Import Scheduler directly, not via host config (#14984)

* Import Scheduler directly, not via host config

We currently schedule asynchronous tasks via the host config. (The host
config is a static/build-time dependency injection system that varies
across different renderers — DOM, native, test, and so on.) Instead of
calling platform APIs like `requestIdleCallback` directly, each renderer
implements a method called `scheduleDeferredCallback`.

We've since discovered that when scheduling tasks, it's crucial that
React work is placed in the same queue as other, non-React work on the
main thread. Otherwise, you easily end up in a starvation scenario where
rendering is constantly interrupted by less important tasks. You need a
centralized coordinator that is used both by React and by other
frameworks and application code. This coordinator must also have a
consistent API across all the different host environments, for
convention's sake and so product code is portable — e.g. so the same
component can work in both React Native and React Native Web.

This turned into the Scheduler package. We will have different builds of
Scheduler for each of our target platforms. With this approach, we treat
Scheduler like a built-in platform primitive that exists wherever React
is supported.

Now that we have this consistent interface, the indirection of the host
config no longer makes sense for the purpose of scheduling tasks. In
fact, we explicitly do not want renderers to scheduled task via any
system except the Scheduler package.

So, this PR removes `scheduleDeferredCallback` and its associated
methods from the host config in favor of directly importing Scheduler.

* Missed an extraneous export

packages/react-art/src/ReactARTHostConfig.js

@@ -343,8 +343,6 @@ export function getChildHostContext() {
 export const scheduleTimeout = setTimeout;
 export const cancelTimeout = clearTimeout;
 export const noTimeout = -1;
-export const schedulePassiveEffects = scheduleDeferredCallback;
-export const cancelPassiveEffects = cancelDeferredCallback;
 
 export function shouldSetTextContent(type, props) {
   return (

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

@@ -310,8 +310,6 @@ export const scheduleTimeout =
 export const cancelTimeout =
   typeof clearTimeout === 'function' ? clearTimeout : (undefined: any);
 export const noTimeout = -1;
-export const schedulePassiveEffects = scheduleDeferredCallback;
-export const cancelPassiveEffects = cancelDeferredCallback;
 
 // -------------------
 //     Mutation

packages/react-native-renderer/src/ReactFabricHostConfig.js

@@ -20,12 +20,6 @@ import {
   warnForStyleProps,
 } from './NativeMethodsMixinUtils';
 import {create, diff} from './ReactNativeAttributePayload';
-import {
-  now as ReactNativeFrameSchedulingNow,
-  cancelDeferredCallback as ReactNativeFrameSchedulingCancelDeferredCallback,
-  scheduleDeferredCallback as ReactNativeFrameSchedulingScheduleDeferredCallback,
-  shouldYield as ReactNativeFrameSchedulingShouldYield,
-} from './ReactNativeFrameScheduling';
 import {get as getViewConfigForType} from 'ReactNativeViewConfigRegistry';
 
 import deepFreezeAndThrowOnMutationInDev from 'deepFreezeAndThrowOnMutationInDev';
@@ -333,16 +327,10 @@ export function shouldSetTextContent(type: string, props: Props): boolean {
 
 // The Fabric renderer is secondary to the existing React Native renderer.
 export const isPrimaryRenderer = false;
-export const now = ReactNativeFrameSchedulingNow;
-export const scheduleDeferredCallback = ReactNativeFrameSchedulingScheduleDeferredCallback;
-export const cancelDeferredCallback = ReactNativeFrameSchedulingCancelDeferredCallback;
-export const shouldYield = ReactNativeFrameSchedulingShouldYield;
 
 export const scheduleTimeout = setTimeout;
 export const cancelTimeout = clearTimeout;
 export const noTimeout = -1;
-export const schedulePassiveEffects = scheduleDeferredCallback;
-export const cancelPassiveEffects = cancelDeferredCallback;
 
 // -------------------
 //     Persistence

packages/react-native-renderer/src/ReactNativeHostConfig.js

@@ -23,12 +23,6 @@ import {
   updateFiberProps,
 } from './ReactNativeComponentTree';
 import ReactNativeFiberHostComponent from './ReactNativeFiberHostComponent';
-import {
-  now as ReactNativeFrameSchedulingNow,
-  cancelDeferredCallback as ReactNativeFrameSchedulingCancelDeferredCallback,
-  scheduleDeferredCallback as ReactNativeFrameSchedulingScheduleDeferredCallback,
-  shouldYield as ReactNativeFrameSchedulingShouldYield,
-} from './ReactNativeFrameScheduling';
 
 export type Type = string;
 export type Props = Object;
@@ -234,17 +228,11 @@ export function resetAfterCommit(containerInfo: Container): void {
   // Noop
 }
 
