Add supportsMicrotasks to the host config (#20809)

* Add `supportsMicrotasks` to the host config

Only certain renderers support scheduling a microtask, so we need a
renderer specific flag that we can toggle. That way it's off for some
renderers and on for others.

I copied the approach we use for the other optional parts of the host
config, like persistent mode and test selectors.

Why isn't the feature flag sufficient?

The feature flag modules, confusingly, are not renderer-specific, at
least when running the our tests against the source files. They are
meant to correspond to a release channel, not a renderer, but we got
confused at some point and haven't cleaned it up.

For example, when we run `yarn test`, Jest loads the flags from the
default `ReactFeatureFlags.js` module, even when we import the React
Native renderer — but in the actual builds, we load a different feature
flag module, `ReactFeatureFlags.native-oss.js.` There's no way in our
current Jest load a different host config for each renderer, because
they all just import the same module. We should solve this by creating
separate Jest project for each renderer, so that the flags loaded when
running against source are the same ones that we use in the
compiled bundles.

The feature flag (`enableDiscreteMicrotasks`) still exists — it's used
to set the React DOM host config's `supportsMicrotasks` flag to `true`.
(Same for React Noop) The important part is that turning on the feature
flag does *not* affect the other renderers, like React Native.

The host config will likely outlive the feature flag, too, since the
feature flag only exists so we can gradually roll it out and measure the
impact in production; once we do, we'll remove it. Whereas the host
config flag may continue to be used to disable the discrete microtask
behavior for RN, because RN will likely use a native (non-JavaScript)
API to schedule its tasks.

* Add `supportsMicrotask` to react-reconciler README

Author: acdlite

packages/react-art/src/ReactARTHostConfig.js

@@ -249,6 +249,7 @@ export * from 'react-reconciler/src/ReactFiberHostConfigWithNoPersistence';
 export * from 'react-reconciler/src/ReactFiberHostConfigWithNoHydration';
 export * from 'react-reconciler/src/ReactFiberHostConfigWithNoScopes';
 export * from 'react-reconciler/src/ReactFiberHostConfigWithNoTestSelectors';
+export * from 'react-reconciler/src/ReactFiberHostConfigWithNoMicrotasks';
 
 export function appendInitialChild(parentInstance, child) {
   if (typeof child === 'string') {
@@ -338,9 +339,6 @@ export function getChildHostContext() {
 export const scheduleTimeout = setTimeout;
 export const cancelTimeout = clearTimeout;
 export const noTimeout = -1;
-export function scheduleMicrotask(callback: Function) {
-  invariant(false, 'Not implemented.');
-}
 
 export function shouldSetTextContent(type, props) {
   return (

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

@@ -67,6 +67,7 @@ import {
   enableCreateEventHandleAPI,
   enableScopeAPI,
   enableNewReconciler,
+  enableDiscreteEventMicroTasks,
 } from 'shared/ReactFeatureFlags';
 import {HostComponent, HostText} from 'react-reconciler/src/ReactWorkTags';
 import {listenToAllSupportedEvents} from '../events/DOMPluginEventSystem';
@@ -399,6 +400,11 @@ export const scheduleTimeout: any =
 export const cancelTimeout: any =
   typeof clearTimeout === 'function' ? clearTimeout : (undefined: any);
 export const noTimeout = -1;
+
+// -------------------
+//     Microtasks
+// -------------------
+export const supportsMicrotasks = enableDiscreteEventMicroTasks;
 export const scheduleMicrotask: any =
   typeof queueMicrotask === 'function'
     ? queueMicrotask

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

@@ -193,6 +193,7 @@ export * from 'react-reconciler/src/ReactFiberHostConfigWithNoMutation';
 export * from 'react-reconciler/src/ReactFiberHostConfigWithNoHydration';
 export * from 'react-reconciler/src/ReactFiberHostConfigWithNoScopes';
 export * from 'react-reconciler/src/ReactFiberHostConfigWithNoTestSelectors';
+export * from 'react-reconciler/src/ReactFiberHostConfigWithNoMicrotasks';
 
 export function appendInitialChild(
   parentInstance: Instance,
@@ -360,9 +361,6 @@ export const warnsIfNotActing = false;
 export const scheduleTimeout = setTimeout;
 export const cancelTimeout = clearTimeout;
 export const noTimeout = -1;
-export function scheduleMicrotask(callback: Function) {
-  invariant(false, 'Not implemented.');
-}
 
 // -------------------
 //     Persistence

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

@@ -97,6 +97,7 @@ export * from 'react-reconciler/src/ReactFiberHostConfigWithNoPersistence';
 export * from 'react-reconciler/src/ReactFiberHostConfigWithNoHydration';
 export * from 'react-reconciler/src/ReactFiberHostConfigWithNoScopes';
 export * from 'react-reconciler/src/ReactFiberHostConfigWithNoTestSelectors';
+export * from 'react-reconciler/src/ReactFiberHostConfigWithNoMicrotasks';
 
 export function appendInitialChild(
   parentInstance: Instance,
@@ -255,9 +256,6 @@ export const warnsIfNotActing = true;
 export const scheduleTimeout = setTimeout;
 export const cancelTimeout = clearTimeout;
 export const noTimeout = -1;
-export function scheduleMicrotask(callback: Function) {
-  invariant(false, 'Not implemented.');
-}
 
 export function shouldSetTextContent(type: string, props: Props): boolean {
   // TODO (bvaughn) Revisit this decision.

