Send notifications to android app using Firebase Functions

Question

I am developing a chat app and so, I need to send notifications that new messages have been received. For that, I am using Firebase Functions. I'm using the sendToDevice function, that needs a token to send a notification. The problem is, I can't seem to retrieve token of the user that sent the message. This is my .js code:

const functions = require('firebase-functions');

const admin = require('firebase-admin');

admin.initializeApp(functions.config().firebase);


exports.sendNotification = functions.database.ref("/chats/{id}/messages/{messageId}/content")
.onWrite((change,context) => {
    var content = change.after.val();

    var payload = {
        data:{
            title: "Stranger has sent you a message",
            text: content
        }
    };

    // Here I need to the ID of the person who sent the message
    // And then compare this Id with the two Ids of the to users that are in the conversation
    // If the two Ids are different, then save the other Id as the token
    // So that I can send a notification to the other user.
    const senderId = database.ref("/chats/{id}/messages/{id}/sender/{senderId}");



    admin.messaging().sendToDevice(senderId, payload)
    .then(function(response){
        console.log("Successfully sent message: ", response);
        return null;
    })
    .catch(function(error){
        console.log("Error sending message: ", error);
    })
});

As you can see, I am checking for any changes in the messages/content child. That as the content of my notification.

Then, I am trying to retrieve the message sender ID so I can know who sent the message and retrieve the other user Id to notify him.

This might be a little confusing so here is my Firebase Realtime Database:

What am I doing wrong so this piece of code works as it should? This is the activity I have in android to receive the message:

class MyFirebaseInstanceId : FirebaseMessagingService() {
    override fun onMessageReceived(p0: RemoteMessage) {
        if(p0.data.size > 0){
            val payload :Map<String, String> = p0.data

            sendNotification(payload)

        }
    }

    private fun sendNotification(payload: Map<String, String>) {
        val builder = NotificationCompat.Builder(this)
        builder.setSmallIcon(R.drawable.common_google_signin_btn_icon_disabled)
        builder.setContentTitle(payload.get("username"))
        builder.setContentText(payload.get("email"))

        val intent = Intent(this, MainActivity::class.java)
        val stackBuilder = TaskStackBuilder.create(this)

        stackBuilder.addNextIntent(intent)

        val resultPendingIntent = stackBuilder.getPendingIntent(0, PendingIntent.FLAG_UPDATE_CURRENT)

        builder.setContentIntent(resultPendingIntent)

        val notificationManager =  (getSystemService(Context.NOTIFICATION_SERVICE)) as NotificationManager

        notificationManager.notify(0, builder.build())
    }
}

Please note that with database.ref("/chats/{id}/messages/{id}/sender/{senderId}") you are not getting a field value, but just a Reference. You need to use the once() method. Also note that you need to return admin.messaging().sendToDevice(senderId, payload)..., see firebase.google.com/docs/functions/terminate-functions — Renaud Tarnec
– Renaud Tarnec, Commented Feb 16, 2020 at 14:03
@RenaudTarnec can you give me an example of how to use .once() function? That awnsers my question and I'd like to give you a green check mark. — André Nogueira
– André Nogueira, Commented Feb 16, 2020 at 14:11

Renaud Tarnec · Accepted Answer · 2020-02-16 14:14:50Z

1

Following our comments above, here is how to use the once() and val() methods in your Cloud Function:

//.....
const refSenderId = database.ref("/chats/{id}/messages/{id}/sender/{senderId}");

return refSenderId.once('value')
 .then(dataSnapshot => { 
     const senderId = dataSnapshot.val();
     return admin.messaging().sendToDevice(senderId, payload)
 })
.then(function(response){
    console.log("Successfully sent message: ", response);
    return null;
})
.catch(function(error){
    console.log("Error sending message: ", error);
    return null;   // <- Note the return here.
})
//.....

answered Feb 16, 2020 at 14:14

Renaud Tarnec

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2 Comments

André Nogueira Over a year ago

Thanks! Quick question, can I use another .once() inside that one to retrieve another value?

Renaud Tarnec Over a year ago

Yes, but you need to correctly chain the different promises returned by the asynchronous methods. developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/… and firebase.google.com/docs/functions/terminate-functions. You can add your new code at the bottom of your question (by editing the question) and I will review it if you want.

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