feat(reactivity): add support for customRef API

packages/reactivity/src/index.ts

@@ -1,4 +1,13 @@
-export { ref, unref, shallowRef, isRef, toRefs, Ref, UnwrapRef } from './ref'
+export {
+  ref,
+  unref,
+  shallowRef,
+  isRef,
+  toRefs,
+  customRef,
+  Ref,
+  UnwrapRef
+} from './ref'
 export {
   reactive,
   isReactive,

packages/reactivity/src/ref.ts

@@ -70,6 +70,31 @@ export function unref<T>(ref: T): T extends Ref<infer V> ? V : T {
   return isRef(ref) ? (ref.value as any) : ref
 }
 
+export type CustomRefFactory<T> = (
+  track: () => void,
+  trigger: () => void
+) => {
+  get: () => T
+  set: (value: T) => void
+}
+
+export function customRef<T>(factory: CustomRefFactory<T>): Ref<T> {
+  const { get, set } = factory(
+    () => track(r, TrackOpTypes.GET, 'value'),
+    () => trigger(r, TriggerOpTypes.SET, 'value')
+  )
+  const r = {
+    _isRef: true,
+    get value() {
+      return get()
+    },
+    set value(v) {
+      set(v)
+    }
+  }
+  return r as any
+}
+
 export function toRefs<T extends object>(
   object: T
 ): { [K in keyof T]: Ref<T[K]> } {