Here a comprehensive guide on how to send push notifications in a React Native application using Firebase Cloud Messaging (FCM).

Prerequisites

Node.js and npm: Make sure you have Node.js and npm installed.
React Native CLI: Install React Native CLI if you haven't already.

   npm install -g react-native-cli

Firebase Project: Create a project on the Firebase Console.
Firebase CLI: Install Firebase CLI to configure your project.

   npm install -g firebase-tools

Step-by-Step Guide

1. Create and Configure Firebase Project

Go to the Firebase Console.
Click on "Add Project" and follow the prompts to create a new project.
Navigate to "Cloud Messaging" and copy the Server key and Sender ID. These will be needed later.

2. Set Up React Native Project

Initialize a new React Native project:

  npx react-native init FirebasePushNotification
  cd FirebasePushNotification

3. Install Required Packages

Install Firebase and FCM libraries:

  npm install @react-native-firebase/app @react-native-firebase/messaging

4. Configure Firebase with Your React Native App

iOS Configuration:
- Navigate to the Firebase Console, select your project, and go to "iOS App."
- Register your app with the iOS bundle ID.
- Download the GoogleService-Info.plist file and place it in the ios/ directory of your React Native project.

  cp GoogleService-Info.plist ios/

Open the ios/YourProjectName.xcworkspace in Xcode.
Drag and drop GoogleService-Info.plist into your project.
Ensure FirebaseAppDelegateProxyEnabled is set to YES in the Info.plist.
- Android Configuration:
Navigate to the Firebase Console, select your project, and go to "Android App."
Register your app with the Android package name.
Download the google-services.json file and place it in the android/app directory.

  cp google-services.json android/app/

Modify android/build.gradle to include the Google services classpath:

dependencies {
    classpath 'com.google.gms:google-services:4.3.10'
}

Update android/app/build.gradle to apply the Google services plugin:
```
apply plugin: 'com.google.gms.google-services'
```

5. Implement Notification Logic

iOS Setup:

Open ios/YourProjectName/AppDelegate.m.
Import Firebase and configure it:

#import <Firebase.h>

In the application:didFinishLaunchingWithOptions method, configure Firebase:

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
    [FIRApp configure];
    return YES;
}

Android Setup:

Modify android/app/src/main/java/com/yourprojectname/MainApplication.java to include Firebase:

import io.invertase.firebase.app.ReactNativeFirebaseAppPackage;

Update the getPackages method:

@Override
protected List<ReactPackage> getPackages() {
    return Arrays.<ReactPackage>asList(
        new MainReactPackage(),
        new ReactNativeFirebaseAppPackage()
    );
}

6. Implement Notification Handling in React Native

Import and configure messaging in your React Native code:

  import messaging from '@react-native-firebase/messaging';

  async function requestUserPermission() {
    const authStatus = await messaging().requestPermission();
    const enabled =
      authStatus === messaging.AuthorizationStatus.AUTHORIZED ||
      authStatus === messaging.AuthorizationStatus.PROVISIONAL;

    if (enabled) {
      console.log('Authorization status:', authStatus);
    }
  }

  useEffect(() => {
    requestUserPermission();

    const unsubscribe = messaging().onMessage(async remoteMessage => {
      console.log('A new FCM message arrived!', JSON.stringify(remoteMessage));
    });

    return unsubscribe;
  }, []);

7. Send a Test Notification

Using Firebase Console:
- Navigate to the "Cloud Messaging" section of your Firebase project.
- Click "Send your first message" and fill in the required fields. Choose your target device (or all devices).
- Click "Send" to dispatch the notification.

Using Firebase Admin SDK (Server-side):

Set up Node.js and install the Firebase Admin SDK:

npm install firebase-admin

Initialize the Admin SDK and send a notification:

const admin = require('firebase-admin');
const serviceAccount = require('./path/to/serviceAccountKey.json');

admin.initializeApp({
  credential: admin.credential.cert(serviceAccount),
  databaseURL: 'https://<your-database-name>.firebaseio.com'
});

const message = {
  notification: {
    title: 'Hello',
    body: 'World'
  },
  token: '<device-token>'
};

admin.messaging().send(message)
  .then((response) => {
    console.log('Successfully sent message:', response);
  })
  .catch((error) => {
    console.log('Error sending message:', error);
  });

Conclusion

You now have a React Native app set up to receive push notifications via Firebase Cloud Messaging. Ensure you handle permissions, foreground and background notifications correctly in your code to provide a seamless user experience.

Sending a push Notification from Firebase