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

@@ -27,7 +27,10 @@ import {
   LegacyRoot,
 } from 'react-reconciler/src/ReactRootTags';
 
-import {enableNativeEventPriorityInference} from 'shared/ReactFeatureFlags';
+import {
+  enableNativeEventPriorityInference,
+  enableDiscreteEventMicroTasks,
+} from 'shared/ReactFeatureFlags';
 import ReactSharedInternals from 'shared/ReactSharedInternals';
 import enqueueTask from 'shared/enqueueTask';
 const {IsSomeRendererActing} = ReactSharedInternals;
@@ -372,6 +375,8 @@ function createReactNoop(reconciler: Function, useMutation: boolean) {
     scheduleTimeout: setTimeout,
     cancelTimeout: clearTimeout,
     noTimeout: -1,
+
+    supportsMicrotasks: enableDiscreteEventMicroTasks,
     scheduleMicrotask:
       typeof queueMicrotask === 'function'
         ? queueMicrotask

packages/react-reconciler/README.md

@@ -203,9 +203,12 @@ 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`.
 
+#### `supportsMicrotask`
+
+Set this to true to indicate that your renderer supports `scheduleMicrotask`. We use microtasks as part of our discrete event implementation in React DOM. If you're not sure if your renderer should support this, you probably should. The option to not implement `scheduleMicrotask` exists so that platforms with more control over user events, like React Native, can choose to use a different mechanism.
 #### `scheduleMicrotask(fn)`
 
-You can proxy this to `queueMicrotask` or its equivalent in your environment.
+Optional. You can proxy this to `queueMicrotask` or its equivalent in your environment.
 
 #### `isPrimaryRenderer`
 

packages/react-reconciler/src/ReactFiberHostConfigWithNoMicrotasks.js

@@ -0,0 +1,26 @@
+/**
+ * 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 invariant from 'shared/invariant';
+
+// Renderers that don't support microtasks
+// can re-export everything from this module.
+
+function shim(...args: any) {
+  invariant(
+    false,
+    'The current renderer does not support microtasks. ' +
+      'This error is likely caused by a bug in React. ' +
+      'Please file an issue.',
+  );
+}
+
+// Test selectors (when unsupported)
+export const supportsMicrotasks = false;
+export const scheduleMicrotask = shim;

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

@@ -93,8 +93,9 @@ import {
   warnsIfNotActing,
   afterActiveInstanceBlur,
   clearContainer,
-  scheduleMicrotask,
   getCurrentEventPriority,
+  supportsMicrotasks,
+  scheduleMicrotask,
 } from './ReactFiberHostConfig';
 
 import {
@@ -219,7 +220,6 @@ import {
   syncNestedUpdateFlag,
 } from './ReactProfilerTimer.new';
 
-import {enableDiscreteEventMicroTasks} from 'shared/ReactFeatureFlags';
 // DEV stuff
 import getComponentName from 'shared/getComponentName';
 import ReactStrictModeWarnings from './ReactStrictModeWarnings.new';
@@ -756,7 +756,7 @@ function ensureRootIsScheduled(root: FiberRoot, currentTime: number) {
       performSyncWorkOnRoot.bind(null, root),
     );
   } else if (
-    enableDiscreteEventMicroTasks &&
+    supportsMicrotasks &&
     newCallbackPriority === InputDiscreteLanePriority
   ) {
     scheduleMicrotask(performSyncWorkOnRoot.bind(null, root));

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

@@ -93,8 +93,9 @@ import {
   warnsIfNotActing,
   afterActiveInstanceBlur,
   clearContainer,
-  scheduleMicrotask,
   getCurrentEventPriority,
+  supportsMicrotasks,
+  scheduleMicrotask,
 } from './ReactFiberHostConfig';
 
 import {
@@ -219,7 +220,6 @@ import {
   syncNestedUpdateFlag,
 } from './ReactProfilerTimer.old';
 
-import {enableDiscreteEventMicroTasks} from 'shared/ReactFeatureFlags';
 // DEV stuff
 import getComponentName from 'shared/getComponentName';
 import ReactStrictModeWarnings from './ReactStrictModeWarnings.old';
@@ -756,7 +756,7 @@ function ensureRootIsScheduled(root: FiberRoot, currentTime: number) {
       performSyncWorkOnRoot.bind(null, root),
     );
   } else if (
-    enableDiscreteEventMicroTasks &&
+    supportsMicrotasks &&
     newCallbackPriority === InputDiscreteLanePriority
   ) {
     scheduleMicrotask(performSyncWorkOnRoot.bind(null, root));

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

@@ -54,7 +54,6 @@ export const shouldSetTextContent = $$$hostConfig.shouldSetTextContent;
 export const createTextInstance = $$$hostConfig.createTextInstance;
 export const scheduleTimeout = $$$hostConfig.scheduleTimeout;
 export const cancelTimeout = $$$hostConfig.cancelTimeout;
-export const scheduleMicrotask = $$$hostConfig.scheduleMicrotask;
 export const noTimeout = $$$hostConfig.noTimeout;
 export const now = $$$hostConfig.now;
 export const isPrimaryRenderer = $$$hostConfig.isPrimaryRenderer;
@@ -75,6 +74,13 @@ export const prepareScopeUpdate = $$$hostConfig.preparePortalMount;
 export const getInstanceFromScope = $$$hostConfig.getInstanceFromScope;
 export const getCurrentEventPriority = $$$hostConfig.getCurrentEventPriority;
 
+// -------------------
+//      Microtasks
+//     (optional)
+// -------------------
+export const supportsMicrotasks = $$$hostConfig.supportsMicrotasks;
+export const scheduleMicrotask = $$$hostConfig.scheduleMicrotask;
+
 // -------------------
 //      Test selectors
 //     (optional)