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Reduce unnecessary calls to useSelector selector #1803

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merged 3 commits into from Sep 4, 2021
Merged

Reduce unnecessary calls to useSelector selector #1803

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7f2ca90
Add a failing tests (and update one test)
sufian-slack Aug 19, 2021
75b2fba
Reduce the number of calls to the provided selector by one
sufian-slack Aug 19, 2021
2ac8bfc
Add test to ensure we are calling selector on state change
sufian-slack Aug 25, 2021
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5 changes: 5 additions & 0 deletions src/hooks/useSelector.js
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Expand Up @@ -65,6 +65,11 @@ function useSelectorWithStoreAndSubscription(
function checkForUpdates() {
try {
const newStoreState = store.getState()
// Avoid calling selector multiple times if the store's state has not changed
if (newStoreState === latestStoreState.current) {
return
}

const newSelectedState = latestSelector.current(newStoreState)

if (equalityFn(newSelectedState, latestSelectedState.current)) {
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37 changes: 35 additions & 2 deletions test/hooks/useSelector.spec.js
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Expand Up @@ -45,14 +45,14 @@ describe('React', () => {
})

expect(result.current).toEqual(0)
expect(selector).toHaveBeenCalledTimes(2)
expect(selector).toHaveBeenCalledTimes(1)

act(() => {
store.dispatch({ type: '' })
})

expect(result.current).toEqual(1)
expect(selector).toHaveBeenCalledTimes(3)
expect(selector).toHaveBeenCalledTimes(2)
})
})

Expand Down Expand Up @@ -246,6 +246,39 @@ describe('React', () => {

expect(renderedItems.length).toBe(1)
})

it('calls selector exactly once on mount and on update', () => {
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✋ can you also add a test that does force the selector to run a second time? This should be doable by rendering a child component inside of a parent, and dispatching inside of a useLayoutEffect in the child. They mount in the same pass, and the child's useLayoutEffect should run before the parent's does.

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Ok, added a test here and in the other PR with a useLayoutEffect that verifies the selector is re-run when state changes during initial mount

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Also, I'm about to head out on holiday and won't be checking back on this until Sep 7th, so don't be alarmed/think this is abandoned if I don't get back to any additional follow-ups. Thank you for all the help!

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Sure, thanks!

store = createStore(({ count } = { count: 0 }) => ({
count: count + 1,
}))

let numCalls = 0
const selector = (s) => {
numCalls += 1
return s.count
}
const renderedItems = []

const Comp = () => {
const value = useSelector(selector)
renderedItems.push(value)
return <div />
}

rtl.render(
<ProviderMock store={store}>
<Comp />
</ProviderMock>
)

expect(numCalls).toBe(1)
expect(renderedItems.length).toEqual(1)

store.dispatch({ type: '' })

expect(numCalls).toBe(2)
expect(renderedItems.length).toEqual(2)
})
})

it('uses the latest selector', () => {
Expand Down