Appearance: Dedupe colorScheme Validation Logic

Summary:
Currently, the implementation of `Appearance` duplicates the validation logic of string `colorScheme` values multiple times.

This leads to more complicated code and also unnecessary work in certain edge cases (e.g. when `NativeAppearance` is not registered).

This refactors `Appearance` to be simpler and to do less work. I've also configured `NativeAppearance.setColorScheme` to be non-nullable because it has existed since 2023.

Changelog:
[Internal]

Differential Revision: D61567881

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

@@ -19,9 +19,8 @@ import NativeAppearance, {
 } from './NativeAppearance';
 import invariant from 'invariant';
 
-type AppearanceListener = (preferences: AppearancePreferences) => void;
 const eventEmitter = new EventEmitter<{
-  change: [AppearancePreferences],
+  change: [{colorScheme: ?ColorSchemeName}],
 }>();
 
 type NativeAppearanceEventDefinitions = {
@@ -32,24 +31,15 @@ if (NativeAppearance != null) {
   new NativeEventEmitter<NativeAppearanceEventDefinitions>(
     NativeAppearance,
   ).addListener('appearanceChanged', (newAppearance: AppearancePreferences) => {
-    const {colorScheme} = newAppearance;
-    invariant(
-      colorScheme === 'dark' || colorScheme === 'light' || colorScheme == null,
-      "Unrecognized color scheme. Did you mean 'dark' or 'light'?",
-    );
+    const colorScheme = toColorScheme(newAppearance.colorScheme);
     eventEmitter.emit('change', {colorScheme});
   });
 }
 
 /**
- * Note: Although color scheme is available immediately, it may change at any
- * time. Any rendering logic or styles that depend on this should try to call
- * this function on every render, rather than caching the value (for example,
- * using inline styles rather than setting a value in a `StyleSheet`).
- *
- * Example: `const colorScheme = Appearance.getColorScheme();`
- *
- * @returns {?ColorSchemeName} Value for the color scheme preference.
+ * Returns the current color scheme preference. This value may change, so the
+ * value should not be cached without either listening to changes or using
+ * the `useColorScheme` hook.
  */
 export function getColorScheme(): ?ColorSchemeName {
   if (__DEV__) {
@@ -59,37 +49,32 @@ export function getColorScheme(): ?ColorSchemeName {
       return 'light';
     }
   }
-
-  // TODO: (hramos) T52919652 Use ?ColorSchemeName once codegen supports union
-  const nativeColorScheme: ?string =
-    NativeAppearance == null ? null : NativeAppearance.getColorScheme() || null;
-  invariant(
-    nativeColorScheme === 'dark' ||
-      nativeColorScheme === 'light' ||
-      nativeColorScheme == null,
-    "Unrecognized color scheme. Did you mean 'dark' or 'light'?",
-  );
-  return nativeColorScheme;
+  return toColorScheme(NativeAppearance?.getColorScheme());
 }
 
+/**
+ * Updates the current color scheme to the supplied value.
+ */
 export function setColorScheme(colorScheme: ?ColorSchemeName): void {
-  const nativeColorScheme = colorScheme == null ? 'unspecified' : colorScheme;
-
-  invariant(
-    colorScheme === 'dark' || colorScheme === 'light' || colorScheme == null,
-    "Unrecognized color scheme. Did you mean 'dark', 'light' or null?",
-  );
-
-  if (NativeAppearance != null && NativeAppearance.setColorScheme != null) {
-    NativeAppearance.setColorScheme(nativeColorScheme);
-  }
+  NativeAppearance?.setColorScheme(toColorScheme(colorScheme) ?? 'unspecified');
 }
 
 /**
  * Add an event handler that is fired when appearance preferences change.
  */
 export function addChangeListener(
-  listener: AppearanceListener,
+  listener: ({colorScheme: ?ColorSchemeName}) => void,
 ): EventSubscription {
   return eventEmitter.addListener('change', listener);
 }
+
+/**
+ * TODO: (hramos) T52919652 Use ?ColorSchemeName once codegen supports union
+ */
+function toColorScheme(colorScheme: ?string): ?ColorSchemeName {
+  invariant(
+    colorScheme === 'dark' || colorScheme === 'light' || colorScheme == null,
+    "Unrecognized color scheme. Did you mean 'dark', 'light' or null?",
+  );
+  return colorScheme;
+}

packages/react-native/Libraries/__tests__/__snapshots__/public-api-test.js.snap

@@ -9155,11 +9155,10 @@ declare export default typeof UTFSequence;
 `;
 
 exports[`public API should not change unintentionally Libraries/Utilities/Appearance.js 1`] = `
-"type AppearanceListener = (preferences: AppearancePreferences) => void;
-declare export function getColorScheme(): ?ColorSchemeName;
+"declare export function getColorScheme(): ?ColorSchemeName;
 declare export function setColorScheme(colorScheme: ?ColorSchemeName): void;
 declare export function addChangeListener(
-  listener: AppearanceListener
+  listener: ({ colorScheme: ?ColorSchemeName }) => void
 ): EventSubscription;
 "
 `;

packages/react-native/src/private/specs/modules/NativeAppearance.js

@@ -14,19 +14,19 @@ import * as TurboModuleRegistry from '../../../../Libraries/TurboModule/TurboMod
 
 export type ColorSchemeName = 'light' | 'dark';
 
-export type AppearancePreferences = {|
+export type AppearancePreferences = {
   // TODO: (hramos) T52919652 Use ?ColorSchemeName once codegen supports union
   // types.
   /* 'light' | 'dark' */
   colorScheme?: ?string,
-|};
+};
 
 export interface Spec extends TurboModule {
   // TODO: (hramos) T52919652 Use ?ColorSchemeName once codegen supports union
   // types.
   /* 'light' | 'dark' */
   +getColorScheme: () => ?string;
-  +setColorScheme?: (colorScheme: string) => void;
+  +setColorScheme: (colorScheme: string) => void;
 
   // RCTEventEmitter
   +addListener: (eventName: string) => void;

packages/rn-tester/js/examples/Appearance/AppearanceExample.js

@@ -8,31 +8,26 @@
  * @flow
  */
 
-import type {
-  AppearancePreferences,
-  ColorSchemeName,
-} from 'react-native/Libraries/Utilities/NativeAppearance';
+import type {ColorSchemeName} from 'react-native/Libraries/Utilities/NativeAppearance';
 
 import {RNTesterThemeContext, themes} from '../../components/RNTesterTheme';
 import * as React from 'react';
 import {useEffect, useState} from 'react';
 import {Appearance, Button, Text, View, useColorScheme} from 'react-native';
 
 function ColorSchemeSubscription() {
-  const [colorScheme, setScheme] = useState<?ColorSchemeName | string>(
+  const [colorScheme, setColorScheme] = useState<?ColorSchemeName | string>(
     Appearance.getColorScheme(),
   );
 
   useEffect(() => {
     const subscription = Appearance.addChangeListener(
-      (preferences: AppearancePreferences) => {
-        const {colorScheme: scheme} = preferences;
-        setScheme(scheme);
+      ({colorScheme: newColorScheme}: {colorScheme: ?ColorSchemeName}) => {
+        setColorScheme(newColorScheme);
       },
     );
-
-    return () => subscription?.remove();
-  }, [setScheme]);
+    return () => subscription.remove();
+  }, [setColorScheme]);
 
   return (
     <RNTesterThemeContext.Consumer>