These changes have been under test in our application using an ESP w TLS, and so far everything looks good

Thanks for this @perigoso. I'm going to make a branch called feature/callbacks and merge this there. Future PRs and development work to the callback system can go there. Once we're certain it's working correctly we can merge it into main.

sounds good 👍

commit e2c1ecf should be cherry picked and merged on master though, as it's a bugfix

hello!
So, is it available somewhere now?

Its not merged into the main repository, but it is available in the fork
aj-tec:callbacks

commit e2c1ecf should be cherry picked and merged on master though, as it's a bugfix

cherry picked into new PR #167

Wow, it's amazing @perigoso, @LiamBindle ! I'm experimenting with mqtt-over-websockets and webassembly (via emscripten), and found that this library perfectly suitable for it (mqtt client running in browser, written in C/C )! It tiny, readable/hackable, easy to integrate - just perfect to use, in comparison to such (huge) alternatives like Qt qmqtt or Eclipse::Paho (which, without huge modification, still not work for me).

Due to event-driven (callback-driven) design of websockets, I'm also incredibly interested in callback-driven design of this library. I'm found this PR, trying to figure such simple things like "how to check is client connected", or "how to subscribe/publish after successful connection" (well, as I see, currently (in master branch) that's only in internal state, without notification to outer space, and this PR fixes it with beautiful generalization).

Callbacks allow us to write C wrapper, which behave beautifully the same as Qt (I'm C programmer which previously uses Qt on my work). Pseudocode:

mqtt::Client client( /*send_buff_size*/ 2028, /*recv_buff_size*/ 2048);
client.onConnect([&client] {
    client.subscribe("/test/topic");
});
client.onDisconnect([] { /*noop or log here */ });
client.onMessage([](const mqtt::Message& msg) {
    /* process msg.topic() / msg.payload() here */
});
client.onError([](mqtt::MQTTErrors error) { /* log with: mqtt_error_str(error); */ } );

client.setHostname("ws://127.0.0.1");
client.setPort(1884); 
client.connectToHost();

while(true) { /* Explicit way, may be hidden in internal thread */
    client.sync();
}

So I'm personally really looking forward to when it will be merged into master (since, as I see, only readme & examples currently not modified to new design) :)