiver: object) {
   ...
   // 数组类型
   const targetIsArray = isArray(target)
   if (!isReadonly && targetIsArray && hasOwn(arrayInstrumentations, key)) {
     return Reflect.get(arrayInstrumentations, key, receiver)
   }
   // 非数组类型
   const res = Reflect.get(target, key, receiver);
   // 对象递归调用
   if (isObject(res)) {
     return isReadonly ? readonly(res) : reactive(res)
   }
   return res
 }
}
// 派发更新
function createSetter() {
 return function set(target: Target, key: string | symbol, value: unknown, receiver: Object) {
   value = toRaw(value)
   oldValue = target[key]
   // 因 ref 数据在 set value 时就已 trigger 依赖了，所以直接赋值 return 即可
   if (!isArray(target) && isRef(oldValue) && !isRef(value)) {
     oldValue.value = value
     return true
   }
   // 对象是否有 key 有 key set，无 key add
   const hadKey = hasOwn(target, key)
   const result = Reflect.set(target, key, value, receiver)
   if (target === toRaw(receiver)) {
     if (!hadKey) {
       trigger(target, TriggerOpTypes.ADD, key, value)
     } else if (hasChanged(value, oldValue)) {
       trigger(target, TriggerOpTypes.SET, key, value, oldValue)
     }
   }
   return result
 }
}

<div id="app">
 <h1>技术摸鱼</h1>
 <p>今天天气真不错</p>
 <div>{{name}}</div>
</div>

import { createElementVNode as _createElementVNode, toDisplayString as _toDisplayString, openBlock as _openBlock, createElementBlock as _createElementBlock, pushScopeId as _pushScopeId, popScopeId as _popScopeId } from "vue"
const _withScopeId = n => (_pushScopeId("scope-id"),n=n(),_popScopeId(),n)
const _hoisted_1 = { id: "app" }
const _hoisted_2 = /*#__PURE__*/ _withScopeId(() => /*#__PURE__*/_createElementVNode("h1", null, "技术摸鱼", -1 /* HOISTED */))
const _hoisted_3 = /*#__PURE__*/ _withScopeId(() => /*#__PURE__*/_createElementVNode("p", null, "今天天气真不错", -1 /* HOISTED */))
export function render(_ctx, _cache, $props, $setup, $data, $options) {
 return (_openBlock(), _createElementBlock("div", _hoisted_1, [
   _hoisted_2,
   _hoisted_3,
   _createElementVNode("div", null, _toDisplayString(_ctx.name), 1 /* TEXT */)
 ]))
}

export const enum PatchFlags {  
 TEXT = 1, // 动态文本内容
 CLASS = 1 << 1, // 动态类名
 STYLE = 1 << 2, // 动态样式
 PROPS = 1 << 3, // 动态属性，不包含类名和样式
 FULL_PROPS = 1 << 4, // 具有动态 key 属性，当 key 改变，需要进行完整的 diff 比较
 HYDRATE_EVENTS = 1 << 5, // 带有监听事件的节点
 STABLE_FRAGMENT = 1 << 6, // 不会改变子节点顺序的 fragment
 KEYED_FRAGMENT = 1 << 7, // 带有 key 属性的 fragment 或部分子节点
 UNKEYED_FRAGMENT = 1 << 8,  // 子节点没有 key 的fragment
 NEED_PATCH = 1 << 9, // 只会进行非 props 的比较
 DYNAMIC_SLOTS = 1 << 10, // 动态的插槽
 HOISTED = -1,  // 静态节点，diff阶段忽略其子节点
 BAIL = -2 // 代表 diff 应该结束
}

<div id="app">
 <h1>技术摸鱼</h1>
 <p>今天天气真不错</p>
 <div>{{name}}</div>
 <span onCLick="() => {}"><span>
</div>

import { createElementVNode as _createElementVNode, toDisplayString as _toDisplayString, openBlock as _openBlock, createElementBlock as _createElementBlock, pushScopeId as _pushScopeId, popScopeId as _popScopeId } from "vue"
const _withScopeId = n => (_pushScopeId("scope-id"),n=n(),_popScopeId(),n)
const _hoisted_1 = { id: "app" }
const _hoisted_2 = /*#__PURE__*/ _withScopeId(() => /*#__PURE__*/_createElementVNode("h1", null, "技术摸鱼", -1 /* HOISTED */))
const _hoisted_3 = /*#__PURE__*/ _withScopeId(() => /*#__PURE__*/_createElementVNode("p", null, "今天天气真不错", -1 /* HOISTED */))
const _hoisted_4 = /*#__PURE__*/ _withScopeId(() => /*#__PURE__*/_createElementVNode("span", { onCLick: "() => {}" }, [
 /*#__PURE__*/_createElementVNode("span")
], -1 /* HOISTED */))
export function render(_ctx, _cache, $props, $setup, $data, $options) {
 return (_openBlock(), _createElementBlock("div", _hoisted_1, [
   _hoisted_2,
   _hoisted_3,
   _createElementVNode("div", null, _toDisplayString(_ctx.name), 1 /* TEXT */),
   _hoisted_4
 ]))
}

