How to send device to device messages using Firebase Cloud Messaging?

Question

After searching the docs I could not find any info on how to send device to device messages using FCM without the use of an external server.

For example, if I was cr

Answer 1

You can use Retrofit. Subscribe devices to topic news. Send notification from one device to other.

public void onClick(View view) {

    HttpLoggingInterceptor logging = new HttpLoggingInterceptor();
    logging.setLevel(HttpLoggingInterceptor.Level.BODY);

    OkHttpClient.Builder httpClient = new OkHttpClient.Builder();
    httpClient.addInterceptor(new Interceptor() {
        @Override
        public okhttp3.Response intercept(Chain chain) throws IOException {
            Request original = chain.request();

            // Request customization: add request headers
            Request.Builder requestBuilder = original.newBuilder()
                    .header("Authorization", "key=legacy server key from FB console"); // <-- this is the important line
            Request request = requestBuilder.build();
            return chain.proceed(request);
        }
    });

    httpClient.addInterceptor(logging);
    OkHttpClient client = httpClient.build();

    Retrofit retrofit = new Retrofit.Builder()
            .baseUrl("https://fcm.googleapis.com")//url of FCM message server
            .client(client)
            .addConverterFactory(GsonConverterFactory.create())//use for convert JSON file into object
            .build();

    // prepare call in Retrofit 2.0
    FirebaseAPI firebaseAPI = retrofit.create(FirebaseAPI.class);

    //for messaging server
    NotifyData notifydata = new NotifyData("Notification title","Notification body");

    Call call2 = firebaseAPI.sendMessage(new Message("topic or deviceID", notifydata));

    call2.enqueue(new Callback() {
        @Override
        public void onResponse(Call call, Response response) {

            Log.d("Response ", "onResponse");
            t1.setText("Notification sent");

        }

        @Override
        public void onFailure(Call call, Throwable t) {
            Log.d("Response ", "onFailure");
            t1.setText("Notification failure");
        }
    });
}

POJOs

public class Message {
    String to;
    NotifyData notification;

    public Message(String to, NotifyData notification) {
        this.to = to;
        this.notification = notification;
    }

}

and

public class NotifyData {
    String title;
    String body;

    public NotifyData(String title, String body ) {

        this.title = title;
        this.body = body;
    }

}

and FirebaseAPI

public interface FirebaseAPI {

    @POST("/fcm/send")
    Call sendMessage(@Body Message message);

}