Fix preact selector stability (#318)

* fix: preact adapter handles unstable selector functions now

* chore: add changeset

Author: crutchcorn

.changeset/fancy-colts-rush.md

@@ -0,0 +1,5 @@
+---
+'@tanstack/preact-store': patch
+---
+
+useSelector handles unstable selector functions now

packages/preact-store/src/useSelector.ts

@@ -142,10 +142,35 @@ export function useSelector<TSource, TSelected = NoInfer<TSource>>(
 
   const getSnapshot = useCallback(() => source.get(), [source])
 
+  // Memoize the selector's output by the source snapshot so that selectors
+  // returning a fresh reference each call (e.g. `(s) => [s.one, s.two]`) still
+  // produce a stable reference for unchanged snapshots. Without this, the
+  // shim's identity-based memo would always miss and trigger an update loop.
+  const selectorMemoRef = useRef<{
+    _hasMemo: boolean
+    _snapshot: TSource | undefined
+    _selected: TSelected | undefined
+  }>({ _hasMemo: false, _snapshot: undefined, _selected: undefined })
+
+  const memoizedSelector = useCallback(
+    (snapshot: TSource): TSelected => {
+      const memo = selectorMemoRef.current
+      if (memo._hasMemo && Object.is(memo._snapshot, snapshot)) {
+        return memo._selected as TSelected
+      }
+      const next = selector(snapshot)
+      memo._hasMemo = true
+      memo._snapshot = snapshot
+      memo._selected = next
+      return next
+    },
+    [selector],
+  )
+
   return useSyncExternalStoreWithSelector(
     subscribe,
     getSnapshot,
-    selector,
+    memoizedSelector,
     compare,
   )
 }

packages/preact-store/tests/index.test.tsx

@@ -800,4 +800,45 @@ describe('shallow', () => {
     const objB = new Date('2025-02-10')
     expect(shallow(objA, objB)).toBe(true)
   })
+
+  test('should handle arrays inside of a useSelector', async () => {
+    const store = createStore({ one: 1, two: 2 })
+
+    function Comp() {
+      const values = useSelector(store, (state) => [state.one, state.two])
+
+      return (
+        <div>
+          <p>Values: {values.join(',')}</p>
+          <button
+            type="button"
+            onClick={() =>
+              store.setState((prev) => ({ ...prev, one: prev.one + 1 }))
+            }
+          >
+            Update one
+          </button>
+          <button
+            type="button"
+            onClick={() =>
+              store.setState((prev) => ({ ...prev, two: prev.two + 1 }))
+            }
+          >
+            Update two
+          </button>
+        </div>
+      )
+    }
+
+    const { getByText } = render(<Comp />)
+    expect(getByText('Values: 1,2')).toBeInTheDocument()
+
+    await user.click(getByText('Update one'))
+
+    await waitFor(() => expect(getByText('Values: 2,2')).toBeInTheDocument())
+
+    await user.click(getByText('Update two'))
+
+    await waitFor(() => expect(getByText('Values: 2,3')).toBeInTheDocument())
+  })
 })

packages/react-store/tests/index.test.tsx

@@ -764,4 +764,45 @@ describe('shallow', () => {
 
     expect(shallow(objA, objB)).toBe(true)
   })
+
+  test('should handle arrays inside of a useSelector', async () => {
+    const store = createStore({ one: 1, two: 2 })
+
+    function Comp() {
+      const values = useSelector(store, (state) => [state.one, state.two])
+
+      return (
+        <div>
+          <p>Values: {values.join(',')}</p>
+          <button
+            type="button"
+            onClick={() =>
+              store.setState((prev) => ({ ...prev, one: prev.one + 1 }))
+            }
+          >
+            Update one
+          </button>
+          <button
+            type="button"
+            onClick={() =>
+              store.setState((prev) => ({ ...prev, two: prev.two + 1 }))
+            }
+          >
+            Update two
+          </button>
+        </div>
+      )
+    }
+
+    const { getByText } = render(<Comp />)
+    expect(getByText('Values: 1,2')).toBeInTheDocument()
+
+    await user.click(getByText('Update one'))
+
+    await waitFor(() => expect(getByText('Values: 2,2')).toBeInTheDocument())
+
+    await user.click(getByText('Update two'))
+
+    await waitFor(() => expect(getByText('Values: 2,3')).toBeInTheDocument())
+  })
 })