Default ROS serialization

mqtt_ros_bridge/bridge_node.py

@@ -5,24 +5,32 @@
 from rclpy.node import Node
 from rclpy.publisher import Publisher
 from rclpy.subscription import Subscription
+from rclpy._rclpy_pybind11 import RMWError
 
 from std_msgs.msg import String
 
 import paho.mqtt.client as MQTT
 
+from mqtt_ros_bridge.serializer import Serializer, ROSDefaultSerializer
+
 
 @dataclass
 class TopicInfo():
     """Metadata about a single topic."""
 
     name: str
+    msg_type: Any
+    serializer: type[Serializer]
     publish_on_ros: bool
 
 
-TOPICS: list[TopicInfo] = [
-    TopicInfo('pub_topic', True),
-    TopicInfo('sub_topic', False)
-]
+TOPICS: dict[str, TopicInfo] = {
+    'pub_topic': TopicInfo('pub_topic', String, ROSDefaultSerializer, True),
+    'sub_topic': TopicInfo('sub_topic', String, ROSDefaultSerializer, False)
+}
+
+MQTT_PORT = 1883
+MQTT_KEEPALIVE = 60
 
 
 class BridgeNode(Node):
@@ -31,45 +39,46 @@ class BridgeNode(Node):
     def __init__(self) -> None:
         super().__init__('mqtt_bridge_node')
 
-        print('Creating MQTT ROS bridge node')
+        self.get_logger().info('Creating MQTT ROS bridge node')
 
         self.mqtt_client = MQTT.Client()
         self.mqtt_client.enable_logger()
-        self.mqtt_client.connect('localhost', port=1883, keepalive=60)
+        self.mqtt_client.connect('localhost', port=MQTT_PORT, keepalive=MQTT_KEEPALIVE)
         self.mqtt_client.loop_start()
 
         self.ros_publishers: dict[str, Publisher] = {}
         self.ros_subscriptions: list[Subscription] = []
 
-        for topic_info in TOPICS:
+        for topic_info in TOPICS.values():
             if topic_info.publish_on_ros:
-                publisher = self.create_publisher(String, topic_info.name, 10)
+                publisher = self.create_publisher(topic_info.msg_type, topic_info.name, 10)
                 self.ros_publishers[topic_info.name] = publisher
                 self.mqtt_client.subscribe(topic_info.name)
             else:
+                callback = self.make_ros_callback(topic_info)
                 subscription = self.create_subscription(
-                    String, topic_info.name, self.make_ros_receiver(topic_info.name), 10)
+                    topic_info.msg_type, topic_info.name, callback, 10)
                 self.ros_subscriptions.append(subscription)
 
-        self.mqtt_client.on_message = self.mqtt_msg_received
+        self.mqtt_client.on_message = self.mqtt_callback
 
-    def make_ros_receiver(self, topic: str):
+    def make_ros_callback(self, topic_info: TopicInfo):
         """
         Create a callback function which re-publishes messages on the same topic in MQTT.
 
         Parameters
         ----------
-        topic : str
-            the topic that the callback will publish on
+        topic_info : TopicInfo
+            information about the topic that the callback will publish on
 
         """
-        def callback(msg: String):
-            self.get_logger().info(f"ROS RECEIVED: Topic: '{topic}' Payload: '{msg}'")
-            self.mqtt_client.publish(topic, msg.data)
+        def callback(msg: topic_info.msg_type):
+            self.get_logger().info(f'ROS RECEIVED: Topic: "{topic_info.name}" Payload: "{msg}"')
+            self.mqtt_client.publish(topic_info.name, topic_info.serializer.serialize(msg))
 
         return callback
 
-    def mqtt_msg_received(self, _client: MQTT.Client, _userdata: Any, mqtt_msg: MQTT.MQTTMessage):
+    def mqtt_callback(self, _client: MQTT.Client, _userdata: Any, mqtt_msg: MQTT.MQTTMessage):
         """
         Re-publish messages from MQTT on the same topic in ROS.
 
@@ -83,12 +92,19 @@ def mqtt_msg_received(self, _client: MQTT.Client, _userdata: Any, mqtt_msg: MQTT
             the message received over MQTT
 
         """
+        topic_info = TOPICS[mqtt_msg.topic]
+
         self.get_logger().info(
-            f"MQTT RECEIVED: Topic: '{mqtt_msg.topic}' Payload: '{mqtt_msg.payload!r}'")
+            f'MQTT RECEIVED: Topic: "{topic_info.name}" Payload: "{mqtt_msg.payload!r}"')
+
+        try:
+            ros_msg = topic_info.serializer.deserialize(mqtt_msg.payload, topic_info.msg_type)
+        except RMWError:
+            self.get_logger().info('Dropping message with bad serialization received' +
+                f'from MQTT on topic "{mqtt_msg.topic}": "{mqtt_msg.payload!r}"')
+            return
 
-        ros_msg = String()
-        ros_msg.data = mqtt_msg.payload.decode('utf-8')
-        self.ros_publishers[mqtt_msg.topic].publish(ros_msg)
+        self.ros_publishers[topic_info.name].publish(ros_msg)
 
 
 def main(args=None):

mqtt_ros_bridge/serializer.py

@@ -0,0 +1,57 @@
+from typing import Any
+from abc import ABC, abstractmethod
+
+from rclpy.serialization import serialize_message, deserialize_message
+
+class Serializer(ABC):
+    """Serializes and deserializes ROS messages for transmission over MQTT."""
+
+    @staticmethod
+    @abstractmethod
+    def serialize(message: Any) -> bytes:
+        """
+        Serialize the provided ROS message to a bytes for MQTT.
+
+        Parameters
+        ----------
+        message : Any
+            the ROS message to serialize
+
+        Returns
+        -------
+        bytes
+            the bytes formed by serializing the ROS message
+
+        """
+
+    @staticmethod
+    @abstractmethod
+    def deserialize(serialized_message: bytes, message_type: Any) -> Any:
+        """
+        Deserialize the provided bytes into a ROS message of the provided type.
+
+        Parameters
+        ----------
+        serialized_message : bytes
+            the bytes generated by serializing a ROS message
+        message_type : Any
+            the type of the message to create
+
+        Returns
+        -------
+        Any
+            the ROS message formed by deserializing the bytes
+
+        """
+
+
+class ROSDefaultSerializer(Serializer):
+    """Serialize and deserialize messages using the default ROS message serializer."""
+
+    @staticmethod
+    def serialize(message: Any) -> bytes:
+        return serialize_message(message)
+
+    @staticmethod
+    def deserialize(serialized_message: bytes, message_type: Any) -> Any:
+        return deserialize_message(serialized_message, message_type)