RN: Improve NativeEventEmitter Flow Types

Reviewed By: fkgozali

Differential Revision: D6050987

fbshipit-source-id: 1c911bed23f7d26800aafed4b7e7c30a1197f64b

Libraries/EventEmitter/NativeEventEmitter.js

@@ -14,43 +14,53 @@
 const EventEmitter = require('EventEmitter');
 const Platform = require('Platform');
 const RCTDeviceEventEmitter = require('RCTDeviceEventEmitter');
+
 const invariant = require('fbjs/lib/invariant');
 
 import type EmitterSubscription from 'EmitterSubscription';
 
+type NativeModule = {
+  +addListener: (eventType: string) => void,
+  +removeListeners: (count: number) => void,
+};
+
 /**
  * Abstract base class for implementing event-emitting modules. This implements
  * a subset of the standard EventEmitter node module API.
  */
 class NativeEventEmitter extends EventEmitter {
-  _nativeModule: Object;
+  _nativeModule: ?NativeModule;
 
-  constructor(nativeModule: Object) {
+  constructor(nativeModule: ?NativeModule) {
     super(RCTDeviceEventEmitter.sharedSubscriber);
     if (Platform.OS === 'ios') {
       invariant(nativeModule, 'Native module cannot be null.');
       this._nativeModule = nativeModule;
     }
   }
 
-  addListener(eventType: string, listener: Function, context: ?Object): EmitterSubscription {
-    if (Platform.OS === 'ios') {
+  addListener(
+    eventType: string,
+    listener: Function,
+    context: ?Object,
+  ): EmitterSubscription {
+    if (this._nativeModule != null) {
       this._nativeModule.addListener(eventType);
     }
     return super.addListener(eventType, listener, context);
   }
 
   removeAllListeners(eventType: string) {
     invariant(eventType, 'eventType argument is required.');
-    if (Platform.OS === 'ios') {
-      const count = this.listeners(eventType).length;
+    const count = this.listeners(eventType).length;
+    if (this._nativeModule != null) {
       this._nativeModule.removeListeners(count);
     }
     super.removeAllListeners(eventType);
   }
 
   removeSubscription(subscription: EmitterSubscription) {
-    if (Platform.OS === 'ios') {
+    if (this._nativeModule != null) {
       this._nativeModule.removeListeners(1);
     }
     super.removeSubscription(subscription);