go - pass addtional parameter to callback

Question

I am using the phao mqtt module to communicate with a mqtt server.

Now I want to subscribe to a topic, for this I need to pass a callback with type MessageHandler. Is it somehow possible to pass an additional parameter to the MessageHandler?

My handler looks like this so far:

var MyMessageHandler mqtt.MessageHandler = func(client mqtt.Client, msg mqtt.Message) {
    fmt.Printf("Received message: %s from topic: %s
", msg.Payload(), msg.Topic())
    # Call here a stuct method
}

Answer 1

You have 2 options:

• define a function that returns mqtt.MessageHandler and accepts your struct
• define a method on your struct and the same signature of mqtt.MessageHandler

type S struct{}

func MessageHandler(s S) mqtt.MessageHandler {
  return func(client mqtt.Client, msg mqtt.Message) {
    // use `s`
    // ...
  }
}

func (s *S) MessageHandler(client mqtt.Client, msg mqtt.Message) {
  // use `s`
  // ...
}

Usage:

func main() {
  s := S{}
  mh := MessageHandler(s)

  mh = s.MessageHandler // same type
  // use mh
  // ...
}