Event callback system

[perigoso]

this is WIP.

Adds an "event" callback system.

There is now a single user callback, this is called from mqtt_sync whenever theres an event relevant for the user, like a publish acknowledged or a publish received, the reconnect callback is now also handled by this.

I think maybe we want to add some sort of flag system, to enable/disable what callbacks we want, for now, everything is enabled as long as the callback is not null.

These changes need a version bump, they break compatibility with the current implementation, i thought baout keeping compatibility, but i think this approach is better and it required the breakage.

note: built on top of #148

TODO:

• modify examples to use new api
• add enable/disable to callbacks

closes #98

[perigoso]

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

[LiamBindle]

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.

[perigoso]

sounds good 👍

[perigoso]

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

[fariouche]

hello!
So, is it available somewhere now?

[perigoso]

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

[perigoso]

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

cherry picked into new PR #167

[inobelar]

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:https://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) :)