Queue discrete events in microtask

packages/react-dom/src/client/ReactDOMHostConfig.js

@@ -392,7 +392,7 @@ export const queueMicrotask: any =
         Promise.resolve(null)
           .then(callback)
           .catch(handleErrorInNextTick)
-    : scheduleTimeout;
+    : scheduleTimeout; // TODO: Determine the best fallback here.
 
 function handleErrorInNextTick(error) {
   setTimeout(() => {

packages/react-dom/src/events/plugins/__tests__/ChangeEventPlugin-test.js

@@ -730,8 +730,15 @@ describe('ChangeEventPlugin', () => {
 
       // Flush callbacks.
       // Now the click update has flushed.
-      expect(Scheduler).toFlushAndYield(['render: ']);
-      expect(input.value).toBe('');
+      if (gate(flags => flags.enableDiscreteEventMicroTasks)) {
+        // Flush microtask queue.
+        await null;
+        expect(Scheduler).toHaveYielded(['render: ']);
+        expect(input.value).toBe('');
+      } else {
+        expect(Scheduler).toFlushAndYield(['render: ']);
+        expect(input.value).toBe('');
+      }
     });
 
     // @gate experimental

packages/react-dom/src/events/plugins/__tests__/SimpleEventPlugin-test.js

@@ -470,11 +470,24 @@ describe('SimpleEventPlugin', function() {
         'High-pri count: 7, Low-pri count: 0',
       ]);
 
-      // At the end, both counters should equal the total number of clicks
-      expect(Scheduler).toFlushAndYield([
-        'High-pri count: 8, Low-pri count: 0',
-        'High-pri count: 8, Low-pri count: 8',
-      ]);
+      if (gate(flags => flags.enableDiscreteEventMicroTasks)) {
+        // Flush the microtask queue
+        await null;
+
+        // At the end, both counters should equal the total number of clicks
+        expect(Scheduler).toHaveYielded([
+          'High-pri count: 8, Low-pri count: 0',
+
+          // TODO: with cancellation, this required another flush?
+          'High-pri count: 8, Low-pri count: 8',
+        ]);
+      } else {
+        // At the end, both counters should equal the total number of clicks
+        expect(Scheduler).toFlushAndYield([
+          'High-pri count: 8, Low-pri count: 0',
+          'High-pri count: 8, Low-pri count: 8',
+        ]);
+      }
       expect(button.textContent).toEqual('High-pri count: 8, Low-pri count: 8');
     });
   });

packages/react-reconciler/README.md

@@ -203,6 +203,10 @@ You can proxy this to `clearTimeout` or its equivalent in your environment.
 
 This is a property (not a function) that should be set to something that can never be a valid timeout ID. For example, you can set it to `-1`.
 
+#### `queueMicrotask(fn)`
+
+You can proxy this to `queueMicrotask` or its equivalent in your environment.
+
 #### `isPrimaryRenderer`
 
 This is a property (not a function) that should be set to `true` if your renderer is the main one on the page. For example, if you're writing a renderer for the Terminal, it makes sense to set it to `true`, but if your renderer is used *on top of* React DOM or some other existing renderer, set it to `false`.

packages/react-reconciler/src/ReactFiberWorkLoop.new.js

@@ -92,6 +92,7 @@ import {
   warnsIfNotActing,
   afterActiveInstanceBlur,
   clearContainer,
+  queueMicrotask,
 } from './ReactFiberHostConfig';
 
 import {
@@ -216,6 +217,7 @@ import {
   syncNestedUpdateFlag,
 } from './ReactProfilerTimer.new';
 
+import {enableDiscreteEventMicroTasks} from 'shared/ReactFeatureFlags';
 // DEV stuff
 import getComponentName from 'shared/getComponentName';
 import ReactStrictModeWarnings from './ReactStrictModeWarnings.new';
@@ -714,21 +716,34 @@ function ensureRootIsScheduled(root: FiberRoot, currentTime: number) {
     // Special case: There's nothing to work on.
     if (existingCallbackNode !== null) {
       cancelCallback(existingCallbackNode);
-      root.callbackNode = null;
-      root.callbackPriority = NoLanePriority;
     }
+    root.callbackNode = null;
+    root.callbackPriority = NoLanePriority;
     return;
   }
 
