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Reuse latest selected state on selector re-run (#1654) #1660

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Mar 22, 2021
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Reuse latest selected state on selector re-run (#1654) #1660

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878271f
Reuse latest selected state on selector re-run (#1654)
otakustay Nov 7, 2020
b53d184
Update src/hooks/useSelector.js
timdorr Dec 3, 2020
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11 changes: 10 additions & 1 deletion src/hooks/useSelector.js
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Original file line number Diff line number Diff line change
Expand Up @@ -33,7 +33,16 @@ function useSelectorWithStoreAndSubscription(
storeState !== latestStoreState.current ||
latestSubscriptionCallbackError.current
) {
selectedState = selector(storeState)
const newSelectedState = selector(storeState)
// ensure latest selected state is reused so that a curtom equality function can result in identical references
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if (
latestSelectedState.current === undefined ||
!equalityFn(newSelectedState, latestSelectedState.current)
) {
selectedState = newSelectedState
} else {
selectedState = latestSelectedState.current
}
} else {
selectedState = latestSelectedState.current
}
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29 changes: 29 additions & 0 deletions test/hooks/useSelector.spec.js
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Expand Up @@ -412,6 +412,35 @@ describe('React', () => {

spy.mockRestore()
})

it('reuse latest selected state on selector re-run', () => {
store = createStore(({ count } = { count: -1 }) => ({
count: count + 1,
}))

const alwaysEqual = () => true
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Does this also maintain the latest state if the equalityFn is defined within the component or inline?

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@nightpool nightpool Dec 3, 2020 •
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Yes—it only cares what the equalityFn returns, not what its identity is. However, having a non-pure equalityFn (i.e., one that closes over variables from its surrounding components) could have very surprising results.


const Comp = () => {
// triggers render on store change
useSelector((s) => s.count)
const array = useSelector(() => [1, 2, 3], alwaysEqual)
renderedItems.push(array)
return <div />
}

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

expect(renderedItems.length).toBe(1)

store.dispatch({ type: '' })

expect(renderedItems.length).toBe(2)
expect(renderedItems[0]).toBe(renderedItems[1])
})
})

describe('error handling for invalid arguments', () => {
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