MQTT

Purpose

The EspMqtt class handles the MQTT functionality by using an extra MQTT Framework called arduino-mqtt. The class EspMqtt acts as an interface in order to facilitate the usage of MQTT. Therefore the class is implemented as a singleton structure, which guarantees that there's just one MQTT instance. With just one connection to the MQTT Broker throughout the whole program.

Functionality

Get an object

Due to the singleton structure there is just one instance of the class throughout the whole program. In order to fetch this instance, do the following:

• Create a new variable with the type EspMqtt
• Call the following method:

static EspMqtt* getInstance()
{
  return &esp_mqtt_instance;
}

Configure the MQTT client

The configuration / setup works on the basis of the use of a structure called EspMqttConfig. This struct consists of the following properties:

struct EspMqttConfig
{
  const char* server_address;
  const char* device_id;
  const char* subscribe_topic;
  const char* publish_topic;
  uint16_t port;
};

Calling the setup() function for the EspConnectionHandler, calls the setup function for MQTT and configures the MQTT client correctly

Connect client to broker

In order to connect the client to the desired MQTT broker use the method:

bool connect();

Register client-side callback function

To get events from a certain topic sent from the MQTT broker the following function works as a interface. This interface enables the user to register a callback function which gets called when new data on a given topic arrives.

template <class T> void registerClient(T subscribe_callback(char* topic, char* payload));

Publish / Subscribe

Publish This method is used by the internal sensors which publish their value automatically to the MQTT broker with an interval set by the EspScheduler

Subscribe This method can be used to re-configure the subscribe topic. It does NOT return any data, sent from the MQTT broker