function patchChildren(n1, n2, container, parentAnchor, parentComponent, parentSuspense, isSVG, optimized) {
 // 获取新老孩子节点
 const c1 = n1 && n1.children
 const c2 = n2.children
 const prevShapeFlag = n1 ? n1.shapeFlag : 0
 const { patchFlag, shapeFlag } = n2
  // 处理 patchFlag 大于 0  
 if(patchFlag > 0) {
   if(patchFlag && PatchFlags.KEYED_FRAGMENT) {
     // 存在 key
     patchKeyedChildren()
     return
   } els if(patchFlag && PatchFlags.UNKEYED_FRAGMENT) {
     // 不存在 key
     patchUnkeyedChildren()
     return
   }
 }
 // 匹配是文本节点（静态）：移除老节点，设置文本节点
 if(shapeFlag && ShapeFlags.TEXT_CHILDREN) {
   if (prevShapeFlag & ShapeFlags.ARRAY_CHILDREN) {
     unmountChildren(c1 as VNode[], parentComponent, parentSuspense)
   }
   if (c2 !== c1) {
     hostSetElementText(container, c2 as string)
   }
 } else {
   // 匹配新老 Vnode 是数组，则全量比较；否则移除当前所有的节点
   if (prevShapeFlag & ShapeFlags.ARRAY_CHILDREN) {
     if (shapeFlag & ShapeFlags.ARRAY_CHILDREN) {
       patchKeyedChildren(c1, c2, container, anchor, parentComponent, parentSuspense,...)
     } else {
       unmountChildren(c1 as VNode[], parentComponent, parentSuspense, true)
     }
   } else {
     if(prevShapeFlag & ShapeFlags.TEXT_CHILDREN) {
       hostSetElementText(container, '')
      }  
     if (shapeFlag & ShapeFlags.ARRAY_CHILDREN) {
       mountChildren(c2 as VNodeArrayChildren, container,anchor,parentComponent,...)
     }
   }
 }
}

function patchUnkeyedChildren(c1, c2, container, parentAnchor, parentComponent, parentSuspense, isSVG, optimized) {
 c1 = c1 || EMPTY_ARR
 c2 = c2 || EMPTY_ARR
 const oldLength = c1.length
 const newLength = c2.length
 const commonLength = Math.min(oldLength, newLength)
 let i
 for(i = 0; i < commonLength; i++) {
   // 如果新 Vnode 已经挂载，则直接 clone 一份，否则新建一个节点
   const nextChild = (c2[i] = optimized ? cloneIfMounted(c2[i] as Vnode)) : normalizeVnode(c2[i])
   patch()
 }
 if(oldLength > newLength) {
   // 移除多余的节点
   unmountedChildren()
 } else {
   // 创建新的节点
   mountChildren()
 }

}

function patchKeyedChildren(c1, c2, container, parentAnchor, parentComponent, parentSuspense, isSVG, optimized) {
 let i = 0;
 const e1 = c1.length - 1
 const e2 = c2.length - 1
 const l2 = c2.length
  // 从头开始遍历，若新老节点是同一节点，执行 patch 更新差异；否则，跳出循环  
 while(i <= e1 && i <= e2) {
   const n1 = c1[i]
   const n2 = c2[i]

   if(isSameVnodeType) {
     patch(n1, n2, container, parentAnchor, parentComponent, parentSuspense, isSvg, optimized)
   } else {
     break
   }
   i++
 }
  // 从尾开始遍历，若新老节点是同一节点，执行 patch 更新差异；否则，跳出循环  
 while(i <= e1 && i <= e2) {
   const n1 = c1[e1]
   const n2 = c2[e2]
   if(isSameVnodeType) {
     patch(n1, n2, container, parentAnchor, parentComponent, parentSuspense, isSvg, optimized)
   } else {
     break
   }
   e1--
   e2--
 }

