feat(reactivity): deferredComputed

Note: this is not exposed as part of Vue API, only as
a lower-level API specific to @vue/reactivity

packages/reactivity/src/deferredComputed.ts

@@ -0,0 +1,88 @@
+import { Dep } from './dep'
+import { ReactiveEffect } from './effect'
+import { ComputedGetter, ComputedRef } from './computed'
+import { ReactiveFlags, toRaw } from './reactive'
+import { trackRefValue, triggerRefValue } from './ref'
+
+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
+          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 */)
+          }
+        }
+      }
+      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
+}

packages/reactivity/src/index.ts

@@ -36,6 +36,7 @@ export {
   ComputedGetter,
   ComputedSetter
 } from './computed'
+export { deferredComputed } from './deferredComputed'
 export {
   effect,
   stop,