mqtt: change 'On' method function signature to match expected interface

Signed-off-by: deadprogram <ron@hybridgroup.com>

examples/mqtt_driver_ping.go

@@ -14,11 +14,11 @@ func main() {
 	helloDriver := mqtt.NewDriver(mqttAdaptor, "hello")
 
 	work := func() {
-		helloDriver.OnData(func(data []byte) {
+		helloDriver.On(mqtt.Data, func(data []byte) {
 			fmt.Println("hello")
 		})
 
-		holaDriver.OnData(func(data []byte) {
+		holaDriver.On(mqtt.Data, func(data []byte) {
 			fmt.Println("hola")
 		})
 

platforms/mqtt/mqtt_driver.go

@@ -2,6 +2,14 @@ package mqtt
 
 import "gobot.io/x/gobot"
 
+const (
+	// Data event when data is available for Driver
+	Data = "data"
+
+	// Error event when error occurs in Driver
+	Error = "error"
+)
+
 // Driver for mqtt
 type Driver struct {
 	name       string
@@ -60,7 +68,9 @@ func (m *Driver) Publish(message []byte) bool {
 	return m.adaptor().Publish(m.topic, message)
 }
 
-// OnData subscribes to the current device topic, and then calls the message handler function when data is received
-func (m *Driver) OnData(f func(s []byte)) bool {
+// On subscribes to data updates for the current device topic,
+// and then calls the message handler function when data is received
+func (m *Driver) On(n string, f func(s []byte)) bool {
+	// TODO: subscribe to Error updates
 	return m.adaptor().On(m.topic, f)
 }