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Point useSubscription to useSyncExternalStore shim #24289

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Apr 11, 2022
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5 changes: 4 additions & 1 deletion packages/use-subscription/package.json
Original file line number Diff line number Diff line change
Expand Up @@ -15,9 +15,12 @@
],
"license": "MIT",
"peerDependencies": {
"react": "^18.0.0"
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"react": "^16.8.0 || ^17.0.0 || ^18.0.0"
},
"devDependencies": {
"rxjs": "^5.5.6"
},
"dependencies": {
"use-sync-external-store": "^1.0.0"
}
}
21 changes: 8 additions & 13 deletions packages/use-subscription/src/__tests__/useSubscription-test.js
Original file line number Diff line number Diff line change
Expand Up @@ -457,17 +457,13 @@ describe('useSubscription', () => {
renderer.update(<Parent observed={observableA} />);

// Flush everything and ensure that the correct subscribable is used
// We expect the new subscribable to finish rendering,
// But then the updated values from the old subscribable should be used.
expect(Scheduler).toFlushAndYield([
'Grandchild: b-0',
'Child: a-2',
'Grandchild: a-2',
'Child: a-2',
'Grandchild: a-2',
]);
expect(log).toEqual([
'Parent.componentDidUpdate:b-0',
'Parent.componentDidUpdate:a-2',
]);
expect(log).toEqual(['Parent.componentDidUpdate:a-2']);
});

// Updates from the new subscribable should be ignored.
Expand Down Expand Up @@ -628,19 +624,18 @@ describe('useSubscription', () => {
} else {
mutate('C');
}
expect(Scheduler).toFlushAndYieldThrough(['render:first:C']);
expect(Scheduler).toFlushAndYieldThrough([
'render:first:C',
'render:second:C',
]);
if (gate(flags => flags.enableSyncDefaultUpdates)) {
React.startTransition(() => {
mutate('D');
});
} else {
mutate('D');
}
expect(Scheduler).toFlushAndYield([
'render:second:C',
'render:first:D',
'render:second:D',
]);
expect(Scheduler).toFlushAndYield(['render:first:D', 'render:second:D']);

// No more pending updates
jest.runAllTimers();
Expand Down
99 changes: 2 additions & 97 deletions packages/use-subscription/src/useSubscription.js
Original file line number Diff line number Diff line change
Expand Up @@ -7,7 +7,7 @@
* @flow
*/

import {useDebugValue, useEffect, useState} from 'react';
import {useSyncExternalStore} from 'use-sync-external-store/shim';

// Hook used for safely managing subscriptions in concurrent mode.
//
Expand All @@ -26,100 +26,5 @@ export function useSubscription<Value>({
getCurrentValue: () => Value,
subscribe: (callback: Function) => () => void,
|}): Value {
// Read the current value from our subscription.
// When this value changes, we'll schedule an update with React.
// It's important to also store the hook params so that we can check for staleness.
// (See the comment in checkForUpdates() below for more info.)
const [state, setState] = useState(() => ({
getCurrentValue,
subscribe,
value: getCurrentValue(),
}));

let valueToReturn = state.value;

// If parameters have changed since our last render, schedule an update with its current value.
if (
state.getCurrentValue !== getCurrentValue ||
state.subscribe !== subscribe
) {
// If the subscription has been updated, we'll schedule another update with React.
// React will process this update immediately, so the old subscription value won't be committed.
// It is still nice to avoid returning a mismatched value though, so let's override the return value.
valueToReturn = getCurrentValue();

setState({
getCurrentValue,
subscribe,
value: valueToReturn,
});
}

// Display the current value for this hook in React DevTools.
useDebugValue(valueToReturn);

// It is important not to subscribe while rendering because this can lead to memory leaks.
// (Learn more at reactjs.org/docs/strict-mode.html#detecting-unexpected-side-effects)
// Instead, we wait until the commit phase to attach our handler.
//
// We intentionally use a passive effect (useEffect) rather than a synchronous one (useLayoutEffect)
// so that we don't stretch the commit phase.
// This also has an added benefit when multiple components are subscribed to the same source:
// It allows each of the event handlers to safely schedule work without potentially removing an another handler.
// (Learn more at https://codesandbox.io/s/k0yvr5970o)
useEffect(() => {
let didUnsubscribe = false;

const checkForUpdates = () => {
// It's possible that this callback will be invoked even after being unsubscribed,
// if it's removed as a result of a subscription event/update.
// In this case, React will log a DEV warning about an update from an unmounted component.
// We can avoid triggering that warning with this check.
if (didUnsubscribe) {
return;
}

// We use a state updater function to avoid scheduling work for a stale source.
// However it's important to eagerly read the currently value,
// so that all scheduled work shares the same value (in the event of multiple subscriptions).
// This avoids visual "tearing" when a mutation happens during a (concurrent) render.
const value = getCurrentValue();

setState(prevState => {
// Ignore values from stale sources!
// Since we subscribe an unsubscribe in a passive effect,
// it's possible that this callback will be invoked for a stale (previous) subscription.
// This check avoids scheduling an update for that stale subscription.
if (
prevState.getCurrentValue !== getCurrentValue ||
prevState.subscribe !== subscribe
) {
return prevState;
}

// Some subscriptions will auto-invoke the handler, even if the value hasn't changed.
// If the value hasn't changed, no update is needed.
// Return state as-is so React can bail out and avoid an unnecessary render.
if (prevState.value === value) {
return prevState;
}

return {...prevState, value};
});
};
const unsubscribe = subscribe(checkForUpdates);

// Because we're subscribing in a passive effect,
// it's possible that an update has occurred between render and our effect handler.
// Check for this and schedule an update if work has occurred.
checkForUpdates();

return () => {
didUnsubscribe = true;
unsubscribe();
};
}, [getCurrentValue, subscribe]);

// Return the current value for our caller to use while rendering.
return valueToReturn;
return useSyncExternalStore(subscribe, getCurrentValue);
}
2 changes: 1 addition & 1 deletion packages/use-sync-external-store/package.json
Original file line number Diff line number Diff line change
Expand Up @@ -19,6 +19,6 @@
],
"license": "MIT",
"peerDependencies": {
"react": "^16.8.0 || ^17.0.0 || ^18.0.0-rc"
"react": "^16.8.0 || ^17.0.0 || ^18.0.0"
}
}