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Remove state queue from useSyncExternalStore
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The userspace shim of useSyncExternalStore uses a useState hook because
it's the only way to trigger a re-render. We don't actually use the
queue to store anything, because we read the current value directly
from the store.

In the native implementation, we can schedule an update on the fiber
directly, without the overhead of a queue.
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@acdlite
acdlite committed Sep 7, 2021
1 parent 06f98c1 commit 220d6df
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Showing 2 changed files with 136 additions and 124 deletions.
130 changes: 68 additions & 62 deletions packages/react-reconciler/src/ReactFiberHooks.new.js
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Original file line number Diff line number Diff line change
Expand Up @@ -41,6 +41,7 @@ import {
} from './ReactTypeOfMode';
import {
NoLane,
SyncLane,
NoLanes,
isSubsetOfLanes,
mergeLanes,
Expand All @@ -49,9 +50,9 @@ import {
isTransitionLane,
markRootEntangled,
markRootMutableRead,
NoTimestamp,
} from './ReactFiberLane.new';
import {
DiscreteEventPriority,
ContinuousEventPriority,
getCurrentUpdatePriority,
setCurrentUpdatePriority,
Expand Down Expand Up @@ -147,7 +148,7 @@ export type Hook = {|
memoizedState: any,
baseState: any,
baseQueue: Update<any, any> | null,
queue: UpdateQueue<any, any> | null,
queue: any,
next: Hook | null,
|};

Expand All @@ -159,6 +160,11 @@ export type Effect = {|
next: Effect,
|};

type StoreInstance<T> = {|
value: T,
getSnapshot: () => T,
|};

export type FunctionComponentUpdateQueue = {|lastEffect: Effect | null|};

type BasicStateAction<S> = (S => S) | S;
Expand Down Expand Up @@ -703,14 +709,15 @@ function mountReducer<S, I, A>(
initialState = ((initialArg: any): S);
}
hook.memoizedState = hook.baseState = initialState;
const queue = (hook.queue = {
const queue: UpdateQueue<S, A> = {
pending: null,
interleaved: null,
lanes: NoLanes,
dispatch: null,
lastRenderedReducer: reducer,
lastRenderedState: (initialState: any),
});
};
hook.queue = queue;
const dispatch: Dispatch<A> = (queue.dispatch = (dispatchAction.bind(
null,
currentlyRenderingFiber,
Expand Down Expand Up @@ -1196,7 +1203,7 @@ function useMutableSource<Source, Snapshot>(
// So if there are interleaved updates, they get pushed to the older queue.
// When this becomes current, the previous queue and dispatch method will be discarded,
// including any interleaving updates that occur.
const newQueue = {
const newQueue: UpdateQueue<Snapshot, BasicStateAction<Snapshot>> = {
pending: null,
interleaved: null,
lanes: NoLanes,
Expand Down Expand Up @@ -1249,86 +1256,86 @@ function mountSyncExternalStore<T>(
getSnapshot: () => T,
): T {
const hook = mountWorkInProgressHook();
return useSyncExternalStore(hook, subscribe, getSnapshot);
// Read the current snapshot from the store on every render. This breaks the
// normal rules of React, and only works because store updates are
// always synchronous.
const nextSnapshot = getSnapshot();
if (__DEV__) {
if (!didWarnUncachedGetSnapshot) {
if (nextSnapshot !== getSnapshot()) {
console.error(
'The result of getSnapshot should be cached to avoid an infinite loop',
);
didWarnUncachedGetSnapshot = true;
}
}
}
hook.memoizedState = nextSnapshot;
const inst: StoreInstance<T> = {
value: nextSnapshot,
getSnapshot,
};
hook.queue = inst;
return useSyncExternalStore(hook, inst, subscribe, getSnapshot, nextSnapshot);
}

function updateSyncExternalStore<T>(
subscribe: (() => void) => () => void,
getSnapshot: () => T,
): T {
const hook = updateWorkInProgressHook();
return useSyncExternalStore(hook, subscribe, getSnapshot);
}

function useSyncExternalStore<T>(
hook: Hook,
subscribe: (() => void) => () => void,
getSnapshot: () => T,
): T {
// TODO: This is a copy-paste of the userspace shim. We can improve the
// built-in implementation using lower-level APIs. We also intend to move
// the tearing checks to an earlier, pre-commit phase so that the layout
// effects always observe a consistent tree.

const dispatcher = ReactCurrentDispatcher.current;

