make lib more user friendly

README.md

@@ -113,32 +113,30 @@ In [firebase console](https://console.firebase.google.com/), you can get `google
 ## Usage
 
 ```javascript
-
-import {DeviceEventEmitter} from 'react-native';
-var FCM = require('react-native-fcm');
-
-componentWillMount() {
-FCM.requestPermissions();
-FCM.getFCMToken().then(data => {
-  console.log(data.token)
-//store fcm token in your server
-});
-this.fcmNotifLsnr = DeviceEventEmitter.addListener('FCMNotificationReceived', (notif) => {
-//there are two parts of notif. notif.notification contains the notification payload, notif.data contains data payload
-});
-this.fcmTokenLsnr = DeviceEventEmitter.addListener('FCMTokenRefreshed', (data) => {
-  console.log(data.token)
-//fcm token may not be available on first load, catch it here
-});
-}
-
-componentWillUnmount() {
-//prevent leak
-this.fcmNotifLsnr.remove();
-this.fcmTokenLsnr.remove();
-}
-
-}
+  import FCM from 'react-native-fcm';
+
+  ...
+  componentWillMount() {
+    FCM.requestPermissions();
+    FCM.getFCMToken().then(token => {
+      console.log(token)
+      // store fcm token in your server
+    });
+    this.notificationUnsubscribe = FCM.on('notification', (notif) => {
+      // there are two parts of notif. notif.notification contains the notification payload, notif.data contains data payload
+    });
+    this.refreshUnsubscribe = FCM.on('refresh', (data) => {
+      console.log(data.token)
+      // fcm token may not be available on first load, catch it here
+    });
+  }
+
+  componentWillUnmount() {
+    // prevent leak
+    this.refreshUnsubscribe();
+    this.notificationUnsubscribe();
+  }
+  ...
 ```
 
 ### Behaviour when sending `notification` and `data` payload through GCM

index.js

-'use strict';
+import {
+  NativeModules,
+  DeviceEventEmitter,
+} from 'react-native';
 
-var React = require('react-native');
-var {NativeModules} = React;
+const eventsMap = {
+  refersh: 'FCMTokenRefreshed',
+  notification: 'FCMNotificationReceived',
+};
 
-var FIRMessaging = NativeModules.RNFIRMessaging;
+const FIRMessaging = NativeModules.RNFIRMessaging;
 
-class FCM {
+const FCM = {};
 
-    static getFCMToken() {
-        return FIRMessaging.getFCMToken();
-    }
+FCM.getFCMToken = function getFCMToken() {
+    return FIRMessaging.getFCMToken();
+};
 
-    static requestPermissions() {
-        return FIRMessaging.requestPermissions();
-    }
+FCM.requestPermissions = function requestPermissions() {
+  return FIRMessaging.requestPermissions();
+};
 
-}
+FCM.on = function on(event, callback) {
+  const nativeEvent = eventsMap[event];
+
+  const listener = DeviceEventEmitter.addListener(nativeEvent, (params) => {
+    callback(params);
+  });
+
+  return function remove() {
+    listener.remove();
+  };
+};
 
 FCM.initialData = FIRMessaging.initialData;
 FCM.initialAction = FIRMessaging.initialAction;
 
-module.exports = FCM;
+module.exports = FCM;
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