Vue3计算属性和异步计算属性方式

export type ComputedGetter<T> = (...args: any[]) => T
export type ComputedSetter<T> = (v: T) => void
export interface WritableComputedOptions<T> {
  get: ComputedGetter<T>
  set: ComputedSetter<T>
}
class ComputedRefImpl<T> {
  public dep?: Dep = undefined
  private _value!: T
  private _dirty = true
  public readonly effect: ReactiveEffect<T>
  public readonly __v_isRef = true
  public readonly [ReactiveFlags.IS_READONLY]: boolean
  constructor(
    getter: ComputedGetter<T>,
    private readonly _setter: ComputedSetter<T>,
    isReadonly: boolean
  ) {
    //内部存储一个ReactiveEffect对象
    this.effect = new ReactiveEffect(getter, () => {
      if (!this._dirty) {
        //标记下次读取将重新计算值
        this._dirty = true
        //触发依赖更新
        triggerRefValue(this)
      }
    })
    this[ReactiveFlags.IS_READONLY] = isReadonly
  }
  get value() {
    // the computed ref may get wrapped by other proxies e.g. readonly() #3376
    const self = toRaw(this)
    //收集依赖
    trackRefValue(self)
    //是否重新计算标记
    if (self._dirty) {
      //重新计算
      self._dirty = false
      self._value = self.effect.run()!
    }
    //直接获取计算好的值
    return self._value
  }
  set value(newValue: T) {
    this._setter(newValue)
  }
}
export function computed<T>(
  getterOrOptions: ComputedGetter<T> | WritableComputedOptions<T>,
  debugOptions?: DebuggerOptions
) {
  let getter: ComputedGetter<T>
  let setter: ComputedSetter<T>
  const onlyGetter = isFunction(getterOrOptions)
  if (onlyGetter) {
    getter = getterOrOptions
    setter = __DEV__
      ? () => {
          console.warn('Write operation failed: computed value is readonly')
        }
      : NOOP
  } else {
    getter = getterOrOptions.get
    setter = getterOrOptions.set
  }
  //只需要关注这儿
  const cRef = new ComputedRefImpl(getter, setter, onlyGetter || !setter)
  if (__DEV__ && debugOptions) {
    cRef.effect.onTrack = debugOptions.onTrack
    cRef.effect.onTrigger = debugOptions.onTrigger
  }
  return cRef as any
}

export function triggerEffects(
  dep: Dep | ReactiveEffect[],
  debuggerEventExtraInfo?: DebuggerEventExtraInfo
) {
  // spread into array for stabilization
  for (const effect of isArray(dep) ? dep : [...dep]) {
    if (effect !== activeEffect || effect.allowRecurse) {
      if (__DEV__ && effect.onTrigger) {
        effect.onTrigger(extend({ effect }, debuggerEventExtraInfo))
      }
      if (effect.scheduler) {
        effect.scheduler()
      } else {
        effect.run()
      }
    }
  }
}

export class ReactiveEffect<T = any> {
  active = true
  deps: Dep[] = []
  // can be attached after creation
  computed?: boolean
  allowRecurse?: boolean
  onStop?: () => void
  // dev only
  onTrack?: (event: DebuggerEvent) => void
  // dev only
  onTrigger?: (event: DebuggerEvent) => void
  constructor(
    public fn: () => T,
    public scheduler: EffectScheduler | null = null,
    scope?: EffectScope | null
  ) {
    recordEffectScope(this, scope)
  }
}

const tick = Promise.resolve()
const queue: any[] = []
let queued = false
const scheduler = (fn: any) => {
  queue.push(fn)
  if (!queued) {
    queued = true
    tick.then(flush)
  }
}
const flush = () => {
  for (let i = 0; i < queue.length; i++) {
    queue[i]()
  }
  queue.length = 0
  queued = false
}
class DeferredComputedRefImpl<T> {
  public dep?: Dep = undefined
  private _value!: T
  private _dirty = true
  public readonly effect: ReactiveEffect<T>
  public readonly __v_isRef = true
  public readonly [ReactiveFlags.IS_READONLY] = true
  constructor(getter: ComputedGetter<T>) {
    let compareTarget: any
    let hasCompareTarget = false
    let scheduled = false
    this.effect = new ReactiveEffect(getter, (computedTrigger?: boolean) => {
      if (this.dep) {
        if (computedTrigger) {
          compareTarget = this._value
          hasCompareTarget = true
        } else if (!scheduled) {
          const valueToCompare = hasCompareTarget ? compareTarget : this._value
          scheduled = true
          hasCompareTarget = false
          //加入执行队列
          scheduler(() => {
            if (this.effect.active && this._get() !== valueToCompare) {
              triggerRefValue(this)
            }
            scheduled = false
          })
        }
        // chained upstream computeds are notified synchronously to ensure
        // value invalidation in case of sync access; normal effects are
        // deferred to be triggered in scheduler.
        for (const e of this.dep) {
          if (e.computed) {
            e.scheduler!(true /* computedTrigger */)
          }
        }
      }
      //保证异步方法获取值时是重新计算的。
      this._dirty = true
    })
    this.effect.computed = true
  }
  private _get() {
    if (this._dirty) {
      this._dirty = false
      return (this._value = this.effect.run()!)
    }
    return this._value
  }
  get value() {
    trackRefValue(this)
    // the computed ref may get wrapped by other proxies e.g. readonly() #3376
    return toRaw(this)._get()
  }
}
export function deferredComputed<T>(getter: () => T): ComputedRef<T> {
  return new DeferredComputedRefImpl(getter) as any
}

for (const e of this.dep) {
  if (e.computed) {
    e.scheduler!(true /* computedTrigger */)
  }
}

    const src = ref(0)
    const c1 = deferredComputed(() => {
      return src.value % 2
    })
    const c2 = deferredComputed(() => {
      return c1.value + 1
    })
    effect(() => {
      c2.value
    })
    src.value = 1
    //同步打印c2.value，输出2
    console.log(c2.value);

if (this.effect.active && this._get() !== valueToCompare) {
    triggerRefValue(this)
}