Add LegacyFunction to preserve callback compatibility

packages/react-native/Libraries/Alert/Alert.js

@@ -27,7 +27,7 @@ export type AlertButtonStyle = 'default' | 'cancel' | 'destructive';
 export type AlertButton = {
   text?: string,
   // $FlowFixMe[unclear-type]
-  onPress?: ?((value?: string) => any) | ?Function,
+  onPress?: ?((value?: string) => any) | ?LegacyFunction,
   isPreferred?: boolean,
   style?: AlertButtonStyle,
   ...

packages/react-native/Libraries/Animated/AnimatedImplementation.js

@@ -559,12 +559,9 @@ const loopImpl = function (
 };
 
 function forkEventImpl(
-  // $FlowFixMe[unclear-type]
-  event: ?AnimatedEvent | ?Function,
-  // $FlowFixMe[unclear-type]
-  listener: Function,
-  // $FlowFixMe[unclear-type]
-): AnimatedEvent | Function {
+  event: ?AnimatedEvent | ?LegacyFunction,
+  listener: LegacyFunction,
+): AnimatedEvent | LegacyFunction {
   if (!event) {
     return listener;
   } else if (event instanceof AnimatedEvent) {
@@ -579,10 +576,8 @@ function forkEventImpl(
 }
 
 function unforkEventImpl(
-  // $FlowFixMe[unclear-type]
-  event: ?AnimatedEvent | ?Function,
-  // $FlowFixMe[unclear-type]
-  listener: Function,
+  event: ?AnimatedEvent | ?LegacyFunction,
+  listener: LegacyFunction,
 ): void {
   if (event && event instanceof AnimatedEvent) {
     event.__removeListener(listener);

packages/react-native/Libraries/PushNotificationIOS/PushNotificationIOS.js

@@ -319,8 +319,7 @@ class PushNotificationIOS {
    *
    * See https://reactnative.dev/docs/pushnotificationios#getapplicationiconbadgenumber
    */
-  // $FlowFixMe[unclear-type]
-  static getApplicationIconBadgeNumber(callback: Function): void {
+  static getApplicationIconBadgeNumber(callback: LegacyFunction): void {
     invariant(
       NativePushNotificationManagerIOS,
       'PushNotificationManager is not available.',
@@ -348,8 +347,7 @@ class PushNotificationIOS {
    *
    * See https://reactnative.dev/docs/pushnotificationios#getscheduledlocalnotifications
    */
-  // $FlowFixMe[unclear-type]
-  static getScheduledLocalNotifications(callback: Function): void {
+  static getScheduledLocalNotifications(callback: LegacyFunction): void {
     invariant(
       NativePushNotificationManagerIOS,
       'PushNotificationManager is not available.',
@@ -365,8 +363,7 @@ class PushNotificationIOS {
    */
   static addEventListener(
     type: PushNotificationEventName,
-    // $FlowFixMe[unclear-type]
-    handler: Function,
+    handler: LegacyFunction,
   ): void {
     invariant(
       type === 'notification' ||

packages/react-native/Libraries/Utilities/Dimensions.js

@@ -111,9 +111,7 @@ class Dimensions {
    */
   static addEventListener(
     type: 'change',
-    /* $FlowFixMe[unclear-type] Callers annotate the payload with assorted
-     * object shapes that any precise signature would reject. */
-    handler: Function,
+    handler: LegacyFunction,
   ): EventSubscription {
     invariant(
       type === 'change',

packages/react-native/flow/LegacyFunction.js

@@ -0,0 +1,13 @@
+/**
+ * Copyright (c) Meta Platforms, Inc. and 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
+ * @format
+ */
+
+// The legacy Flow `Function` type, which is `any`. The generated TypeScript API
+// (scripts/js-api/build-types) translates this type to `Function`.
+declare type LegacyFunction = any;

scripts/js-api/build-types/transforms/flow/__tests__/replaceLegacyFunctionWithFunction-test.js

