深入了解Vue3中侦听器watcher的实现原理

export default { 
  watch: { 
    a(newVal, oldVal) { 
      console.log('new: %s,00 old: %s', newVal, oldVal) 
    } 
  } 
} 

const unwatch = vm.$watch('a', function(newVal, oldVal) { 
  console.log('new: %s, old: %s', newVal, oldVal) 
}) 

import { reactive, watch } from 'vue' 
const state = reactive({ count: 0 }) 
watch(() => state.count, (count, prevCount) => { 
  // 当 state.count 更新，会触发此回调函数 
}) 

import { ref, watch } from 'vue' 
const count = ref(0) 
watch(count, (count, prevCount) => { 
  // 当 count.value 更新，会触发此回调函数 
}) 

import { ref, watch } from 'vue' 
const count = ref(0) 
const count2 = ref(1) 
watch([count, count2], ([count, count2], [prevCount, prevCount2]) => { 
  // 当 count.value 或者 count2.value 更新，会触发此回调函数 
}) 

function watch(source, cb, options) { 
  if ((process.env.NODE_ENV !== 'production') && !isFunction(cb)) { 
    warn(``watch(fn, options?)` signature has been moved to a separate API. ` + 
      `Use `watchEffect(fn, options?)` instead. `watch` now only ` + 
      `supports `watch(source, cb, options?) signature.`) 
  } 
  return doWatch(source, cb, options) 
} 
function doWatch(source, cb, { immediate, deep, flush, onTrack, onTrigger } = EMPTY_OBJ) { 
  // 标准化 source 
  // 构造 applyCb 回调函数 
  // 创建 scheduler 时序执行函数 
  // 创建 effect 副作用函数 
  // 返回侦听器销毁函数 
}    

// source 不合法的时候会报警告 
const warnInvalidSource = (s) => { 
  warn(`Invalid watch source: `, s, `A watch source can only be a getter/effect function, a ref, ` + 
    `a reactive object, or an array of these types.`) 
} 
// 当前组件实例 
const instance = currentInstance 
let getter 
if (isArray(source)) { 
  getter = () => source.map(s => { 
    if (isRef(s)) { 
      return s.value 
    } 
    else if (isReactive(s)) { 
      return traverse(s) 
    } 
    else if (isFunction(s)) { 
      return callWithErrorHandling(s, instance, 2 /* WATCH_GETTER */) 
    } 
    else { 
      (process.env.NODE_ENV !== 'production') && warnInvalidSource(s) 
    } 
  }) 
} 
else if (isRef(source)) { 
  getter = () => source.value 
} 
else if (isReactive(source)) { 
  getter = () => source 
  deep = true 
} 
else if (isFunction(source)) { 
  if (cb) { 
    // getter with cb 
    getter = () => callWithErrorHandling(source, instance, 2 /* WATCH_GETTER */) 
  } 
  else { 
    // watchEffect 的逻辑 
  } 
} 
else { 
  getter = NOOP 
  (process.env.NODE_ENV !== 'production') && warnInvalidSource(source) 
} 
if (cb && deep) { 
  const baseGetter = getter 
  getter = () => traverse(baseGetter()) 
} 

import { reactive, watch } from 'vue' 
const state = reactive({ 
  count: { 
    a: { 
      b: 1 
    } 
  } 
}) 
watch(state.count, (count, prevCount) => { 
  console.log(count) 
}) 
state.count.a.b = 2  

watch(state.count.a, (newVal, oldVal) => { 
  console.log(newVal) 
}) 
state.count.a.b = 2 

watch(() => state.count.a.b, (newVal, oldVal) => { 
  console.log(newVal) 
}) 
state.count.a.b = 2 

let cleanup 
// 注册无效回调函数 
const onInvalidate = (fn) => { 
  cleanup = runner.options.onStop = () => { 
    callWithErrorHandling(fn, instance, 4 /* WATCH_CLEANUP */) 
  } 
} 
// 旧值初始值 
let oldValue = isArray(source) ? [] : INITIAL_WATCHER_VALUE /*{}*/ 
// 回调函数 
const applyCb = cb 
  ? () => { 
    // 组件销毁，则直接返回 
    if (instance && instance.isUnmounted) { 
      return 
    } 
    // 求得新值 
    const newValue = runner() 
    if (deep || hasChanged(newValue, oldValue)) { 
      // 执行清理函数 
      if (cleanup) { 
        cleanup() 
      } 
      callWithAsyncErrorHandling(cb, instance, 3 /* WATCH_CALLBACK */, [ 
        newValue, 
        // 第一次更改时传递旧值为 undefined 
        oldValue === INITIAL_WATCHER_VALUE ? undefined : oldValue, 
        onInvalidate 
      ]) 
      // 更新旧值 
      oldValue = newValue 
    } 
  } 
  : void 0 

