Understanding callbacks

When we declare a function, we pass this function as an argument to another function or method, or we assign this function to an attribute, and then some code calls this function at some time; this mechanism is known as callback. The name callback is used because the code calls back a function at some time.

The code declares three functions that we will specify as callbacks:

• on_connect: This function will be called when the MQTT client receives a CONNACK response from the MQTT server, that is, when a connection has been successfully established with the MQTT server.

• on_subscribe: This function will be called when the MQTT client receives a SUBACK response from the MQTT server, that is, when a subscription has been successfully completed.

• on_message: This function will be called when the MQTT client receives a PUBLISH message from the MQTT server. Whenever the MQTT server publishes a message, based on the subscriptions for the client, this function will be called.

The...