Receive messages on a C++ client app

To receive simple downstream messages, each client app needs to implement the methods on the firebase::messaging::Listener API.

Initialize FCM

Before you can use FCM to get access to your registration token or receive messages it must be initialized.

To initialize FCM, call ::firebase::messaging::Initialize and supply it with your ::firebase::App object as well as an implementation of the ::firebase::messaging::Listener class.

MyListener my_listener_implementation;
::firebase::messaging::Initialize(app, &my_listener_implementation);

Access the registration token

On initial startup of your app, the FCM SDK generates a registration token for the client app instance. If you want to target single devices, or create device groups for FCM, you'll need to access this token.

You can access the token's value through the ::firebase::messaging::Listener::OnTokenReceived virtual function.

void OnTokenReceived(const char* token) {
  LogMessage("The registration token is `%s`", token);

  // TODO: If necessary send token to application server.
}

Receive and handle messages

To receive messages, your Listener class must implement the OnMessage virtual function.

Override OnMessage

By overriding the method ::firebase::messaging::Listener::OnMessage, you can perform actions based on the received message and get the message data:

void OnMessage(const ::firebase::messaging::Message& message) {
  LogMessage(TAG, "From: %s", message.from.c_str());
  LogMessage(TAG, "Message ID: %s", message.message_id.c_str());
}

Messages can represent different kinds of incoming data. Most commonly, messages are sent to the app after being initiated by the developer. Messages are also sent to you app to represent message sent events, message send error events, and messages deleted events. These special events can be differentiated by checking the Message::message_type field.

Messages Deleted

Sent to your app when the FCM server deletes pending messages. Message::message_type will be "deleted_messages". Messages may be delete due to:

1. Too many messages stored on the FCM server.

   This can occur when an app's servers send a bunch of non-collapsible messages to FCM servers while the device is offline.

2. The device hasn't connected in a long time and the app server has recently (within the last 4 weeks) sent a message to the app on that device.

   It is recommended that the app do a full sync with the app server after receiving this call.