Only most recent transition is pending, per queue

When multiple transitions update the same queue, only the most recent
one should be considered pending.

Example: If I switch tabs multiple times, only the last tab I click
should display a pending state (e.g. an inline spinner).

Author: acdlite

packages/react-reconciler/src/ReactFiberHooks.js

@@ -17,6 +17,7 @@ import type {Fiber} from './ReactFiber';
 import type {ExpirationTime} from './ReactFiberExpirationTime';
 import type {HookEffectTag} from './ReactHookEffectTags';
 import type {SuspenseConfig} from './ReactFiberSuspenseConfig';
+import type {TransitionInstance} from './ReactFiberTransition';
 import type {ReactPriorityLevel} from './SchedulerWithReactIntegration';
 
 import ReactSharedInternals from 'shared/ReactSharedInternals';
@@ -53,11 +54,11 @@ import is from 'shared/objectIs';
 import {markWorkInProgressReceivedUpdate} from './ReactFiberBeginWork';
 import {requestCurrentSuspenseConfig} from './ReactFiberSuspenseConfig';
 import {
-  UserBlockingPriority,
-  NormalPriority,
-  runWithPriority,
-  getCurrentPriorityLevel,
-} from './SchedulerWithReactIntegration';
+  startTransition,
+  requestCurrentTransition,
+  cancelPendingTransition,
+} from './ReactFiberTransition';
+import {getCurrentPriorityLevel} from './SchedulerWithReactIntegration';
 
 const {ReactCurrentDispatcher, ReactCurrentBatchConfig} = ReactSharedInternals;
 
@@ -117,14 +118,11 @@ type Update<S, A> = {
 type UpdateQueue<S, A> = {
   last: Update<S, A> | null,
   dispatch: (A => mixed) | null,
+  pendingTransition: TransitionInstance | null,
   lastRenderedReducer: ((S, A) => S) | null,
   lastRenderedState: S | null,
 };
 
-type TransitionInstance = {|
-  pendingExpirationTime: ExpirationTime,
-|};
-
 export type HookType =
   | 'useState'
   | 'useReducer'
@@ -667,6 +665,7 @@ function mountReducer<S, I, A>(
   const queue = (hook.queue = {
     last: null,
     dispatch: null,
+    pendingTransition: null,
     lastRenderedReducer: reducer,
     lastRenderedState: (initialState: any),
   });
@@ -839,6 +838,7 @@ function mountState<S>(
   const queue = (hook.queue = {
     last: null,
     dispatch: null,
+    pendingTransition: null,
     lastRenderedReducer: basicStateReducer,
     lastRenderedState: (initialState: any),
   });
@@ -1176,89 +1176,14 @@ function updateDeferredValue<T>(
   return prevValue;
 }
 
