core/packages/reactivity/src/computed.ts

import { ReactiveEffect } from './effect'
import { Ref, trackRefValue, triggerRefValue } from './ref'
import { isFunction, NOOP } from '@vue/shared'
import { ReactiveFlags, toRaw } from './reactive'

export interface ComputedRef<T = any> extends WritableComputedRef<T> {
  readonly value: T
  defer?: (fn: () => void) => void
}

export interface WritableComputedRef<T> extends Ref<T> {
  readonly effect: ReactiveEffect<T>
}

export type ComputedGetter<T> = (ctx?: any) => T
export type ComputedSetter<T> = (v: T) => void

export interface WritableComputedOptions<T> {
  get: ComputedGetter<T>
  set: ComputedSetter<T>
}

type ComputedScheduler = (fn: () => void) => void
let scheduler: ComputedScheduler | undefined

/**
 * Set a scheduler for deferring computed computations
 */
export const setComputedScheduler = (s: ComputedScheduler | undefined) => {
  scheduler = s
}

class ComputedRefImpl<T> {
  public dep?: Set<ReactiveEffect> = 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
  ) {
    let deferFn: () => void
    let scheduled = false
    this.effect = new ReactiveEffect(getter, () => {
      if (!this._dirty) {
        this._dirty = true
        if (scheduler) {
          if (!scheduled) {
            scheduled = true
            scheduler(
              deferFn ||
                (deferFn = () => {
                  scheduled = false
                  if (this._dirty) {
                    this._dirty = false
                    const newValue = this.effect.run()!
                    if (this._value !== newValue) {
                      this._value = newValue
                      triggerRefValue(this)
                    }
                  } else {
                    triggerRefValue(this)
                  }
                })
            )
          }
        } else {
          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)
    if (self._dirty) {
      self._value = self.effect.run()!
      self._dirty = false
    }
    trackRefValue(self)
    return self._value
  }

  set value(newValue: T) {
    this._setter(newValue)
  }
}

export function computed<T>(getter: ComputedGetter<T>): ComputedRef<T>
export function computed<T>(
  options: WritableComputedOptions<T>
): WritableComputedRef<T>
export function computed<T>(
  getterOrOptions: ComputedGetter<T> | WritableComputedOptions<T>
) {
  let getter: ComputedGetter<T>
  let setter: ComputedSetter<T>

  if (isFunction(getterOrOptions)) {
    getter = getterOrOptions
    setter = __DEV__
      ? () => {
          console.warn('Write operation failed: computed value is readonly')
        }
      : NOOP
  } else {
    getter = getterOrOptions.get
    setter = getterOrOptions.set
  }

  return new ComputedRefImpl(
    getter,
    setter,
    isFunction(getterOrOptions) || !getterOrOptions.set
  ) as any
}