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+Fixation events sending
+=======================
+
+Observers are attached to pipeline steps to be notified when a method is called.
+
+## observers.py
+
+For this use case we need to enable [Ivy bus communication](https://gitlab.com/ivybus/ivy-python/) to log ArUco detection results (on *ArUcoCamera.on_watch* call) and fixation identification with AOI matching (on *ArUcoCamera.on_look* call).
+
+```python
+import logging
+
+from argaze import DataFeatures, GazeFeatures
+
+from ivy.std_api import *
+from ivy.ivy import IvyIllegalStateError
+
+
+class IvyBus(DataFeatures.PipelineStepObject):
+	"""Handle Ivy bus."""
+
+	@DataFeatures.PipelineStepInit
+	def __init__(self, **kwargs):
+
+		self.__bus = None
+
+	@property
+	def bus(self) -> str:
+		return self.__bus
+
+	@bus.setter
+	def bus(self, bus: str):
+		self.__bus = bus
+
+	@DataFeatures.PipelineStepEnter
+	def __enter__(self, parent = None):
+
+		# Enable Ivy bus
+		IvyInit(self.name)
+		IvyStart(self.__bus)
+
+		return self
+
+	@DataFeatures.PipelineStepExit
+	def __exit__(self, exception_type, exception_value, exception_traceback):
+
+		# Stop Ivy bus
+		IvyStop()
+
+
+class ArUcoCameraLogger(DataFeatures.PipelineStepObject):
+	"""Log ArUcoCamera activity."""
+
+	@DataFeatures.PipelineStepInit
+	def __init__(self, **kwargs):
+
+		self._last_markers_number = None
+
+	def on_watch(self, timestamp, aruco_camera, exception):
+		"""Report ArUco markers detection info on Ivy bus."""
+
+		# Wait for number of detected marker changes
+		if aruco_camera.aruco_detector.detected_markers_number() != self._last_markers_number:
+
+			self._last_markers_number = aruco_camera.aruco_detector.detected_markers_number()
+
+			output = f'ArUcoDetection MarkersNumber={self._last_markers_number}'
+
+			# Send Ivy message
+			IvySendMsg(output)
+
+			logging.debug('%i %s', timestamp, output)
+
+	def on_look(self, timestamp, aruco_camera, exception):
+		"""Report fixation and metrics on Ivy bus."""
+
+		# Select 'Main' layer
+		main_layer = aruco_camera.layers['Main']
+
+		if GazeFeatures.is_fixation(aruco_camera.last_gaze_movement()):
+
+			fixation = aruco_camera.last_gaze_movement()
+
+			# Output in progress fixation data
+			if not fixation.is_finished():
+
+				output = f'FixationInProgress Start={fixation[0].timestamp} Duration={fixation.duration} AOI={main_layer.last_looked_aoi_name()} Probabilities={main_layer.aoi_matcher.looked_probabilities()}'
+
+				# Send Ivy message
+				IvySendMsg(output)
+				
+				logging.debug('%i %s %s %s', timestamp, aruco_camera.last_gaze_position().value, aruco_camera.name, output)
+						
+			# Output finished fixation data
+			else:
+
+				output = f'FixationEnd Start={fixation[0].timestamp} Duration={fixation.duration} AOI={main_layer.aoi_matcher.looked_aoi_name()} Probabilities={main_layer.aoi_matcher.looked_probabilities()}'
+
+				# Send Ivy message
+				IvySendMsg(output)
+				
+				logging.debug('%i %s %s %s', timestamp, aruco_camera.last_gaze_position().value, aruco_camera.name, output)
+```