fix(reactivity): should not trigger watch on computed ref when value is unchanged

fix #2231

packages/runtime-core/__tests__/apiWatch.spec.ts

@@ -13,7 +13,8 @@ import {
   DebuggerEvent,
   TrackOpTypes,
   TriggerOpTypes,
-  triggerRef
+  triggerRef,
+  shallowRef
 } from '@vue/reactivity'
 
 // reference: https://vue-composition-api-rfc.netlify.com/api.html#watch
@@ -750,8 +751,8 @@ describe('api: watch', () => {
     expect(calls).toBe(1)
   })
 
-  test('should force trigger on triggerRef when watching a ref', async () => {
-    const v = ref({ a: 1 })
+  test('should force trigger on triggerRef when watching a shallow ref', async () => {
+    const v = shallowRef({ a: 1 })
     let sideEffect = 0
     watch(v, obj => {
       sideEffect = obj.a
@@ -785,4 +786,17 @@ describe('api: watch', () => {
     await nextTick()
     expect(spy).toHaveBeenCalledTimes(1)
   })
+
+  // #2231
+  test('computed refs should not trigger watch if value has no change', async () => {
+    const spy = jest.fn()
+    const source = ref(0)
+    const price = computed(() => source.value === 0)
+    watch(price, spy)
+    source.value++
+    await nextTick()
+    source.value++
+    await nextTick()
+    expect(spy).toHaveBeenCalledTimes(1)
+  })
 })