   // Check if there's an existing task. We may be able to reuse it.
-  if (existingCallbackNode !== null) {
-    const existingCallbackPriority = root.callbackPriority;
-    if (existingCallbackPriority === newCallbackPriority) {
-      // The priority hasn't changed. We can reuse the existing task. Exit.
-      return;
+  const existingCallbackPriority = root.callbackPriority;
+  if (existingCallbackPriority === newCallbackPriority) {
+    if (__DEV__) {
+      // If we're going to re-use an existing task, it needs to exist.
+      // Assume that discrete update microtasks are non-cancellable and null.
+      // TODO: Temporary until we confirm this warning is not fired.
+      if (
+        existingCallbackNode == null &&
+        existingCallbackPriority !== InputDiscreteLanePriority
+      ) {
+        console.error(
+          'Expected scheduled callback to exist. This error is likely caused by a bug in React. Please file an issue.',
+        );
+      }
     }
-    // The priority changed. Cancel the existing callback. We'll schedule a new
-    // one below.
+    // The priority hasn't changed. We can reuse the existing task. Exit.
+    return;
+  }
+
+  if (existingCallbackNode != null) {
+    // Cancel the existing callback. We'll schedule a new one below.
     cancelCallback(existingCallbackNode);
   }
 
@@ -737,6 +752,8 @@ function ensureRootIsScheduled(root: FiberRoot, currentTime: number) {
   if (newCallbackPriority === SyncLanePriority) {
     // Special case: Sync React callbacks are scheduled on a special
     // internal queue
+
+    // TODO: After enableDiscreteEventMicroTasks lands, we can remove the fake node.
     newCallbackNode = scheduleSyncCallback(
       performSyncWorkOnRoot.bind(null, root),
     );
@@ -745,6 +762,12 @@ function ensureRootIsScheduled(root: FiberRoot, currentTime: number) {
       ImmediateSchedulerPriority,
       performSyncWorkOnRoot.bind(null, root),
     );
+  } else if (
+    enableDiscreteEventMicroTasks &&
+    newCallbackPriority === InputDiscreteLanePriority
+  ) {
+    queueMicrotask(performSyncWorkOnRoot.bind(null, root));
+    newCallbackNode = null;
   } else {
     const schedulerPriorityLevel = lanePriorityToSchedulerPriority(
       newCallbackPriority,
@@ -1871,6 +1894,7 @@ function commitRootImpl(root, renderPriorityLevel) {
   // commitRoot never returns a continuation; it always finishes synchronously.
   // So we can clear these now to allow a new callback to be scheduled.
   root.callbackNode = null;
+  root.callbackPriority = NoLanePriority;
 
   // Update the first and last pending times on this root. The new first
   // pending time is whatever is left on the root fiber.

packages/react-reconciler/src/ReactFiberWorkLoop.old.js

@@ -34,6 +34,7 @@ import {
   disableSchedulerTimeoutInWorkLoop,
   enableDoubleInvokingEffects,
   skipUnmountedBoundaries,
+  enableDiscreteEventMicroTasks,
 } from 'shared/ReactFeatureFlags';
 import ReactSharedInternals from 'shared/ReactSharedInternals';
 import invariant from 'shared/invariant';
@@ -92,6 +93,7 @@ import {
   warnsIfNotActing,
   afterActiveInstanceBlur,
   clearContainer,
+  queueMicrotask,
 } from './ReactFiberHostConfig';
 
 import {
@@ -696,21 +698,34 @@ function ensureRootIsScheduled(root: FiberRoot, currentTime: number) {
     // Special case: There's nothing to work on.
     if (existingCallbackNode !== null) {
       cancelCallback(existingCallbackNode);
-      root.callbackNode = null;
-      root.callbackPriority = NoLanePriority;
     }
+    root.callbackNode = null;
+    root.callbackPriority = NoLanePriority;
     return;
   }
 