-export const now = ReactNativeFrameSchedulingNow;
 export const isPrimaryRenderer = true;
-export const scheduleDeferredCallback = ReactNativeFrameSchedulingScheduleDeferredCallback;
-export const cancelDeferredCallback = ReactNativeFrameSchedulingCancelDeferredCallback;
-export const shouldYield = ReactNativeFrameSchedulingShouldYield;
 
 export const scheduleTimeout = setTimeout;
 export const cancelTimeout = clearTimeout;
 export const noTimeout = -1;
-export const schedulePassiveEffects = scheduleDeferredCallback;
-export const cancelPassiveEffects = cancelDeferredCallback;
 
 export function shouldDeprioritizeSubtree(type: string, props: Props): boolean {
   return false;

packages/react-noop-renderer/src/createReactNoop.js

@@ -304,18 +304,10 @@ function createReactNoop(reconciler: Function, useMutation: boolean) {
       return inst;
     },
 
-    scheduleDeferredCallback: Scheduler.unstable_scheduleCallback,
-    cancelDeferredCallback: Scheduler.unstable_cancelCallback,
-
-    shouldYield: Scheduler.unstable_shouldYield,
-
     scheduleTimeout: setTimeout,
     cancelTimeout: clearTimeout,
     noTimeout: -1,
 
-    schedulePassiveEffects: Scheduler.unstable_scheduleCallback,
-    cancelPassiveEffects: Scheduler.unstable_cancelCallback,
-
     prepareForCommit(): void {},
 
     resetAfterCommit(): void {},

packages/react-reconciler/src/ReactFiberScheduler.js

@@ -14,9 +14,7 @@ import type {Interaction} from 'scheduler/src/Tracing';
 
 // Intentionally not named imports because Rollup would use dynamic dispatch for
 // CommonJS interop named imports.
-// TODO: We're not using this import anymore, but I've left this here so we
-// don't accidentally use named imports when we add it back.
-// import * as Scheduler from 'scheduler';
+import * as Scheduler from 'scheduler';
 import {
   __interactionsRef,
   __subscriberRef,
@@ -78,17 +76,11 @@ import {
   setCurrentFiber,
 } from './ReactCurrentFiber';
 import {
-  now,
-  scheduleDeferredCallback,
-  cancelDeferredCallback,
-  shouldYield,
   prepareForCommit,
   resetAfterCommit,
   scheduleTimeout,
   cancelTimeout,
   noTimeout,
-  schedulePassiveEffects,
-  cancelPassiveEffects,
 } from './ReactFiberHostConfig';
 import {
   markPendingPriorityLevel,
@@ -172,6 +164,15 @@ import {
 } from './ReactFiberCommitWork';
 import {ContextOnlyDispatcher} from './ReactFiberHooks';
 
+// Intentionally not named imports because Rollup would use dynamic dispatch for
+// CommonJS interop named imports.
+const {
+  unstable_scheduleCallback: scheduleCallback,
+  unstable_cancelCallback: cancelCallback,
+  unstable_shouldYield: shouldYield,
+  unstable_now: now,
+} = Scheduler;
+
 export type Thenable = {
   then(resolve: () => mixed, reject?: () => mixed): mixed,
 };
@@ -598,7 +599,7 @@ function markLegacyErrorBoundaryAsFailed(instance: mixed) {
 
 function flushPassiveEffects() {
   if (passiveEffectCallbackHandle !== null) {
-    cancelPassiveEffects(passiveEffectCallbackHandle);
+    cancelCallback(passiveEffectCallbackHandle);
   }
   if (passiveEffectCallback !== null) {
     // We call the scheduled callback instead of commitPassiveEffects directly
@@ -807,7 +808,7 @@ function commitRoot(root: FiberRoot, finishedWork: Fiber): void {
       // here because that code is still in flux.
       callback = Scheduler_tracing_wrap(callback);
     }
-    passiveEffectCallbackHandle = schedulePassiveEffects(callback);
+    passiveEffectCallbackHandle = scheduleCallback(callback);
     passiveEffectCallback = callback;
   }
 