// Read the current snapshot from the store on every render. Again, this
// breaks the rules of React, and only works here because of specific
// implementation details, most importantly that updates are
// Read the current snapshot from the store on every render. This breaks the
// normal rules of React, and only works because store updates are
// always synchronous.
const value = getSnapshot();
const nextSnapshot = getSnapshot();
if (__DEV__) {
if (!didWarnUncachedGetSnapshot) {
if (value !== getSnapshot()) {
if (nextSnapshot !== getSnapshot()) {
console.error(
'The result of getSnapshot should be cached to avoid an infinite loop',
);
didWarnUncachedGetSnapshot = true;
}
}
}
const prevSnapshot = hook.memoizedState;
if (!is(prevSnapshot, nextSnapshot)) {
hook.memoizedState = nextSnapshot;
markWorkInProgressReceivedUpdate();
}
const inst = hook.queue;
return useSyncExternalStore(hook, inst, subscribe, getSnapshot, nextSnapshot);
}

// Because updates are synchronous, we don't queue them. Instead we force a
// re-render whenever the subscribed state changes by updating an some
// arbitrary useState hook. Then, during render, we call getSnapshot to read
// the current value.
//
// Because we don't actually use the state returned by the useState hook, we
// can save a bit of memory by storing other stuff in that slot.
//
// To implement the early bailout, we need to track some things on a mutable
// object. Usually, we would put that in a useRef hook, but we can stash it in
// our useState hook instead.
//
// To force a re-render, we call forceUpdate({inst}). That works because the
// new object always fails an equality check.
const [{inst}, forceUpdate] = dispatcher.useState({
inst: {value, getSnapshot},
});
function useSyncExternalStore<T>(
hook: Hook,
inst: StoreInstance<T>,
subscribe: (() => void) => () => void,
getSnapshot: () => T,
nextSnapshot: T,
): T {
const fiber = currentlyRenderingFiber;
const dispatcher = ReactCurrentDispatcher.current;

// Track the latest getSnapshot function with a ref. This needs to be updated
// in the layout phase so we can access it during the tearing check that
// happens on subscribe.
// TODO: Circumvent SSR warning
dispatcher.useLayoutEffect(() => {
inst.value = value;
inst.value = nextSnapshot;
inst.getSnapshot = getSnapshot;

// Whenever getSnapshot or subscribe changes, we need to check in the
// commit phase if there was an interleaved mutation. In concurrent mode
// this can happen all the time, but even in synchronous mode, an earlier
// effect may have mutated the store.
// TODO: Move the tearing checks to an earlier, pre-commit phase so that the
// layout effects always observe a consistent tree.
if (checkIfSnapshotChanged(inst)) {
// Force a re-render.
const prevTransition = ReactCurrentBatchConfig.transition;
const prevPriority = getCurrentUpdatePriority();
ReactCurrentBatchConfig.transition = 0;
setCurrentUpdatePriority(DiscreteEventPriority);
forceUpdate({inst});
setCurrentUpdatePriority(prevPriority);
ReactCurrentBatchConfig.transition = prevTransition;
forceStoreRerender(fiber);
}
}, [subscribe, value, getSnapshot]);
}, [subscribe, nextSnapshot, getSnapshot]);

dispatcher.useEffect(() => {
const handleStoreChange = () => {
Expand All @@ -1341,13 +1348,7 @@ function useSyncExternalStore<T>(
// read from the store.
if (checkIfSnapshotChanged(inst)) {
// Force a re-render.
const prevTransition = ReactCurrentBatchConfig.transition;
const prevPriority = getCurrentUpdatePriority();
ReactCurrentBatchConfig.transition = 0;
setCurrentUpdatePriority(DiscreteEventPriority);
forceUpdate({inst});
setCurrentUpdatePriority(prevPriority);
ReactCurrentBatchConfig.transition = prevTransition;
forceStoreRerender(fiber);
}
};
// Check for changes right before subscribing. Subsequent changes will be
Expand All @@ -1357,7 +1358,7 @@ function useSyncExternalStore<T>(
return subscribe(handleStoreChange);
}, [subscribe]);

return value;
return nextSnapshot;
}

function checkIfSnapshotChanged(inst) {
Expand All @@ -1371,6 +1372,10 @@ function checkIfSnapshotChanged(inst) {
}
}

function forceStoreRerender(fiber) {
scheduleUpdateOnFiber(fiber, SyncLane, NoTimestamp);
}

function mountState<S>(
initialState: (() => S) | S,
): [S, Dispatch<BasicStateAction<S>>] {
Expand All @@ -1380,14 +1385,15 @@ function mountState<S>(
initialState = initialState();
}
hook.memoizedState = hook.baseState = initialState;
const queue = (hook.queue = {
const queue: UpdateQueue<S, BasicStateAction<S>> = {
pending: null,
interleaved: null,
lanes: NoLanes,
dispatch: null,
lastRenderedReducer: basicStateReducer,
lastRenderedState: (initialState: any),
});
};
hook.queue = queue;
const dispatch: Dispatch<
BasicStateAction<S>,
> = (queue.dispatch = (dispatchAction.bind(
Expand Down
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