   // Check if there's an existing task. We may be able to reuse it.
-  if (existingCallbackNode !== null) {
-    const existingCallbackPriority = root.callbackPriority;
-    if (existingCallbackPriority === newCallbackPriority) {
-      // The priority hasn't changed. We can reuse the existing task. Exit.
-      return;
+  const existingCallbackPriority = root.callbackPriority;
+  if (existingCallbackPriority === newCallbackPriority) {
+    if (__DEV__) {
+      // If we're going to re-use an existing task, it needs to exist.
+      // Assume that discrete update microtasks are non-cancellable and null.
+      // TODO: Temporary until we confirm this warning is not fired.
+      if (
+        existingCallbackNode == null &&
+        existingCallbackPriority !== InputDiscreteLanePriority
+      ) {
+        console.error(
+          'Expected scheduled callback to exist. This error is likely caused by a bug in React. Please file an issue.',
+        );
+      }
     }
-    // The priority changed. Cancel the existing callback. We'll schedule a new
-    // one below.
+    // The priority hasn't changed. We can reuse the existing task. Exit.
+    return;
+  }
+
+  if (existingCallbackNode != null) {
+    // Cancel the existing callback. We'll schedule a new one below.
     cancelCallback(existingCallbackNode);
   }
 
@@ -719,6 +734,8 @@ function ensureRootIsScheduled(root: FiberRoot, currentTime: number) {
   if (newCallbackPriority === SyncLanePriority) {
     // Special case: Sync React callbacks are scheduled on a special
     // internal queue
+
+    // TODO: After enableDiscreteEventMicroTasks lands, we can remove the fake node.
     newCallbackNode = scheduleSyncCallback(
       performSyncWorkOnRoot.bind(null, root),
     );
@@ -727,6 +744,12 @@ function ensureRootIsScheduled(root: FiberRoot, currentTime: number) {
       ImmediateSchedulerPriority,
       performSyncWorkOnRoot.bind(null, root),
     );
+  } else if (
+    enableDiscreteEventMicroTasks &&
+    newCallbackPriority === InputDiscreteLanePriority
+  ) {
+    queueMicrotask(performSyncWorkOnRoot.bind(null, root));
+    newCallbackNode = null;
   } else {
     const schedulerPriorityLevel = lanePriorityToSchedulerPriority(
       newCallbackPriority,
@@ -1851,6 +1874,7 @@ function commitRootImpl(root, renderPriorityLevel) {
   // commitRoot never returns a continuation; it always finishes synchronously.
   // So we can clear these now to allow a new callback to be scheduled.
   root.callbackNode = null;
+  root.callbackPriority = NoLanePriority;
 
   // Update the first and last pending times on this root. The new first
   // pending time is whatever is left on the root fiber.

packages/react-reconciler/src/__tests__/ReactSuspenseWithNoopRenderer-test.js

@@ -3528,6 +3528,7 @@ describe('ReactSuspenseWithNoopRenderer', () => {
   });
 
   // @gate enableCache
+  // @gate !enableDiscreteEventMicroTasks
   it('regression: empty render at high priority causes update to be dropped', async () => {
     // Reproduces a bug where flushDiscreteUpdates starts a new (empty) render
     // pass which cancels a scheduled timeout and causes the fallback never to

packages/react-test-renderer/src/ReactTestHostConfig.js

@@ -228,7 +228,7 @@ export const queueMicrotask =
         Promise.resolve(null)
           .then(callback)
           .catch(handleErrorInNextTick)
-    : scheduleTimeout;
+    : scheduleTimeout; // TODO: Determine the best fallback here.
 
 function handleErrorInNextTick(error) {
   setTimeout(() => {

packages/shared/ReactFeatureFlags.js

@@ -152,3 +152,5 @@ export const disableSchedulerTimeoutInWorkLoop = false;
 
 // Experiment to simplify/improve how transitions are scheduled
 export const enableTransitionEntanglement = false;
+
+export const enableDiscreteEventMicroTasks = false;

packages/shared/forks/ReactFeatureFlags.native-fb.js

@@ -59,6 +59,7 @@ export const enableUseRefAccessWarning = false;
 export const enableRecursiveCommitTraversal = false;
 export const disableSchedulerTimeoutInWorkLoop = false;
 export const enableTransitionEntanglement = false;
+export const enableDiscreteEventMicroTasks = false;
 