 // 仅存在需要新增的节点
 if(i > e1) {    
   if(i <= e2) {
     const nextPos = e2 + 1
     const anchor = nextPos < l2 ? c2[nextPos] : parentAnchor
     while(i <= e2) {
       patch(null, c2[i], container, parentAnchor, parentComponent, parentSuspense, isSvg, optimized)
     }
   }
 }
 // 仅存在需要删除的节点
 else if(i > e2) {
   while(i <= e1) {
     unmount(c1[i], parentComponent, parentSuspense, true)    
   }
 }
 // 新旧节点均未遍历完
 // [i ... e1 + 1]: a b [c d e] f g
 // [i ... e2 + 1]: a b [e d c h] f g
 // i = 2, e1 = 4, e2 = 5
 else {
   const s1 = i
   const s2 = i
   // 缓存新 Vnode 剩余节点 上例即{e: 2, d: 3, c: 4, h: 5}
   const keyToNewIndexMap = new Map()
   for (i = s2; i <= e2; i++) {
     const nextChild = (c2[i] = optimized
         ? cloneIfMounted(c2[i] as VNode)
         : normalizeVNode(c2[i]))

     if (nextChild.key != null) {
       if (__DEV__ && keyToNewIndexMap.has(nextChild.key)) {
         warn(
           `Duplicate keys found during update:`,
            JSON.stringify(nextChild.key),
           `Make sure keys are unique.`
         )
       }
       keyToNewIndexMap.set(nextChild.key, i)
     }
   }
 }
 let j = 0
 // 记录即将 patch 的 新 Vnode 数量
 let patched = 0
 // 新 Vnode 剩余节点长度
 const toBePatched = e2 - s2 + 1
 // 是否移动标识
 let moved = false
 let maxNewindexSoFar = 0
 // 初始化 新老节点的对应关系（用于后续最大递增序列算法）
 const newIndexToOldIndexMap = new Array(toBePatched)
 for (i = 0; i < toBePatched; i++) newIndexToOldIndexMap[i] = 0

 // 遍历老 Vnode 剩余节点
 for (i = s1; i <= e1; i++) {
   const prevChild = c1[i]
   // 代表当前新 Vnode 都已patch，剩余旧 Vnode 移除即可
   if (patched >= toBePatched) {
     unmount(prevChild, parentComponent, parentSuspense, true)
     continue
   }
   let newIndex
   // 旧 Vnode 存在 key，则从 keyToNewIndexMap 获取
   if (prevChild.key != null) {
     newIndex = keyToNewIndexMap.get(prevChild.key)
   // 旧 Vnode 不存在 key，则遍历新 Vnode 获取
   } else {
     for (j = s2; j <= e2; j++) {
       if (newIndexToOldIndexMap[j - s2] === 0 && isSameVNodeType(prevChild, c2[j] as VNode)){
          newIndex = j
          break
       }
      }            
  }
  // 删除、更新节点
  // 新 Vnode 没有 当前节点，移除
  if (newIndex === undefined) {
    unmount(prevChild, parentComponent, parentSuspense, true)
  } else {
    // 旧 Vnode 的下标位置 + 1，存储到对应 新 Vnode 的 Map 中
    // + 1 处理是为了防止数组首位下标是 0 的情况，因为这里的 0 代表需创建新节点
    newIndexToOldIndexMap[newIndex - s2] = i + 1
    // 若不是连续递增，则代表需要移动
    if (newIndex >= maxNewIndexSoFar) {
      maxNewIndexSoFar = newIndex
    } else {
      moved = true
    }
    patch(prevChild,c2[newIndex],...)
    patched++
  }
 }
 // 遍历结束，newIndexToOldIndexMap = {0:5, 1:4, 2:3, 3:0}
 // 新建、移动节点
 const increasingNewIndexSequence = moved
 // 获取最长递增序列
 ? getSequence(newIndexToOldIndexMap)
 : EMPTY_ARR
 j = increasingNewIndexSequence.length - 1
 for (i = toBePatched - 1; i >= 0; i--) {
   const nextIndex = s2 + i
   const nextChild = c2[nextIndex] as VNode
   const anchor = extIndex + 1 < l2 ? (c2[nextIndex + 1] as VNode).el : parentAnchor
   // 0 新建 Vnode
   if (newIndexToOldIndexMap[i] === 0) {
     patch(null,nextChild,...)
   } else if (moved) {
     // 移动节点
     if (j < 0 || i !== increasingNewIndexSequence[j]) {
       move(nextChild, container, anchor, MoveType.REORDER)
     } else {
       j--
     }
   }
 }
}

import Vue from 'vue'
Vue.nextTick(() => {
 // 一些和DOM有关的东西
})

import { nextTick } from 'vue'
nextTick(() => {
 // 一些和DOM有关的东西
})

// 自定义runtime-render函数
import { createApp } from './runtime-render'
import App from './src/App'
createApp(App).mount('#app')