@@ -1978,7 +1979,7 @@ function scheduleCallbackWithExpirationTime(
       if (callbackID !== null) {
         // Existing callback has insufficient timeout. Cancel and schedule a
         // new one.
-        cancelDeferredCallback(callbackID);
+        cancelCallback(callbackID);
       }
     }
     // The request callback timer is already running. Don't start a new one.
@@ -1990,7 +1991,7 @@ function scheduleCallbackWithExpirationTime(
   const currentMs = now() - originalStartTimeMs;
   const expirationTimeMs = expirationTimeToMs(expirationTime);
   const timeout = expirationTimeMs - currentMs;
-  callbackID = scheduleDeferredCallback(performAsyncWork, {timeout});
+  callbackID = scheduleCallback(performAsyncWork, {timeout});
 }
 
 // For every call to renderRoot, one of onFatal, onComplete, onSuspend, and

packages/react-reconciler/src/ReactProfilerTimer.js

@@ -11,7 +11,11 @@ import type {Fiber} from './ReactFiber';
 
 import {enableProfilerTimer} from 'shared/ReactFeatureFlags';
 
-import {now} from './ReactFiberHostConfig';
+// Intentionally not named imports because Rollup would use dynamic dispatch for
+// CommonJS interop named imports.
+import * as Scheduler from 'scheduler';
+
+const {unstable_now: now} = Scheduler;
 
 export type ProfilerTimer = {
   getCommitTime(): number,

packages/react-reconciler/src/forks/ReactFiberHostConfig.custom.js

@@ -51,14 +51,9 @@ export const shouldSetTextContent = $$$hostConfig.shouldSetTextContent;
 export const shouldDeprioritizeSubtree =
   $$$hostConfig.shouldDeprioritizeSubtree;
 export const createTextInstance = $$$hostConfig.createTextInstance;
-export const scheduleDeferredCallback = $$$hostConfig.scheduleDeferredCallback;
-export const cancelDeferredCallback = $$$hostConfig.cancelDeferredCallback;
-export const shouldYield = $$$hostConfig.shouldYield;
 export const scheduleTimeout = $$$hostConfig.setTimeout;
 export const cancelTimeout = $$$hostConfig.clearTimeout;
 export const noTimeout = $$$hostConfig.noTimeout;
-export const schedulePassiveEffects = $$$hostConfig.schedulePassiveEffects;
-export const cancelPassiveEffects = $$$hostConfig.cancelPassiveEffects;
 export const now = $$$hostConfig.now;
 export const isPrimaryRenderer = $$$hostConfig.isPrimaryRenderer;
 export const supportsMutation = $$$hostConfig.supportsMutation;

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

@@ -7,7 +7,6 @@
  * @flow
  */
 
-import * as Scheduler from 'scheduler/unstable_mock';
 import warning from 'shared/warning';
 
 export type Type = string;
@@ -195,18 +194,10 @@ export function createTextInstance(
 }
 
 export const isPrimaryRenderer = false;
-// This approach enables `now` to be mocked by tests,
-// Even after the reconciler has initialized and read host config values.
-export const now = Scheduler.unstable_now;
-export const scheduleDeferredCallback = Scheduler.unstable_scheduleCallback;
-export const cancelDeferredCallback = Scheduler.unstable_cancelCallback;
-export const shouldYield = Scheduler.unstable_shouldYield;
 
 export const scheduleTimeout = setTimeout;
 export const cancelTimeout = clearTimeout;
 export const noTimeout = -1;
-export const schedulePassiveEffects = Scheduler.unstable_scheduleCallback;
-export const cancelPassiveEffects = Scheduler.unstable_cancelCallback;
 
 // -------------------
 //     Mutation

packages/react/src/__tests__/ReactProfiler-test.internal.js

@@ -261,7 +261,7 @@ describe('Profiler', () => {
       it('does not record times for components outside of Profiler tree', () => {
         // Mock the Scheduler module so we can track how many times the current
         // time is read
-        jest.mock('scheduler/unstable_mock', obj => {
+        jest.mock('scheduler', obj => {
           const ActualScheduler = require.requireActual(
             'scheduler/unstable_mock',
           );
@@ -300,8 +300,10 @@ describe('Profiler', () => {
           'read current time',
         ]);
 
-        // Remove mock
-        jest.unmock('scheduler/unstable_mock');
+        // Restore original mock
+        jest.mock('scheduler', () =>
+          require.requireActual('scheduler/unstable_mock'),
+        );
       });
 
       it('logs render times for both mount and update', () => {