Add a demonstration in examples/hooks on how to subscribe to a topic and publish messages directly within the hook. (#333)

examples/hooks/main.go

@@ -28,7 +28,10 @@ func main() {
 		done <- true
 	}()
 
-	server := mqtt.New(nil)
+	server := mqtt.New(&mqtt.Options{
+		InlineClient: true, // you must enable inline client to use direct publishing and subscribing.
+	})
+
 	_ = server.AddHook(new(auth.AllowHook), nil)
 	tcp := listeners.NewTCP("t1", ":1883", nil)
 	err := server.AddListener(tcp)
@@ -36,7 +39,11 @@ func main() {
 		log.Fatal(err)
 	}
 
-	err = server.AddHook(new(ExampleHook), map[string]any{})
+	// Add custom hook (ExampleHook) to the server
+	err = server.AddHook(new(ExampleHook), &ExampleHookOptions{
+		Server: server,
+	})
+
 	if err != nil {
 		log.Fatal(err)
 	}
@@ -87,8 +94,14 @@ func main() {
 	server.Log.Info("main.go finished")
 }
 
+// Options contains configuration settings for the hook.
+type ExampleHookOptions struct {
+	Server *mqtt.Server
+}
+
 type ExampleHook struct {
 	mqtt.HookBase
+	config *ExampleHookOptions
 }
 
 func (h *ExampleHook) ID() string {
@@ -108,11 +121,34 @@ func (h *ExampleHook) Provides(b byte) bool {
 
 func (h *ExampleHook) Init(config any) error {
 	h.Log.Info("initialised")
+	if _, ok := config.(*ExampleHookOptions); !ok && config != nil {
+		return mqtt.ErrInvalidConfigType
+	}
+
+	h.config = config.(*ExampleHookOptions)
+	if h.config.Server == nil {
+		return mqtt.ErrInvalidConfigType
+	}
 	return nil
 }
 
+// subscribeCallback handles messages for subscribed topics
+func (h *ExampleHook) subscribeCallback(cl *mqtt.Client, sub packets.Subscription, pk packets.Packet) {
+	h.Log.Info("hook subscribed message", "client", cl.ID, "topic", pk.TopicName)
+}
+
 func (h *ExampleHook) OnConnect(cl *mqtt.Client, pk packets.Packet) error {
 	h.Log.Info("client connected", "client", cl.ID)
+
+	// Example demonstrating how to subscribe to a topic within the hook.
+	h.config.Server.Subscribe("hook/direct/publish", 1, h.subscribeCallback)
+
+	// Example demonstrating how to publish a message within the hook
+	err := h.config.Server.Publish("hook/direct/publish", []byte("packet hook message"), false, 0)
+	if err != nil {
+		h.Log.Error("hook.publish", "error", err)
+	}
+
 	return nil
 }