add subscribe/unsubscribe

Author: Eugen Freiter

src/main/java/io/github/hapjava/accessories/TemperatureSensor.java

@@ -70,4 +70,14 @@ default TemperatureUnit getTemperatureUnit() {
   default void setTemperatureUnit(TemperatureUnit unit) {
     // override depending on the thermostat if required.
   }
+
+  /**
+   * subscribe to unit changes.
+   *
+   * @param callback callback
+   */
+  default void subscribeTemperatureUnit(final HomekitCharacteristicChangeCallback callback) {}
+
+  /** unsubscribe from unit changes. */
+  default void unsubscribeTemperatureUnit() {}
 }

src/main/java/io/github/hapjava/impl/characteristics/thermostat/TemperatureUnitsCharacteristic.java

@@ -29,8 +29,12 @@ protected CompletableFuture<Integer> getValue() {
   }
 
   @Override
-  public void subscribe(final HomekitCharacteristicChangeCallback callback) {}
+  public void subscribe(final HomekitCharacteristicChangeCallback callback) {
+    thermostat.subscribeTemperatureUnit(callback);
+  }
 
   @Override
-  public void unsubscribe() {}
+  public void unsubscribe() {
+    thermostat.unsubscribeTemperatureUnit();
+  }
 }