const invoke = (fn) => fn() 
let scheduler 
if (flush === 'sync') { 
  // 同步 
  scheduler = invoke 
} 
else if (flush === 'pre') { 
  scheduler = job => { 
    if (!instance || instance.isMounted) { 
      // 进入异步队列，组件更新前执行 
      queueJob(job) 
    } 
    else { 
      // 如果组件还没挂载，则同步执行确保在组件挂载前 
      job() 
    } 
  } 
} 
else { 
  // 进入异步队列，组件更新后执行 
  scheduler = job => queuePostRenderEffect(job, instance && instance.suspense) 
} 

const runner = effect(getter, { 
  // 延时执行 
  lazy: true, 
  // computed effect 可以优先于普通的 effect 先运行，比如组件渲染的 effect 
  computed: true, 
  onTrack, 
  onTrigger, 
  scheduler: applyCb ? () => scheduler(applyCb) : scheduler 
}) 
// 在组件实例中记录这个 effect 
recordInstanceBoundEffect(runner) 
// 初次执行 
if (applyCb) { 
  if (immediate) { 
    applyCb() 
  } 
  else { 
    // 求旧值 
    oldValue = runner() 
  } 
} 
else { 
  // 没有 cb 的情况 
  runner() 
} 

return () => { 
  stop(runner) 
  if (instance) { 
    // 移除组件 effects 对这个 runner 的引用 
    remove(instance.effects, runner) 
  } 
} 
function stop(effect) { 
  if (effect.active) { 
    cleanup(effect) 
    if (effect.options.onStop) { 
      effect.options.onStop() 
    } 
    effect.active = false 
  } 
} 

import { reactive, watch } from 'vue' 
const state = reactive({ count: 0 }) 
watch(() => state.count, (count, prevCount) => { 
  console.log(count) 
}) 
state.count++ 
state.count++ 
state.count++ 

// 异步任务队列 
const queue = [] 
// 队列任务执行完后执行的回调函数队列 
const postFlushCbs = [] 
function queueJob(job) { 
  if (!queue.includes(job)) { 
    queue.push(job) 
    queueFlush() 
  } 
} 
function queuePostFlushCb(cb) { 
  if (!isArray(cb)) { 
    postFlushCbs.push(cb) 
  } 
  else { 
    // 如果是数组，把它拍平成一维 
    postFlushCbs.push(...cb) 
  } 
  queueFlush() 
} 

const p = Promise.resolve() 
// 异步任务队列是否正在执行 
let isFlushing = false 
// 异步任务队列是否等待执行 
let isFlushPending = false 
function nextTick(fn) { 
  return fn ? p.then(fn) : p 
} 
function queueFlush() { 
  if (!isFlushing && !isFlushPending) { 
    isFlushPending = true 
    nextTick(flushJobs) 
  } 
} 

const getId = (job) => (job.id == null ? Infinity : job.id) 
function flushJobs(seen) { 
  isFlushPending = false 
  isFlushing = true 
  let job 
  if ((process.env.NODE_ENV !== 'production')) { 
    seen = seen || new Map() 
  } 
  // 组件的更新是先父后子 
  // 如果一个组件在父组件更新过程中卸载，它自身的更新应该被跳过 
  queue.sort((a, b) => getId(a) - getId(b)) 
  while ((job = queue.shift()) !== undefined) { 
    if (job === null) { 
      continue 
    } 
    if ((process.env.NODE_ENV !== 'production')) { 
      checkRecursiveUpdates(seen, job) 
    } 
    callWithErrorHandling(job, null, 14 /* SCHEDULER */) 
  } 
  flushPostFlushCbs(seen) 
  isFlushing = false 
  // 一些 postFlushCb 执行过程中会再次添加异步任务，递归 flushJobs 会把它们都执行完毕 
  if (queue.length || postFlushCbs.length) { 
    flushJobs(seen) 
  } 
} 

