Missing user context in MQTT and QoS callbacks

[pavel-benes]

Hi,

I am using MQTT-TLS library from C++ instance. Unfortunately one function pointer is not enough to reference C++ class member function. I have solved this by introducing one extra argument 'userData' to initialization and callback methods. More details are here:

pavel-benes@3e4d89f
pavel-benes@24f5d82

The usage pattern is to provide MQTT-TLS library with pointer to static function and there use the (cast) userData value to reference the right class instance.

Do you think this would fit into you library? Since this is the only difference in my fork, I would like to abandon it and start to use your version.

Thanks a lot,

Regards,

Pavel

[hirotakaster]

Hi @pavel-benes ,

Thank you for your diff.

I think you want to use the user specified define data when MQTT client get a data with. But "userData" is not modified by MQTT class. If you want to use this pointer data, I think it's better don't depend on callback function.
I would like to please show me your sample source code with this userData function.

And thank you for your comment about the getByte().

Thank you

[pavel-benes]

Hi,

thank you for the quick response. Sorry about not providing the use case in the first place ... Here is the fragment of usage:

// class header
class AzureIotHub {
 ...
    MQTT *mqttClient;
...
    bool connect(const char *hostName, const char *deviceId, const char *sasToken);
    static void messageCallback(char *topic, uint8_t *payload, unsigned int length,void* userData=NULL);
    static void qosCallback(unsigned int msgId, void* userData=NULL);    
}

// class implementation
bool AzureIotHub::connect(const char *hostName, const char *deviceId, const char *sasToken) {
    ...
    //HINT Pass 'this' as 'userData' parameter to MQTT class
    mqttClient = new MQTT((char *) hostName, 8883, &AzureIotHub::messageCallback, 512, this);
    ...
    mqttClient->addQosCallback(&AzureIotHub::qosCallback);
    ...
}

//HINT Use the previously set 'userData' as pointer to AzureIotHub class instance
void AzureIotHub::messageCallback(char *topic, uint8_t *payload, unsigned int length, void* userData) {
    if (userData != NULL) {
        ((AzureIotHub*) userData)->handleMessage(topic, payload, length);
    } else {
        ERROR("NULL user data.");
    }
}

void AzureIotHub::qosCallback(unsigned int msgId, void* userData) {
    if (userData != NULL) {
        ((AzureIotHub*) userData)->handleQosConfirmation(msgId);
    } else {
        ERROR("NULL user data.");
    }
}

This indirection with the userData parameter allows me to call C++ member function as callback from MQTT library. It is not a critical feature to have, it can be solved for example by making AzureIotHub a singleton or define some listener class within MQTT library, but this approach with userData is (arguably) a little bit more flexible.

Thanks,

Regards,
Pavel