(mqtt-subscribe t topic qos)

mqtt-subscribe subscribes to topic from the MQTT broker.

Parameter	Description
t	MQTT broker
topic	Topic name string
qos	QOS level: 0 fire-and-forget, 1 send-at-least-once , 2 send-exactly-once

Example

Example 1: Make a connection to a mosquitto broker running on localhost:1883, subscribe to two topics, publish some data to them and wait for the results to show. Then unsubscribe from one topic and show that it's handler is no longer called. Finally close the connection properly.

> (define m (make-mqtt 'host "127.0.0.1" 'port 1883 
                        'handler (lambda (topic val) 
                          (for-each display (list topic ": " val "\n")))))
> (thread-sleep! 0.5)
> (mqtt-subscribe m "Test1" 2)
#t
> (mqtt-subscribe m "Test2" 2)
#t
> (mqtt-publish m "Test1" 123 2 0)
#t
> (mqtt-publish m "Test2" 123456 2 0)
#t
> (thread-sleep! 1.)
Test1: 123
Test2: 123456
> (mqtt-unsubscribe m "Test1")
#t
> (mqtt-publish m "Test1" "blah" 2 0)
#t
> (mqtt-publish m "Test2" "LN1" 2 0)
#t
> (thread-sleep! 1.)
Test2: LN1
> (mqtt-destroy m)
#t