-function startTransition(fiber, instance, config, callback) {
-  let resolvedConfig: SuspenseConfig | null =
-    config === undefined ? null : config;
-
-  // TODO: runWithPriority shouldn't be necessary here. React should manage its
-  // own concept of priority, and only consult Scheduler for updates that are
-  // scheduled from outside a React context.
-  const priorityLevel = getCurrentPriorityLevel();
-  runWithPriority(
-    priorityLevel < UserBlockingPriority ? UserBlockingPriority : priorityLevel,
-    () => {
-      const currentTime = requestCurrentTimeForUpdate();
-      const expirationTime = computeExpirationForFiber(
-        currentTime,
-        fiber,
-        null,
-      );
-      scheduleWork(fiber, expirationTime);
-    },
-  );
-  runWithPriority(
-    priorityLevel > NormalPriority ? NormalPriority : priorityLevel,
-    () => {
-      const currentTime = requestCurrentTimeForUpdate();
-      let expirationTime = computeExpirationForFiber(
-        currentTime,
-        fiber,
-        resolvedConfig,
-      );
-      // Set the expiration time at which the pending transition will finish.
-      // Because there's only a single transition per useTransition hook, we
-      // don't need a queue here; we can cheat by only tracking the most
-      // recently scheduled transition.
-      // TODO: This trick depends on transition expiration times being
-      // monotonically decreasing in priority, but since expiration times
-      // currently correspond to `timeoutMs`, that might not be true if
-      // `timeoutMs` changes to something smaller before the previous transition
-      // resolves. But this is a temporary edge case, since we're about to
-      // remove the correspondence between `timeoutMs` and the expiration time.
-      const oldPendingExpirationTime = instance.pendingExpirationTime;
-      while (
-        oldPendingExpirationTime !== NoWork &&
-        oldPendingExpirationTime <= expirationTime
-      ) {
-        // Temporary hack to make pendingExpirationTime monotonically decreasing
-        if (resolvedConfig === null) {
-          resolvedConfig = {
-            timeoutMs: 5250,
-          };
-        } else {
-          resolvedConfig = {
-            timeoutMs: (resolvedConfig.timeoutMs | 0 || 5000) + 250,
-            busyDelayMs: resolvedConfig.busyDelayMs,
-            busyMinDurationMs: resolvedConfig.busyMinDurationMs,
-          };
-        }
-        expirationTime = computeExpirationForFiber(
-          currentTime,
-          fiber,
-          resolvedConfig,
-        );
-      }
-      instance.pendingExpirationTime = expirationTime;
-
-      scheduleWork(fiber, expirationTime);
-      const previousConfig = ReactCurrentBatchConfig.suspense;
-      ReactCurrentBatchConfig.suspense = resolvedConfig;
-      try {
-        callback();
-      } finally {
-        ReactCurrentBatchConfig.suspense = previousConfig;
-      }
-    },
-  );
-}
-
 function mountTransition(
   config: SuspenseConfig | void | null,
 ): [(() => void) => void, boolean] {
   const hook = mountWorkInProgressHook();
-
+  const fiber = ((currentlyRenderingFiber: any): Fiber);
   const instance: TransitionInstance = {
     pendingExpirationTime: NoWork,
+    fiber,
   };
   // TODO: Intentionally storing this on the queue field to avoid adding a new/
   // one; `queue` should be a union.
@@ -1270,15 +1195,9 @@ function mountTransition(
   // Then we don't have to recompute the callback whenever it changes. However,
   // if we don't end up changing the API, we should at least optimize this
   // to use the same hook instead of a separate hook just for the callback.
-  const start = mountCallback(
-    startTransition.bind(
-      null,
-      ((currentlyRenderingFiber: any): Fiber),
-      instance,
-      config,
-    ),
-    [config],
-  );
+  const start = mountCallback(startTransition.bind(null, instance, config), [
+    config,
+  ]);
 
   const resolvedExpirationTime = NoWork;
   hook.memoizedState = {
@@ -1374,15 +1293,9 @@ function updateTransition(
     resolvedExpirationTime: newResolvedExpirationTime,
   };
 
-  const start = updateCallback(
-    startTransition.bind(
-      null,
-      ((currentlyRenderingFiber: any): Fiber),
-      instance,
-      config,
-    ),
-    [config],
-  );
+  const start = updateCallback(startTransition.bind(null, instance, config), [
+    config,
+  ]);
 
   return [start, newIsPending];
 }
@@ -1444,6 +1357,7 @@ function dispatchAction<S, A>(
   } else {
     const currentTime = requestCurrentTimeForUpdate();
     const suspenseConfig = requestCurrentSuspenseConfig();
+    const transition = requestCurrentTransition();
     const expirationTime = computeExpirationForFiber(
       currentTime,
       fiber,
@@ -1524,6 +1438,20 @@ function dispatchAction<S, A>(
         warnIfNotCurrentlyActingUpdatesInDev(fiber);
       }
     }
+
+    if (transition !== null) {
+      const prevPendingTransition = queue.pendingTransition;
+      if (transition !== prevPendingTransition) {
+        queue.pendingTransition = transition;
+        if (prevPendingTransition !== null) {
+          // There's already a pending transition on this queue. The new
+          // transition supersedes the old one. Turn of the `isPending` state
+          // of the previous transition.
+          cancelPendingTransition(prevPendingTransition);
+        }
+      }
+    }
+
     scheduleWork(fiber, expirationTime);
   }
 }

packages/react-reconciler/src/ReactFiberSuspenseConfig.js

@@ -9,6 +9,7 @@
 
 import ReactSharedInternals from 'shared/ReactSharedInternals';
 
+// TODO: Remove React.unstable_withSuspenseConfig and move this to the renderer
 const {ReactCurrentBatchConfig} = ReactSharedInternals;
 
