Import dynamic RN flags from external module

Internal feature flags that we wish to control with a GK can now be
imported from an external module, which I've called
"ReactNativeInternalFeatureFlags".

We'll need to add this module to the downstream repo.

We can't yet use this in our tests, because we don't have a test
configuration that runs against the React Native feature flags fork. We
should set up that up the same way we did for www.

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

@@ -0,0 +1,29 @@
+/**
+ * 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 strict
+ */
+
+import typeof * as ExportsType from './ReactFeatureFlags.native-fb-dynamic';
+import typeof * as DynamicFlagsType from 'ReactNativeInternalFeatureFlags';
+
+// In xplat, these flags are controlled by GKs. Because most GKs have some
+// population running in either mode, we should run our tests that way, too,
+//
+// Use __VARIANT__ to simulate a GK. The tests will be run twice: once
+// with the __VARIANT__ set to `true`, and once set to `false`.
+//
+// TODO: __VARIANT__ isn't supported for React Native flags yet. You can set the
+// flag here but it won't be set to `true` in any of our test runs. Need to
+// update the test configuration.
+
+export const enablePersistentOffscreenHostContainer = __VARIANT__;
+
+// Flow magic to verify the exports of this file match the original version.
+// eslint-disable-next-line no-unused-vars
+type Check<_X, Y: _X, X: Y = _X> = null;
+// eslint-disable-next-line no-unused-expressions
+(null: Check<ExportsType, DynamicFlagsType>);

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

@@ -10,6 +10,14 @@
 import typeof * as FeatureFlagsType from 'shared/ReactFeatureFlags';
 import typeof * as ExportsType from './ReactFeatureFlags.native-fb';
 
+// Re-export dynamic flags from the internal module. Intentionally using *
+// because this import is compiled to a `require` call.
+import * as dynamicFlags from 'ReactNativeInternalFeatureFlags';
+
+// We destructure each value before re-exporting to avoid a dynamic look-up on
+// the exports object every time a flag is read.
+export const {enablePersistentOffscreenHostContainer} = dynamicFlags;
+
 // The rest of the flags are static for better dead code elimination.
 export const enableDebugTracing = false;
 export const enableSchedulingProfiler = false;
@@ -60,8 +68,6 @@ export const disableSchedulerTimeoutInWorkLoop = false;
 export const enableLazyContextPropagation = false;
 export const enableSyncDefaultUpdates = true;
 export const allowConcurrentByDefault = true;
-// TODO: Import this from internal ReactNativeFeatureFlags instead
-export const enablePersistentOffscreenHostContainer = __EXPERIMENTAL__;
 
 // Flow magic to verify the exports of this file match the original version.
 // eslint-disable-next-line no-unused-vars

scripts/flow/config/flowconfig

@@ -34,6 +34,7 @@
 ./scripts/flow/react-devtools.js
 ./scripts/flow/react-native-host-hooks.js
 ./scripts/flow/react-relay-hooks.js
+./scripts/flow/xplat.js
 
 [lints]
 untyped-type-import=error

scripts/flow/xplat.js

@@ -0,0 +1,12 @@
+/**
+ * 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
+ */
+
+declare module 'ReactNativeInternalFeatureFlags' {
+  declare export var enablePersistentOffscreenHostContainer: boolean;
+}

scripts/rollup/bundles.js

@@ -79,7 +79,7 @@ const bundles = [
     moduleType: ISOMORPHIC,
     entry: 'react',
     global: 'React',
-    externals: [],
+    externals: ['ReactNativeInternalFeatureFlags'],
   },
 
   /******* Isomorphic Shared Subset *******/
@@ -105,7 +105,7 @@ const bundles = [
     moduleType: ISOMORPHIC,
     entry: 'react/jsx-runtime',
     global: 'JSXRuntime',
-    externals: ['react'],
+    externals: ['react', 'ReactNativeInternalFeatureFlags'],
   },
 
   /******* React JSX DEV Runtime *******/
@@ -124,7 +124,7 @@ const bundles = [
     moduleType: ISOMORPHIC,
     entry: 'react/jsx-dev-runtime',
     global: 'JSXDEVRuntime',
-    externals: ['react'],
+    externals: ['react', 'ReactNativeInternalFeatureFlags'],
   },
 
   /******* React Fetch Browser (experimental, new) *******/
@@ -372,6 +372,7 @@ const bundles = [
       'react',
       'ReactFlightNativeRelayServerIntegration',
       'JSResourceReferenceImpl',
+      'ReactNativeInternalFeatureFlags',
     ],
   },
 
@@ -385,6 +386,7 @@ const bundles = [
       'react',
       'ReactFlightNativeRelayClientIntegration',
       'JSResourceReferenceImpl',
+      'ReactNativeInternalFeatureFlags',
     ],
   },
 
@@ -432,7 +434,7 @@ const bundles = [
     moduleType: RENDERER,
     entry: 'react-native-renderer',
     global: 'ReactNativeRenderer',
-    externals: ['react-native'],
+    externals: ['react-native', 'ReactNativeInternalFeatureFlags'],
     babel: opts =>
       Object.assign({}, opts, {
         plugins: opts.plugins.concat([
@@ -462,7 +464,7 @@ const bundles = [
     moduleType: RENDERER,
     entry: 'react-native-renderer/fabric',
     global: 'ReactFabric',
-    externals: ['react-native'],
+    externals: ['react-native', 'ReactNativeInternalFeatureFlags'],
     babel: opts =>
       Object.assign({}, opts, {
         plugins: opts.plugins.concat([
@@ -499,7 +501,12 @@ const bundles = [
     moduleType: RENDERER,
     entry: 'react-test-renderer',
     global: 'ReactTestRenderer',
-    externals: ['react', 'scheduler', 'scheduler/unstable_mock'],
+    externals: [
+      'react',
+      'scheduler',
+      'scheduler/unstable_mock',
+      'ReactNativeInternalFeatureFlags',
+    ],
     babel: opts =>
       Object.assign({}, opts, {
         plugins: opts.plugins.concat([
@@ -692,7 +699,7 @@ const bundles = [
     moduleType: ISOMORPHIC,
     entry: 'scheduler',
     global: 'Scheduler',
-    externals: [],
+    externals: ['ReactNativeInternalFeatureFlags'],
   },
 
   /******* React Scheduler Mock (experimental) *******/
@@ -710,7 +717,7 @@ const bundles = [
     moduleType: ISOMORPHIC,
     entry: 'scheduler/unstable_mock',
     global: 'SchedulerMock',
-    externals: [],
+    externals: ['ReactNativeInternalFeatureFlags'],
   },
 
   /******* React Scheduler Post Task (experimental) *******/

scripts/rollup/modules.js

@@ -21,6 +21,7 @@ const importSideEffects = Object.freeze({
   'react-fetch/node': HAS_NO_SIDE_EFFECTS_ON_IMPORT,
   'react-dom': HAS_NO_SIDE_EFFECTS_ON_IMPORT,
   url: HAS_NO_SIDE_EFFECTS_ON_IMPORT,
+  ReactNativeInternalFeatureFlags: HAS_NO_SIDE_EFFECTS_ON_IMPORT,
 });
 
 // Bundles exporting globals that other modules rely on.
@@ -31,6 +32,7 @@ const knownGlobals = Object.freeze({
   'react-interactions/events/tap': 'ReactEventsTap',
   scheduler: 'Scheduler',
   'scheduler/unstable_mock': 'SchedulerMock',
+  ReactNativeInternalFeatureFlags: 'ReactNativeInternalFeatureFlags',
 });
 
 // Given ['react'] in bundle externals, returns { 'react': 'React' }.