Appearance: stop caching colorScheme in JS

Summary:
At times, the OS updates the color scheme without any active listener on RN side, e.g. if all RCTRootView's in iOS have been deallocated, no one will tell JS that the color scheme changes. So let's just always ask native side for the latest value.

Changelog: [Internal]

Reviewed By: yungsters

Differential Revision: D18117334

fbshipit-source-id: e8564fb284c5720061592ba72e5b4907e5b48853

Libraries/Utilities/Appearance.js

@@ -21,18 +21,6 @@ import invariant from 'invariant';
 type AppearanceListener = (preferences: AppearancePreferences) => void;
 const eventEmitter = new EventEmitter();
 
-// 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'?",
-);
-
-let currentColorScheme: ?ColorSchemeName = nativeColorScheme;
-
 if (NativeAppearance) {
   const nativeEventEmitter = new NativeEventEmitter(NativeAppearance);
   nativeEventEmitter.addListener(
@@ -45,7 +33,6 @@ if (NativeAppearance) {
           colorScheme == null,
         "Unrecognized color scheme. Did you mean 'dark' or 'light'?",
       );
-      currentColorScheme = colorScheme;
       eventEmitter.emit('change', {colorScheme});
     },
   );
@@ -63,7 +50,18 @@ module.exports = {
    * @returns {?ColorSchemeName} Value for the color scheme preference.
    */
   getColorScheme(): ?ColorSchemeName {
-    return currentColorScheme;
+    // 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;
   },
   /**
    * Add an event handler that is fired when appearance preferences change.