【真香系列】Vue-Next 源码第四章

Demo

在第一篇文章中使用了下面这个 demo，其中用 setTimeout 模拟了一个更新，我们知道 Vue3 中使用 Proxy 代替了 Object.defineProperty 做响应拦截，在 Proxy setter 中会有一个 trigger 的触发，这个就是更新的入口。

const { reactive } = Vue
const App = {
  template: `<h1>{{state.message}}</h1>`,
  setup() {
    const state = reactive({ message: 'World' })
    setTimeout(() => {
      state.message = 1
    }, 3000)
    return {
      state
    }
  }
}
const app = Vue.createApp(App)
app.mount('#app')

trigger

// packages/reactivity/src/effect.ts
export function trigger(
  target: object,
  type: TriggerOpTypes,
  key?: unknown,
  newValue?: unknown,
  oldValue?: unknown,
  oldTarget?: Map<unknown, unknown> | Set<unknown>
) {
  const depsMap = targetMap.get(target)
  if (!depsMap) {
    // never been tracked
    return
  }

  const effects = new Set<ReactiveEffect>()
  const add = (effectsToAdd: Set<ReactiveEffect> | undefined) => {
    if (effectsToAdd) {
      effectsToAdd.forEach(effect => {
        if (effect !== activeEffect || effect.options.allowRecurse) {
          effects.add(effect)
        }
      })
    }
  }

  if (type === TriggerOpTypes.CLEAR) {
    // collection being cleared
    // trigger all effects for target
    depsMap.forEach(add)
  } else if (key === 'length' && isArray(target)) {
    depsMap.forEach((dep, key) => {
      if (key === 'length' || key >= (newValue as number)) {
        add(dep)
      }
    })
  } else {
    // schedule runs for SET | ADD | DELETE
    if (key !== void 0) {
      add(depsMap.get(key))
    }

    // also run for iteration key on ADD | DELETE | Map.SET
    switch (type) {
      case TriggerOpTypes.ADD:
        if (!isArray(target)) {
          add(depsMap.get(ITERATE_KEY))
          if (isMap(target)) {
            add(depsMap.get(MAP_KEY_ITERATE_KEY))
          }
        } else if (isIntegerKey(key)) {
          // new index added to array -> length changes
          add(depsMap.get('length'))
        }
        break
      case TriggerOpTypes.DELETE:
        if (!isArray(target)) {
          add(depsMap.get(ITERATE_KEY))
          if (isMap(target)) {
            add(depsMap.get(MAP_KEY_ITERATE_KEY))
          }
        }
        break
      case TriggerOpTypes.SET:
        if (isMap(target)) {
          add(depsMap.get(ITERATE_KEY))
        }
        break
    }
  }

  const run = (effect: ReactiveEffect) => {
    if (__DEV__ && effect.options.onTrigger) {
      effect.options.onTrigger({
        effect,
        target,
        key,
        type,
        newValue,
        oldValue,
        oldTarget
      })
    }
    if (effect.options.scheduler) {
      effect.options.scheduler(effect)
    } else {
      effect()
    }
  }

  effects.forEach(run)
}

我们先看下 targetMap 的结构

首先会先在 targetMap 里查找更新目标的依赖，然后声明 effects set，调用 add 方法去把所有更新目标相关的 value 取出来并放入 effects，value 就是前面文章里提到过的 reactiveEffect 函数，最后遍历 effects 去执行 scheduler(queueJob) 进入任务调度流程。

const run = (effect: ReactiveEffect) => {
  effect.options.scheduler(effect)
}

effects.forEach(run)

queueJob

queueJob 把 reactiveEffect 放入更新任务队列中。

export function queueJob(job: SchedulerJob) {
  // the dedupe search uses the startIndex argument of Array.includes()
  // by default the search index includes the current job that is being run
  // so it cannot recursively trigger itself again.
  // if the job is a watch() callback, the search will start with a +1 index to
  // allow it recursively trigger itself - it is the user's responsibility to
  // ensure it doesn't end up in an infinite loop.
  if (
    (!queue.length ||
      !queue.includes(
        job,
        isFlushing && job.allowRecurse ? flushIndex + 1 : flushIndex
      )) &&
    job !== currentPreFlushParentJob
  ) {
    queue.push(job)
    queueFlush()
  }
}

queueFlush

创建微任务，flushJobs 执行时会清除微任务队列。

// packages/runtime-core/src/scheduler.ts
function queueFlush() {
  if (!isFlushing && !isFlushPending) {
    isFlushPending = true
    currentFlushPromise = resolvedPromise.then(flushJobs)
  }
}

flushJobs

执行当前更新任务。

// packages/runtime-core/src/scheduler.ts
function flushJobs(seen?: CountMap) {
  isFlushPending = false
  isFlushing = true
  if (__DEV__) {
    seen = seen || new Map()
  }

  flushPreFlushCbs(seen)

  // Sort queue before flush.
  // This ensures that:
  // 1. Components are updated from parent to child. (because parent is always
  //    created before the child so its render effect will have smaller
  //    priority number)
  // 2. If a component is unmounted during a parent component's update,
  //    its update can be skipped.
  // Jobs can never be null before flush starts, since they are only invalidated
  // during execution of another flushed job.
  queue.sort((a, b) => getId(a!) - getId(b!))

  try {
    for (flushIndex = 0; flushIndex < queue.length; flushIndex++) {
      const job = queue[flushIndex]
      if (job) {
        if (__DEV__) {
          checkRecursiveUpdates(seen!, job)
        }
        callWithErrorHandling(job, null, ErrorCodes.SCHEDULER)
      }
    }
  } finally {
    flushIndex = 0
    queue.length = 0

    flushPostFlushCbs(seen)

    isFlushing = false
    currentFlushPromise = null
    // some postFlushCb queued jobs!
    // keep flushing until it drains.
    if (queue.length || pendingPostFlushCbs.length) {
      flushJobs(seen)
    }
  }
}

callWithErrorHandling

执行 reactiveEffect，也就回到了第二篇文章里提到的 componentEffect，也就是更新时会执行的函数，完成页面渲染。

// packages/runtime-core/src/errorHandling.ts
export function callWithErrorHandling(
  fn: Function,
  instance: ComponentInternalInstance | null,
  type: ErrorTypes,
  args?: unknown[]
) {
  let res
  try {
    res = args ? fn(...args) : fn()
  } catch (err) {
    handleError(err, instance, type)
  }
  return res
}

总结

更新流程里最重要的部分是任务调度系统，在第五篇文章中会详细说明。至此初始化和更新流程先介绍到这里，后续我会反复更新第 2 - 4 篇文章，争取把更细节的地方补充进去。