fix(reactivity): prevent overwriting next property during batch pro…

…cessing

packages/reactivity/__tests__/watch.spec.ts

@@ -260,4 +260,21 @@ describe('watch', () => {
  src.value = 10
  expect(spy).toHaveBeenCalledTimes(2)
  })
+
+ test('should ensure correct execution order in batch processing', () => {
+ const dummy: number[] = []
+ const n1 = ref(0)
+ const n2 = ref(0)
+ const sum = computed(() => n1.value + n2.value)
+ watch(n1, () => {
+ dummy.push(1)
+ n2.value++
+ })
+ watch(sum, () => dummy.push(2))
+ watch(n1, () => dummy.push(3))
+
+ n1.value++
+
+ expect(dummy).toEqual([1, 2, 3])
+ })
 })

packages/reactivity/src/effect.ts

@@ -237,6 +237,9 @@ let batchedSub: Subscriber | undefined
 
 export function batch(sub: Subscriber): void {
  sub.flags |= EffectFlags.NOTIFIED
+ // If sub.next is set, the subscriber is already in a batch,
+ // return to avoid overwriting it and losing previous subscriptions.
+ if (sub.next) return
  sub.next = batchedSub
  batchedSub = sub
 }