 export type SuspenseConfig = {|

packages/react-reconciler/src/ReactFiberTransition.js

@@ -0,0 +1,140 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @flow
+ */
+
+import type {Fiber} from './ReactFiber';
+import type {ExpirationTime} from './ReactFiberExpirationTime';
+import type {SuspenseConfig} from './ReactFiberSuspenseConfig';
+
+import ReactSharedInternals from 'shared/ReactSharedInternals';
+
+import {
+  UserBlockingPriority,
+  NormalPriority,
+  runWithPriority,
+  getCurrentPriorityLevel,
+} from './SchedulerWithReactIntegration';
+import {
+  scheduleUpdateOnFiber,
+  computeExpirationForFiber,
+  requestCurrentTimeForUpdate,
+} from './ReactFiberWorkLoop';
+import {NoWork} from './ReactFiberExpirationTime';
+
+const {ReactCurrentBatchConfig} = ReactSharedInternals;
+
+export type TransitionInstance = {|
+  pendingExpirationTime: ExpirationTime,
+  fiber: Fiber,
+|};
+
+// Inside `startTransition`, this is the transition instance that corresponds to
+// the `useTransition` hook.
+let currentTransition: TransitionInstance | null = null;
+
+// Inside `startTransition`, this is the expiration time of the update that
+// turns on `isPending`. We also use it to turn off the `isPending` of previous
+// transitions, if they exists.
+let userBlockingExpirationTime = NoWork;
+
+export function requestCurrentTransition(): TransitionInstance | null {
+  return currentTransition;
+}
+
+export function startTransition(
+  transitionInstance: TransitionInstance,
+  config: SuspenseConfig | null | void,
+  callback: () => void,
+) {
+  const fiber = transitionInstance.fiber;
+
+  let resolvedConfig: SuspenseConfig | null =
+    config === undefined ? null : config;
+
+  // TODO: runWithPriority shouldn't be necessary here. React should manage its
+  // own concept of priority, and only consult Scheduler for updates that are
+  // scheduled from outside a React context.
+  const priorityLevel = getCurrentPriorityLevel();
+  runWithPriority(
+    priorityLevel < UserBlockingPriority ? UserBlockingPriority : priorityLevel,
+    () => {
+      const currentTime = requestCurrentTimeForUpdate();
+      userBlockingExpirationTime = computeExpirationForFiber(
+        currentTime,
+        fiber,
+        null,
+      );
+      scheduleUpdateOnFiber(fiber, userBlockingExpirationTime);
+    },
+  );
+  runWithPriority(
+    priorityLevel > NormalPriority ? NormalPriority : priorityLevel,
+    () => {
+      const currentTime = requestCurrentTimeForUpdate();
+      let expirationTime = computeExpirationForFiber(
+        currentTime,
+        fiber,
+        resolvedConfig,
+      );
+      // Set the expiration time at which the pending transition will finish.
+      // Because there's only a single transition per useTransition hook, we
+      // don't need a queue here; we can cheat by only tracking the most
+      // recently scheduled transition.
+      // TODO: This trick depends on transition expiration times being
+      // monotonically decreasing in priority, but since expiration times
+      // currently correspond to `timeoutMs`, that might not be true if
+      // `timeoutMs` changes to something smaller before the previous transition
+      // resolves. But this is a temporary edge case, since we're about to
+      // remove the correspondence between `timeoutMs` and the expiration time.
+      const oldPendingExpirationTime = transitionInstance.pendingExpirationTime;
+      while (
+        oldPendingExpirationTime !== NoWork &&
+        oldPendingExpirationTime <= expirationTime
+      ) {
+        // Temporary hack to make pendingExpirationTime monotonically decreasing
+        if (resolvedConfig === null) {
+          resolvedConfig = {
+            timeoutMs: 5250,
+          };
+        } else {
+          resolvedConfig = {
+            timeoutMs: (resolvedConfig.timeoutMs | 0 || 5000) + 250,
+            busyDelayMs: resolvedConfig.busyDelayMs,
+            busyMinDurationMs: resolvedConfig.busyMinDurationMs,
+          };
+        }
+        expirationTime = computeExpirationForFiber(
+          currentTime,
+          fiber,
+          resolvedConfig,
+        );
+      }
+      transitionInstance.pendingExpirationTime = expirationTime;
+
+      scheduleUpdateOnFiber(fiber, expirationTime);
+      const previousConfig = ReactCurrentBatchConfig.suspense;
+      const previousTransition = currentTransition;
+      ReactCurrentBatchConfig.suspense = resolvedConfig;
+      currentTransition = transitionInstance;
+      try {
+        callback();
+      } finally {
+        ReactCurrentBatchConfig.suspense = previousConfig;
+        currentTransition = previousTransition;
+        userBlockingExpirationTime = NoWork;
+      }
+    },
+  );
+}
+
+export function cancelPendingTransition(prevTransition: TransitionInstance) {
+  // Turn off the `isPending` state of the previous transition, at the same
+  // priority we use to turn on the `isPending` state of the current transition.
+  prevTransition.pendingExpirationTime = NoWork;
+  scheduleUpdateOnFiber(prevTransition.fiber, userBlockingExpirationTime);
+}