Support class member as an onMessage callback

[corporateanon]

I have a class which uses MQTTClient. I want to pass a pointer to class method as a callback to onMessage. Could you please create an overloaded onMessage method which would accept a "context" (a pointer to a class instance) and a callback (a pointer to a method of this class)?

Otherwise it is almost impossible for me to have a callback which would have an access to the rest of my class instance methods.

class App
{
  //...skipped
  public:
    void messageHandler(String &topic, String &payload);
    void setup();
    MQTTClient client;
}

//---

void App::setup()
{
  //...skipped
  client.onMessage(this, App::onMessage);
}

[256dpi]

I'm resistant to changing the callback signatures again and breaking already existing code.

Would a void * ref property on the MQTTClient class suffice? Together with the advanced callback you could get a reference to your custom class:

void messageReceived(MQTTClient *client, char topic[], char payload[], int payload_length) {
  auto parent = (*App)client.ref;
}

[corporateanon]

Yes, it would also be very nice!

[abcd-ca]

I would also benefit from a ref on the simple callback signature

[chris-zen]

@256dpi one solution that wouldn't break backward compatibility is to add a new overloaded onMessage that has a lambda as a parameter. Lambdas allow to capture other variables from the environment, for example this which would solve the problem with class members. The interface would be something like:

#include <functional>

void onMessage(std::function<void(String&, String&)> callback) {}

[256dpi]

Added the ref property in d0c2820.

[ramboerik]

Thanks for implementing this, found it very useful!

[jenschr]

Thanks for making this library and adding this feature! It wasn't instantly obvious to me how to use it, so I made a bare-bones example showing how to implement a wrapper class using MQTTClientCallbackAdvancedFunction and ref https://github.com/jenschr/BasicMqttClassExample Feel free to link to it in the docs.

[256dpi]

@jenschr Thanks for letting me know! If you want, we can also add the example to the examples within this repository. You could check out the existing examples, and submit a PR for one with your changes.

[jenschr]

Yeah. PR is the way to go. I can make it an Arduino project before pushing, so it's in line with the other examples.