Guard against missing native module

Reviewed By: martinbigio

Differential Revision: D4579114

fbshipit-source-id: a2e9f634748e1c56eb9867bfb9e3e66e9cdc5e93

Libraries/AppState/AppState.js

@@ -11,6 +11,7 @@
  */
 'use strict';
 
+const EventEmitter = require('EventEmitter');
 const NativeEventEmitter = require('NativeEventEmitter');
 const NativeModules = require('NativeModules');
 const RCTAppState = NativeModules.AppState;
@@ -25,6 +26,9 @@ const invariant = require('fbjs/lib/invariant');
  * AppState is frequently used to determine the intent and proper behavior when
  * handling push notifications.
  *
+ * This module depends on the native RCTAppState module. If you don't include it,
+ * `AppState.isAvailable` will return `false`, and any method calls will throw.
+ *
  * ### App States
  *
  *  - `active` - The app is running in the foreground
@@ -86,6 +90,7 @@ class AppState extends NativeEventEmitter {
 
   _eventHandlers: Object;
   currentState: ?string;
+  isAvailable: boolean = true;
 
   constructor() {
     super(RCTAppState);
@@ -173,6 +178,45 @@ class AppState extends NativeEventEmitter {
   }
 }
 
-AppState = new AppState();
+function throwMissingNativeModule() {
+  invariant(
+    false,
+    'Cannot use AppState module when native RCTAppState is not included in the build.\n' +
+    'Either include it, or check AppState.isAvailable before calling any methods.'
+  );
+}
+
+class MissingNativeAppStateShim extends EventEmitter {
+  // AppState
+  isAvailable: boolean = false;
+  currentState: ?string = null;
+
+  addEventListener() {
+    throwMissingNativeModule();
+  }
+
+  removeEventListener() {
+    throwMissingNativeModule();
+  }
+
+  // EventEmitter
+  addListener() {
+    throwMissingNativeModule();
+  }
+
+  removeAllListeners() {
+    throwMissingNativeModule();
+  }
+
+  removeSubscription() {
+    throwMissingNativeModule();
+  }
+}
+
+// Guard against missing native module by throwing on first method call.
+// Keep the API the same so Flow doesn't complain.
+const appState = RCTAppState
+  ? new AppState()
+  : new MissingNativeAppStateShim();
 
-module.exports = AppState;
+module.exports = appState;

Libraries/Core/Devtools/setupDevtools.js

@@ -20,6 +20,9 @@ if (__DEV__) {
   connectToDevTools({
     isAppActive() {
       // Don't steal the DevTools from currently active app.
+      // Note: if you add any AppState subscriptions to this file,
+      // you will also need to guard against `AppState.isAvailable`,
+      // or the code will throw for bundles that don't have it.
       return AppState.currentState !== 'background';
     },
     // Special case: Genymotion is running on a different host.