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I'm trying to understand the usage and behaviour of the various callback mechanisms in the Paho MQTT C++ library when using the async_client, specifically:

  • mqtt::callback
  • mqtt::iaction listener
  • mqtt::connection_handler

What I'm trying to achieve: A blocking initialise method that waits for successful connection and successful subscription to a list of topics (a single topic to begin with), plus automatic re-connection and re-subscription to those topics should the connection drop out.

The 'async_subscribe' sample (https://github.com/eclipse/paho.mqtt.cpp/blob/master/src/samples/async_subscribe.cpp) suggests I should subscribe to topics in mqtt::callback::connected. However, it does not show how to then wait for the subscription to successfully complete. My understanding is that I should not call 'wait' on the subscribe token inside the connected callback. If I set a private var _subscribeToken on which to wait, it seems there is no guarantee whether it has been set after waiting for the connect token to complete, i.e. (pseudo-code):

void Callback::connected(const std::string& cause){
    _subscribeToken = _client->subscribe(topic, qos, nullptr, action_listener);
}

in initialise:

token_ptr connectToken = connect(connectOpts);
connectToken->wait();

_subscribeToken->wait(); // but _subscribeToken may still be null at this point.

What's the point of connectToken->wait if it doesn't guarantee the callback has been called yet? It's quite likely I'm misunderstanding something, but I'd like to be pointed in the right direction!

Steve Magness
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1 Answers1

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With some further digging into the C++/C libraries I was able to ascertain that Callback::connected is called after Token::on_success (and therefore after the wait has already completed).

It seems the docs and samples aren't great in this area. I've raised an issue in the Github project: https://github.com/eclipse/paho.mqtt.cpp/issues/256.

The solution was to:

  • Call subscribe from the on_success callback from the connect token (first ensuring the token_type == CONNECT)
  • In Callback::connected if cause is "automatic reconnect" I post a message back to the main thread that we need to resubscribe to topics. From the main thread I can then handle the reconnection (subscribe and wait on subscription tokens to complete).
Steve Magness
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