1 files changed, 29 insertions, 4 deletions
diff --git a/catbus.go b/catbus.go
index 66cab2e..c8cf033 100644
--- a/catbus.go
+++ b/catbus.go
@@ -19,6 +19,8 @@ type (
 	client struct {
 		mqtt mqtt.Client
 
+		subscribeEveryMessage bool
+
 		payloadByTopicMu sync.Mutex
 		payloadByTopic   map[string]string
 
@@ -27,12 +29,20 @@ type (
 
 		onconnectDelay  time.Duration
 		onconnectJitter time.Duration
+
+		// syncCallbacks makes callbacks synchronous.
+		// ONLY FOR TESTING.
+		syncCallbacks bool
 	}
 
 	ClientOptions struct {
 		DisconnectHandler func(Client, error)
 		ConnectHandler    func(Client)
 
+		// SubscribeEveryMessage determines if the Subscribe callback will be called for all incoming messages.
+		// When SubscribeEveryMessage is false, the Subscribe callback will only trigger when the value changes.
+		SubscribeEveryMessage bool
+
 		// Publish previously seen or default values on connecting after OnconnectDelay ± [0,OnconnectJitter).
 		OnconnectDelay  time.Duration
 		OnconnectJitter time.Duration
@@ -56,6 +66,8 @@ const (
 
 func NewClient(brokerURI string, options ClientOptions) Client {
 	client := &client{
+		subscribeEveryMessage: options.SubscribeEveryMessage,
+
 		payloadByTopic:        map[string]string{},
 		onconnectTimerByTopic: map[string]*time.Timer{},
 
@@ -107,10 +119,17 @@ func (c *client) Connect() error {
 
 // Subscribe subscribes to a Catbus MQTT topic.
 func (c *client) Subscribe(topic string, f MessageHandler) error {
-	return c.mqtt.Subscribe(topic, atLeastOnce, func(_ mqtt.Client, msg mqtt.Message) {
-		c.storePayload(msg.Topic(), Retention(msg.Retained()), string(msg.Payload()))
-
-		go f(c, messageFromMQTTMessage(msg))
+	return c.mqtt.Subscribe(topic, atLeastOnce, func(_ mqtt.Client, raw mqtt.Message) {
+		msg := messageFromMQTTMessage(raw)
+
+		if c.subscribeEveryMessage || msg.Payload != c.payloadForTopic(topic) {
+			c.storePayload(msg.Topic, msg.Retention, msg.Payload)
+			if c.syncCallbacks {
+				f(c, msg)
+			} else {
+				go f(c, msg)
+			}
+		}
 	}).Error()
 }
 
@@ -129,6 +148,12 @@ func (c *client) jitteredOnconnectDelay() time.Duration {
 	return c.onconnectDelay - jitter
 }
 
+func (c *client) payloadForTopic(topic string) string {
+	c.payloadByTopicMu.Lock()
+	defer c.payloadByTopicMu.Unlock()
+	return c.payloadByTopic[topic]
+}
+
 func (c *client) storePayload(topic string, retention Retention, payload string) {
 	c.payloadByTopicMu.Lock()
 	defer c.payloadByTopicMu.Unlock()