@@ -0,0 +1,62 @@
+/**
+ * Copyright (c) Meta Platforms, Inc. and 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-local
+ * @format
+ */
+
+const translateSourceFile = require('../../../translateSourceFile');
+const replaceLegacyFunctionWithFunction = require('../replaceLegacyFunctionWithFunction');
+const {parse, print} = require('flow-transform');
+
+const prettierOptions = {parser: 'babel'};
+
+async function translate(code: string): Promise<string> {
+  const parsed = await parse(code);
+  const result = await replaceLegacyFunctionWithFunction(parsed);
+  return print(result.ast, result.mutatedCode, prettierOptions);
+}
+
+describe('replaceLegacyFunctionWithFunction', () => {
+  test('replaces LegacyFunction in return annotations', async () => {
+    expect(await translate('function foo(): LegacyFunction {}')).toBe(
+      'function foo(): Function {}\n',
+    );
+  });
+
+  test('replaces LegacyFunction in nullable unions', async () => {
+    expect(
+      await translate('let foo: ?((value?: string) => any) | ?LegacyFunction;'),
+    ).toBe('let foo: ?((value?: string) => any) | ?Function;\n');
+  });
+
+  test('replaces LegacyFunction in type aliases', async () => {
+    expect(await translate('type Foo = LegacyFunction;')).toBe(
+      'type Foo = Function;\n',
+    );
+  });
+
+  test('preserves unrelated and qualified types', async () => {
+    const code = 'type Foo = Function | Other.LegacyFunction;';
+    expect(await translate(code)).toBe(code + '\n');
+  });
+
+  test('preserves Function in the generated TypeScript API', async () => {
+    const source = `export type Callbacks = {
+      onPress?: ?((value?: string) => any) | ?LegacyFunction,
+      listener: LegacyFunction,
+    };
+    export function forkEvent(event: ?LegacyFunction): LegacyFunction {
+      return event;
+    }`;
+    const result = await translateSourceFile(source, __filename);
+    const original = await translateSourceFile(
+      source.replaceAll('LegacyFunction', 'Function'),
+      __filename,
+    );
+    expect(result).toEqual(original);
+  });
+});

scripts/js-api/build-types/transforms/flow/replaceLegacyFunctionWithFunction.js

@@ -0,0 +1,34 @@
+/**
+ * Copyright (c) Meta Platforms, Inc. and 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-local
+ * @format
+ */
+
+import type {TransformVisitor} from 'flow-transform';
+import type {ParseResult} from 'flow-transform/dist/transform/parse';
+import type {TransformASTResult} from 'flow-transform/dist/transform/transformAST';
+
+const {transformAST} = require('flow-transform/dist/transform/transformAST');
+
+const visitors: TransformVisitor = context => ({
+  GenericTypeAnnotation(node): void {
+    if (node.id.type === 'Identifier' && node.id.name === 'LegacyFunction') {
+      context.modifyNodeInPlace(node.id, {name: 'Function'});
+    }
+  },
+});
+
+/**
+ * Replaces LegacyFunction with Function to preserve the TypeScript API.
+ */
+async function replaceLegacyFunctionWithFunction(
+  source: ParseResult,
+): Promise<TransformASTResult> {
+  return transformAST(source, visitors);
+}
+
+module.exports = replaceLegacyFunctionWithFunction;

scripts/js-api/build-types/translateSourceFile.js

@@ -24,6 +24,7 @@ const preTransforms: Array<PreTransformFn> = [
   require('./transforms/flow/replaceRequiresWithImports'),
   require('./transforms/flow/replaceEmptyWithNever'),
   require('./transforms/flow/replaceStringishWithString'),
+  require('./transforms/flow/replaceLegacyFunctionWithFunction'),
   require('./transforms/flow/replaceLegacyObjectWithObject'),
   require('./transforms/flow/replaceNullablePropertiesWithUndefined'),
   require('./transforms/flow/reattachDocComments'),