 // Flow magic to verify the exports of this file match the original version.
 // eslint-disable-next-line no-unused-vars

packages/shared/forks/ReactFeatureFlags.native-oss.js

@@ -58,6 +58,7 @@ export const enableUseRefAccessWarning = false;
 export const enableRecursiveCommitTraversal = false;
 export const disableSchedulerTimeoutInWorkLoop = false;
 export const enableTransitionEntanglement = false;
+export const enableDiscreteEventMicroTasks = false;
 
 // Flow magic to verify the exports of this file match the original version.
 // eslint-disable-next-line no-unused-vars

packages/shared/forks/ReactFeatureFlags.test-renderer.js

@@ -58,6 +58,7 @@ export const enableUseRefAccessWarning = false;
 export const enableRecursiveCommitTraversal = false;
 export const disableSchedulerTimeoutInWorkLoop = false;
 export const enableTransitionEntanglement = false;
+export const enableDiscreteEventMicroTasks = false;
 
 // Flow magic to verify the exports of this file match the original version.
 // eslint-disable-next-line no-unused-vars

packages/shared/forks/ReactFeatureFlags.test-renderer.native.js

@@ -58,6 +58,7 @@ export const enableUseRefAccessWarning = false;
 export const enableRecursiveCommitTraversal = false;
 export const disableSchedulerTimeoutInWorkLoop = false;
 export const enableTransitionEntanglement = false;
+export const enableDiscreteEventMicroTasks = false;
 
 // Flow magic to verify the exports of this file match the original version.
 // eslint-disable-next-line no-unused-vars

packages/shared/forks/ReactFeatureFlags.test-renderer.www.js

@@ -58,6 +58,7 @@ export const enableUseRefAccessWarning = false;
 export const enableRecursiveCommitTraversal = false;
 export const disableSchedulerTimeoutInWorkLoop = false;
 export const enableTransitionEntanglement = false;
+export const enableDiscreteEventMicroTasks = false;
 
 // Flow magic to verify the exports of this file match the original version.
 // eslint-disable-next-line no-unused-vars

packages/shared/forks/ReactFeatureFlags.testing.js

@@ -58,6 +58,7 @@ export const enableUseRefAccessWarning = false;
 export const enableRecursiveCommitTraversal = false;
 export const disableSchedulerTimeoutInWorkLoop = false;
 export const enableTransitionEntanglement = false;
+export const enableDiscreteEventMicroTasks = false;
 
 // Flow magic to verify the exports of this file match the original version.
 // eslint-disable-next-line no-unused-vars

packages/shared/forks/ReactFeatureFlags.testing.www.js

@@ -58,6 +58,7 @@ export const enableUseRefAccessWarning = false;
 export const enableRecursiveCommitTraversal = false;
 export const disableSchedulerTimeoutInWorkLoop = false;
 export const enableTransitionEntanglement = false;
+export const enableDiscreteEventMicroTasks = false;
 
 // Flow magic to verify the exports of this file match the original version.
 // eslint-disable-next-line no-unused-vars

packages/shared/forks/ReactFeatureFlags.www-dynamic.js

@@ -56,3 +56,4 @@ export const enableUseRefAccessWarning = __VARIANT__;
 export const enableProfilerNestedUpdateScheduledHook = __VARIANT__;
 export const disableSchedulerTimeoutInWorkLoop = __VARIANT__;
 export const enableTransitionEntanglement = __VARIANT__;
+export const enableDiscreteEventMicroTasks = __VARIANT__;

packages/shared/forks/ReactFeatureFlags.www.js

@@ -32,6 +32,7 @@ export const {
   disableNativeComponentFrames,
   disableSchedulerTimeoutInWorkLoop,
   enableTransitionEntanglement,
+  enableDiscreteEventMicroTasks,
 } = dynamicFeatureFlags;
 
 // On WWW, __EXPERIMENTAL__ is used for a new modern build.