Add support for sending FCM messages to topics

[erimicel]

I noticed that while FCM device tokens are supported, there’s currently no way to send messages to a topic (e.g. /topics/news). This is part of the official FCM API and would be very useful when broadcasting notifications to a group of users.

Reference:
FCM documentation

Feature request:
Add support for passing a topic parameter when sending via FCM. Although this requires subscribing/unsubscribing users to Firebase Topics, which might not align with the existing notification model, it would be great to have the option available.

Example payload:

POST https://fcm.googleapis.com/v1/projects/myproject-b5ae1/messages:send HTTP/1.1

Content-Type: application/json
Authorization: Bearer ya29.ElqKBGN2Ri_Uz...HnS_uNreA
{
  "message":{
    "topic" : "foo-bar",
    "notification" : {
      "body" : "This is a Firebase Cloud Messaging Topic Message!",
      "title" : "FCM Message"
      }
   }
}

[intrip]

There are a few reasons Topics aren’t first-class:

• They're FCM-only
• As you mentioned, they don't align well with the current workflow
• You can already broadcast today by fanning out to multiple devices in the library:

devices = [device_a, device_b, ...]
notification.deliver_later_to(devices)

That said, if you're managing Firebase Topic subscriptions yourself, you can send to a topic via the .with_google payload override:

notification = ApplicationPushNotification.with_google(topic: "topic").new(title: "hello")
# This will generate the following payload:
ActionNativePush.service_for("google", notification).send(:payload_from, notification)
 {
  message: {
    data: {},
    android: {notification: {title: "hello"}, priority: "high"},
    topic: "topic"
  }
}
# Send it via
ActionNativePush.service_for("google", notification).push(notification)

[erimicel]

Thank you for the details. I guess it makes sense, anyone who wants to use the topic can manage it themselves and override if needed.

I will close this issue