Add support for using std::function callbacks instead of C-pointers

[chris-hatton]

When writing more structured Arduino code, it may be desirable to have an instance of MqttClient be encapsulated inside another object e.g. MyMqttIntegration. In this case, the exclusive use of C-pointers for message callbacks becomes a problem since a member function of MyMqttIntegration cannot be presented as a C-pointer. It is possible to cast a captureless lambda as a C-pointer, but this has it's own limitations.

This PR provides a solution: the current C-pointer API remains the default, but by defining MQTT_CLIENT_STD_FUNCTION_CALLBACK the library swaps to using std::functions. Such a callback can then be defined by a lambda or by using std::bind on a member function.

Example setting of message callback when MQTT_CLIENT_STD_FUNCTION_CALLBACK is defined:

MqttClient::MessageCallback callback = [this](MqttClient *client, int messageSize) {
    Serial.print("Received message with topic: " + client->messageTopic());
};
mqttClient->onMessage(callback);

Including a pointer to the source MqttClient affords a little extra flexibility in case multiple MqttClients are used which share the same callback function.

[CLAassistant]

All committers have signed the CLA.

[chris-hatton]

I signed the CLA a month ago and checks all pass - do you need anything else from me to be able to merge this? Thanks.

[aentinger]

I signed the CLA a month ago and checks all pass - do you need anything else from me to be able to merge this? Thanks.

Hi @chris-hatton :wave: Doing both does not guarantee a merge into that repository. How do you intend to provide this define to the library? It's unproblematic for e.g. platform.io but not available in the Arduino tooling - by design.

[JDuchniewicz]

Relying on a standalone function as a callback is a sound solution albeit limited. In order to circumvent this I have to rely to heavy hacks on an embedded platform which does not have std::function

[chris-hatton]

It's unproblematic for e.g. platform.io but not available in the Arduino tooling - by design.

@aentinger Yes, my own use case is with platform.io - perhaps including this optional improvement for the benefit of other platform.io users, while not impacting Arduino users, can be considered? Also wondering; is the 'define' restriction still present in Arduino IDE 2.0?

[aentinger]

Okay. You've got me there. Merging it in 👍

[Johboh]

Thank you for this!