function flushPostFlushCbs(seen) { 
  if (postFlushCbs.length) { 
    // 拷贝副本 
    const cbs = [...new Set(postFlushCbs)] 
    postFlushCbs.length = 0 
    if ((process.env.NODE_ENV !== 'production')) { 
      seen = seen || new Map() 
    } 
    for (let i = 0; i < cbs.length; i++) { 
      if ((process.env.NODE_ENV !== 'production')) {                                                       
        checkRecursiveUpdates(seen, cbs[i]) 
      } 
      cbs[i]() 
    } 
  } 
} 

import { reactive, watch } from 'vue' 
const state = reactive({ count: 0 }) 
watch(() => state.count, (count, prevCount) => { 
  state.count++ 
  console.log(count) 
}) 
state.count++ 

const RECURSION_LIMIT = 100 
function checkRecursiveUpdates(seen, fn) { 
  if (!seen.has(fn)) { 
    seen.set(fn, 1) 
  } 
  else { 
    const count = seen.get(fn) 
    if (count > RECURSION_LIMIT) { 
      throw new Error('Maximum recursive updates exceeded. ' + 
        "You may have code that is mutating state in your component's " + 
        'render function or updated hook or watcher source function.') 
    } 
    else { 
      seen.set(fn, count + 1) 
    } 
  } 
} 

function queueFlush() { 
  if (!isFlushing) { 
    isFlushing = true 
    nextTick(flushJobs) 
  } 
} 
function flushJobs(seen) { 
  let job 
  if ((process.env.NODE_ENV !== 'production')) { 
    seen = seen || new Map() 
  } 
  queue.sort((a, b) => getId(a) - getId(b)) 
  while ((job = queue.shift()) !== undefined) { 
    if (job === null) { 
      continue 
    } 
    if ((process.env.NODE_ENV !== 'production')) { 
      checkRecursiveUpdates(seen, job) 
    } 
    callWithErrorHandling(job, null, 14 /* SCHEDULER */) 
  } 
  flushPostFlushCbs(seen) 
  if (queue.length || postFlushCbs.length) { 
    flushJobs(seen) 
  } 
  isFlushing = false 
} 

import { ref, watchEffect } from 'vue' 
const count = ref(0) 
watchEffect(() => console.log(count.value)) 
count.value++ 

function watchEffect(effect, options) { 
  return doWatch(effect, null, options); 
} 
function doWatch(source, cb, { immediate, deep, flush, onTrack, onTrigger } = EMPTY_OBJ) { 
  instance = currentInstance; 
  let getter; 
  if (isFunction(source)) { 
    getter = () => { 
      if (instance && instance.isUnmounted) { 
        return; 
      } 
       // 执行清理函数 
      if (cleanup) { 
        cleanup(); 
      } 
      // 执行 source 函数，传入 onInvalidate 作为参数 
      return callWithErrorHandling(source, instance, 3 /* WATCH_CALLBACK */, [onInvalidate]); 
    }; 
  } 
  let cleanup; 
  const onInvalidate = (fn) => { 
    cleanup = runner.options.onStop = () => { 
      callWithErrorHandling(fn, instance, 4 /* WATCH_CLEANUP */); 
    }; 
  }; 
  let scheduler; 
  // 创建 scheduler 
  if (flush === 'sync') { 
    scheduler = invoke; 
  } 
  else if (flush === 'pre') { 
    scheduler = job => { 
      if (!instance || instance.isMounted) { 
        queueJob(job); 
      } 
      else { 
        job(); 
      } 
    }; 
  } 
  else { 
    scheduler = job => queuePostRenderEffect(job, instance && instance.suspense); 
  } 
  // 创建 runner 
  const runner = effect(getter, { 
    lazy: true, 
    computed: true, 
    onTrack, 
    onTrigger, 
    scheduler 
  }); 
  recordInstanceBoundEffect(runner); 
   
  // 立即执行 runner 
  runner(); 
   
  // 返回销毁函数 
  return () => { 
    stop(runner); 
    if (instance) { 
      remove(instance.effects, runner); 
    } 
  }; 
} 

import {ref, watchEffect } from 'vue' 
const id = ref(0) 
watchEffect(onInvalidate => { 
  // 执行异步操作 
  const token = performAsyncOperation(id.value) 
  onInvalidate(() => { 
    // 如果 id 发生变化或者 watcher 停止了，则执行逻辑取消前面的异步操作 
    token.cancel() 
  }) 
}) 

const onInvalidate = (fn) => { 
  cleanup = runner.options.onStop = () => { 
    callWithErrorHandling(fn, instance, 4 /* WATCH_CLEANUP */); 
  }; 
};