How to send the push notification using Topic Messaging GCM

Aman Gupta

14 December 2016

“GCM topic messaging allows your app server to send a message to multiple devices that have opted into a particular topic.”

Based on the publish/subscribe model, topic messaging supports unlimited subscriptions per app.

The app server sends messages with payloads up to 2KB to the topic.

GCM introduces topic messaging, a way of sending messages to many clients.

An application can create one or more topics and register clients to each topic. Then messages are sent for a specific topic, GCM informing all clients who have been registered for it. This way one can reach a large number of clients, even all of them.

Developers can choose any topic name that matches the regular expression, "/topics/[a-zA-Z0-9-_.~%]+"

How to Send the topic messages from the server

GCM topic is very similar to sending messages to an individual device.

The app server sets to with /topics/yourTopicMessage

We can send data messages to a topic by HTTP and XMPP protocols.

HTTP POST Request:

https://gcm-http.googleapis.com/gcm/send
Content-Type:application/json
Authorization:key=AIzaSyDaIaL....3gOx_YQgTxWpQ

{
  "to": "/topics/mobikul",
  "data": {
    "message": "This is a GCM Topic Message by mobikul",
   }
}

https://gcm-http.googleapis.com/gcm/send

Content-Type:application/json

Authorization:key=AIzaSyDaIaL....3gOx_YQgTxWpQ

{

"to": "/topics/mobikul",

"data": {

"message": "This is a GCM Topic Message by mobikul",

}

XMPP Message:

<message id="">
  <gcm xmlns="google:mobile:data">
  {
      "to": "/topics/mobikul",
      "message_id": "m-1363452849222" ,
      "data": {
          "message":"This is a GCM Topic Message by mobikul"
      }
  }
  </gcm>
</message>

{

"to": "/topics/mobikul",

"message_id": "m-1363452849222" ,

"data": {

"message":"This is a GCM Topic Message by mobikul"

}

</gcm>

</message>

Expect up to 30 seconds of delay before the GCM Connection Server returns a success or failure response to the topic send requests.

How to Subscribe to a topic

To subscribe to a topic, the client app calls GCM PubSub subscribe() with the GCM registration token and topic name.

private void subscribeTopics(String token) throws IOException {
    GcmPubSub pubSub = GcmPubSub.getInstance(this);
    for (String topic : TOPICS) {
        pubSub.subscribe(token, "/topics/" + topic, null);
    }
}

private void subscribeTopics(String token) throws IOException {

GcmPubSub pubSub = GcmPubSub.getInstance(this);

for (String topic : TOPICS) {

pubSub.subscribe(token, "/topics/" + topic, null);

}

How to receive and handle topic messages

@Override
public void onMessageReceived(String from, Bundle data) {
    String message = data.getString("message");
    Log.d(TAG, "From: " + from);
    Log.d(TAG, "Message: " + message);

    if (from.startsWith("/topics/")) {
        // message received from some topic.
    } else {
        // normal downstream message.
    }

    // ...
}

@Override

public void onMessageReceived(String from, Bundle data) {

String message = data.getString("message");

Log.d(TAG, "From: " + from);

Log.d(TAG, "Message: " + message);

if (from.startsWith("/topics/")) {

// message received from some topic.

} else {

// normal downstream message.

}

// ...

}

The GCMReceiver and GCMListenerService services is simplify use for message handling on Android client apps.