Fix useSyncExternalStoreWithSelector to not call selectors when snapshot hasn't changed #27776
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Summary
Current implementation of
useSyncExternalStoreWithSelectorcalls selectors even if snapshot hasn't changed.Details
Up to 50% of all dispatched actions in the apps that use
redux-sagaorredux-thunkcould be ones that don't change the redux state but start some async logic.Current
reduximplementation notifies listeners when any action is called, even if it does not change the state, and this behavior won't change.Current
react-reduximplementation usesuseSyncExternalStoreWithSelectorfrom React, which notifies subscribers for each such action even if state hasn't changed, causing all active selectors to be re-evaluated, which is actually a bug ofuseSyncExternalStoreWithSelector.In huge apps there can be hundreds of subscribed selectors and dozens of actions per second, so this leads to additional CPU usage for no good reason. Especially noticable in
react-nativeapps on old android devices.How did you test this change?
I added a test which fails in current version, and fixed it:
... const container = document.createElement('div'); const root = createRoot(container); await act(() => root.render(<App />)); assertLog(['App', 'Selector', 'A0']); await act(() => store.set(store.getState())); await act(() => store.set({a: 0})); assertLog(['Selector']); await act(() => store.set(store.getState())); await act(() => store.set(store.getState())); assertLog([]); // without the fix here we got ["Selector", "Selector"]