[Fix-5538]: Assign observer's current Result when optimistic read occurs

packages/query-core/src/queryObserver.ts

@@ -240,7 +240,30 @@ export class QueryObserver<
   ): QueryObserverResult<TData, TError> {
     const query = this.client.getQueryCache().build(this.client, options)
 
-    return this.createResult(query, options)
+    const result = this.createResult(query, options)
+
+    if (shouldAssignObserverCurrentProperties(this, result, options)) {
+      // this assigns the optimistic result to the current Observer
+      // because if the query function changes, useQuery will be performing
+      // an effect where it would fetch again.
+      // When the fetch finishes, we perform a deep data cloning in order
+      // to reuse objects references. This deep data clone is performed against
+      // the `observer.currentResult.data` property
+      // When QueryKey changes, we refresh the query and get new `optimistic`
+      // result, while we leave the `observer.currentResult`, so when new data
+      // arrives, it finds the old `observer.currentResult` which is related
+      // to the old QueryKey. Which means that currentResult and selectData are
+      // out of sync already.
+      // To solve this, we move the cursor of the currentResult everytime
+      // an observer reads an optimistic value.
+
+      // When keeping the previous data, the result doesn't change until new
+      // data arrives.
+      this.currentResult = result
+      this.currentResultOptions = this.options
+      this.currentResultState = this.currentQuery.state
+    }
+    return result
   }
 
   getCurrentResult(): QueryObserverResult<TData, TError> {
@@ -764,3 +787,51 @@ function isStale(
 ): boolean {
   return query.isStaleByTime(options.staleTime)
 }
+
+// this function would decide if we will update the observer's 'current'
+// properties after an optimistic reading via getOptimisticResult
+function shouldAssignObserverCurrentProperties<
+  TQueryFnData = unknown,
+  TError = unknown,
+  TData = TQueryFnData,
+  TQueryData = TQueryFnData,
+  TQueryKey extends QueryKey = QueryKey,
+>(
+  observer: QueryObserver<TQueryFnData, TError, TData, TQueryData, TQueryKey>,
+  optimisticResult: QueryObserverResult<TData, TError>,
+  options: DefaultedQueryObserverOptions<
+    TQueryFnData,
+    TError,
+    TData,
+    TQueryData,
+    TQueryKey
+  >,
+) {
+  // it is important to keep this condition like this for three reasons:
+  // 1. It will get removed in the v5
+  // 2. it reads: don't update the properties if we want to keep the previous
+  // data.
+  // 3. The opposite condition (!options.keepPreviousData) would fallthrough
+  // and will result in a bad decision
+  if (options.keepPreviousData) {
+    return false
+  }
+
+  // if the newly created result isn't what the observer is holding as current,
+  // then we'll need to update the properties as well
+  if (observer.getCurrentResult() !== optimisticResult) {
+    return true
+  }
+
+  // this means we want to put some placeholder data when pending and queryKey
+  // changed.
+  if (options.placeholderData !== undefined) {
+    // re-assign properties only if current data is placeholder data
+    // which means that data did not arrive yet, so, if there is some cached data
+    // we need to "prepare" to receive it
+    return optimisticResult.isPlaceholderData
+  }
+
+  // basically, just keep previous properties if nothing changed
+  return false
+}

packages/react-query-persist-client/src/__tests__/PersistQueryClientProvider.test.tsx

@@ -325,7 +325,7 @@ describe('PersistQueryClientProvider', () => {
     await waitFor(() => rendered.getByText('data: null'))
     await waitFor(() => rendered.getByText('data: hydrated'))
 
-    expect(states).toHaveLength(3)
+    expect(states).toHaveLength(2)
 
     expect(fetched).toBe(false)
 
