index.js

const functions = require('firebase-functions');
    // Firebase admin
    const admin = require('firebase-admin');
    admin.initializeApp(functions.config().firebase);
// The snapshot to the user's tokens.
    //     /* Trigger when a new document is created */ 
    exports.createdMatchNotification = functions.firestore
// Add the document path to your collection mine is called matches
    .document('match/{documentId}')
    .onCreate((snap, context) => {
        const user_id = context.params.user_id;
// Notifications builder
    const payload = {
        notification:{
            title : 'Message from Cloud',
            body : 'This is your body'
        },
        topic: 'newMatch'
    };
    return admin.messaging().send(payload);
});
 /**
     Its advisable to use shared preferences,you can have EG a switch where the user can
    subscribe/unsubscribe to notifications. So find a way to implement this in your Shared Prefs
    Dont forget to set up FCM in your app
     
     */
  FirebaseMessaging.getInstance().subscribeToTopic("newMatch")
                        .addOnCompleteListener { task ->
                            var msg =     "Subscription to get new match alerts successful"

                            if (!task.isSuccessful) {
                                msg = "Subscription to new match alerts failed"
                            }
                            Timber.d(msg)
                            Toast.makeText(baseContext, msg, Toast.LENGTH_SHORT).show()
                        }