I hope it helps:
void parse(String description) {
def response = interfaces.mqtt.parseMessage(description)
if (response.get('topic') == "Topic_you_are_using"{
if (response.get('payload').contains("on")){
sendEvent(name: "switch", value: "on")
}
if (responce.get('payload').contains("off")){
sendEvent(name: "switch", value: "off")
}
}
}