@@ -340,9 +340,6 @@ describe('PersistQueryClientProvider', () => {
       fetchStatus: 'idle',
       data: 'hydrated',
     })
-
-    // #5443 seems like we get an extra render now ...
-    expect(states[1]).toStrictEqual(states[2])
   })
 
   test('should call onSuccess after successful restoring', async () => {

packages/react-query/src/__tests__/useQuery.test.tsx

@@ -914,17 +914,15 @@ describe('useQuery', () => {
     // required to make sure no additional renders are happening after data is successfully fetched for the second time
     await sleep(100)
 
-    expect(states.length).toBe(5)
+    expect(states.length).toBe(4)
     // First load
     expect(states[0]).toMatchObject({ isLoading: true, isSuccess: false })
     // First success
     expect(states[1]).toMatchObject({ isLoading: false, isSuccess: true })
     // Remove
     expect(states[2]).toMatchObject({ isLoading: true, isSuccess: false })
-    // Hook state update
-    expect(states[3]).toMatchObject({ isLoading: true, isSuccess: false })
     // Second success
-    expect(states[4]).toMatchObject({ isLoading: false, isSuccess: true })
+    expect(states[3]).toMatchObject({ isLoading: false, isSuccess: true })
   })
 
   it('should fetch when refetchOnMount is false and nothing has been fetched yet', async () => {
@@ -3650,6 +3648,7 @@ describe('useQuery', () => {
           )
           act(() => setPrefetched(true))
         }
+
         prefetch()
       }, [])
 
@@ -5879,6 +5878,7 @@ describe('useQuery', () => {
         </div>
       )
     }
+
     const rendered = renderWithClient(queryClient, <Page />)
     const fetchBtn = rendered.getByRole('button', { name: 'refetch' })
     await waitFor(() => rendered.getByText('data: 1'))
@@ -5916,8 +5916,68 @@ describe('useQuery', () => {
         </div>
       )
     }
+
     const rendered = renderWithClient(queryClient, <Page />)
     await waitFor(() => rendered.getByText('status: success'))
     await waitFor(() => rendered.getByText('data: 1'))
   })
+  it('should reuse same data object reference when queryKey changes back to some cached data', async () => {
+    const spy = jest.fn()
+
+    async function fetchNumber(id: number) {
+      await sleep(5)
+      return { numbers: { current: { id } } }
+    }
+    function Test() {
+      const [id, setId] = React.useState(1)
+
+      const { data } = useQuery({
+        select: selector,
+        queryKey: ['user', id],
+        queryFn: () => fetchNumber(id),
+      })
+
+      React.useEffect(() => {
+        spy(data)
+      }, [data])
+
+      return (
+        <div>
+          <button name="1" onClick={() => setId(1)}>
+            1
+          </button>
+          <button name="2" onClick={() => setId(2)}>
+            2
+          </button>
+          <span>Rendered Id: {data?.id}</span>
+        </div>
+      )
+    }
+
+    function selector(data: any) {
+      return data.numbers.current
+    }
+
+    const rendered = renderWithClient(queryClient, <Test />)
+    expect(spy).toHaveBeenCalledTimes(1)
+
+    spy.mockClear()
+    await waitFor(() => rendered.getByText('Rendered Id: 1'))
+    expect(spy).toHaveBeenCalledTimes(1)
+
+    spy.mockClear()
+    fireEvent.click(rendered.getByRole('button', { name: /2/ }))
+    await waitFor(() => rendered.getByText('Rendered Id: 2'))
+    expect(spy).toHaveBeenCalledTimes(2) // called with undefined because id changed
+
+    spy.mockClear()
+    fireEvent.click(rendered.getByRole('button', { name: /1/ }))
+    await waitFor(() => rendered.getByText('Rendered Id: 1'))
+    expect(spy).toHaveBeenCalledTimes(1)
+
+    spy.mockClear()
+    fireEvent.click(rendered.getByRole('button', { name: /2/ }))
+    await waitFor(() => rendered.getByText('Rendered Id: 2'))
+    expect(spy).toHaveBeenCalledTimes(1)
+  })
 })