deserialize SerializedMessage into arbitrary message type on runtime?

[nyxaria]

Hello,

I am just starting to use ROS2 and am looking into creating a generic subscriber which can subscribe to any topic, get rclcpp::SerializedMessage and then convert it to the message_type of the topic that the message came from.

So far I have the generic subscriber working and have a SerializedMessage, but am confused how to go about converting it to a normal ros message.

My thought is to use the Serialization.deserialize_message. To do this, I would need a function which does "std_msgs::msg::String" -> std_msgs::msg::String (ie string name of message_type -> actual message_type class). Is there something which achieves this?

What I'm ultimately trying to achieve is to get a key-value vector of all the data inside any message.

Furthermore, for this approach I would need to create a Serialization object for each message_type. Is there a better way?

Many thanks

[Karsten1987]

There's pretty much exactly what you need inside rosbag2_transport:

• https://github.com/ros2/rosbag2/blob/master/rosbag2_transport/src/rosbag2_transport/generic_publisher.hpp
• https://github.com/ros2/rosbag2/blob/master/rosbag2_transport/src/rosbag2_transport/generic_subscription.hpp

There was work started in here to factor this out of rosbag and make it available within the C++ library.

[nyxaria]

Thank you @Karsten1987, I have used a generic subscriber to get SerializedMessages, but I'm unsure how to extract the data from it into a key value pair array.

[nyxaria]

I have found this code from here

    my_pkg::msg::Msg msg;
    auto ros_message = std::make_shared<rosbag2_introspection_message_t>();
    ros_message->time_stamp = 0;
    ros_message->message = nullptr;
    ros_message->allocator = rcutils_get_default_allocator();
    ros_message->message = &msg;
    auto type_support = rosbag2::get_typesupport("my_pkg/msg/Msg", "rosidl_typesupport_cpp");

    rosbag2::SerializationFormatConverterFactory factory;
    std::unique_ptr<rosbag2::converter_interfaces::SerializationFormatDeserializer> cdr_deserializer_;
    cdr_deserializer_ = factory.load_deserializer("cdr");

    cdr_deserializer_->deserialize(msg, type_support, ros_message);

Is there some way to make the type of msg set from the message_type string (ie "my_pkg/msg/Msg")?

Maybe I'm stuck in a XY problem, I'm not even sure if this is needed and if I can somehow extract the key value data directly from the SerializedMessage. What I basically need to achieve is generic subscriber -> JSON representation of any message

[Karsten1987]

does the snippet from the demos help you here? It's converting an incoming serialized message into a ros2 message.
https://github.com/ros2/demos/blob/master/demo_nodes_cpp/src/topics/listener_serialized_message.cpp#L56-L63

I feel though your ticket here is actually better suited for a question at https://answers.ros.org/questions/

[nyxaria]

I asked on ros answers, but did not get anywhere. The demo you posted is ALMOST what I want -- in that code, you know the data type is going to be std_msgs::msg::String, but what if you only had the message type as a string from node.get_topic_names_and_types()? Can you change this using MessageT = std_msgs::msg::String; in this demo into using MessageT = class_from_string("std_msgs::msg::String")?

Thanks @Karsten1987 for your help, I'm new to ROS2 and I'm just trying to read the source code to figure this stuff out but there is a lot of ground to cover.

[Karsten1987]

The rclcpp::Serialization class takes a pointer to a message type support: https://github.com/ros2/rclcpp/blob/master/rclcpp/include/rclcpp/serialization.hpp#L56

So the question really boils down to how to get this typesupport from a string. That's a little bit tricky, but can be done with the typesupport helper functions we've implemented within rosbag2: https://github.com/ros2/rosbag2/blob/master/rosbag2_cpp/include/rosbag2_cpp/typesupport_helpers.hpp
Example use case: https://github.com/ros2/rosbag2/blob/master/rosbag2_cpp/test/rosbag2_cpp/test_typesupport_helpers.cpp#L79-L82

[clalancette]

@nyxaria @Karsten1987 Can we close this issue now?

[nyxaria]

Yes, thank you for your help

[agoeckner]

Hello @nyxaria. Would you be able to post your completed code?

Also, is there a way to do this using ROS2 Eloquent? It seems that Eloquent does not have the rclcpp::Serialization class mentioned above by @Karsten1987 .

[Karsten1987]

The functionality was already available in Eloquent, unfortunately just not in a clean C++ API:
https://github.com/ros2/demos/blob/eloquent/demo_nodes_cpp/src/topics/talker_serialized_message.cpp#L58-L112

[agoeckner]

Thanks for the response, @Karsten1987! That's very helpful. My issue right now is that I don't know the message type at compile-time. I only have a string describing the type, like "sensor_msgs/msg/BatteryState", from which I derive the rosidl_message_type_support_t object. How can I create a message object given only the rosidl_message_type_support_t?

[Karsten1987]

That should be answered via the previous comment in here. It's part of the typesupport helpers within rosbag2, where we load the message type based on a string.
It's essentially a dynamic lookup of the library where we expect the symbol for the rosidl_message_type_support_t to be present.

[Uhrendoktor]

For any one else looking into this in the future wanting a concrete example. I've written something along the lines in a recent project, where I edit frame_ids in Headers of arbitrary message at runtime: https://github.com/Uhrendoktor/tf2_remapper/tree/main

[xhy279]

For any one else looking into this in the future wanting a concrete example. I've written something along the lines in a recent project, where I edit frame_ids in Headers of arbitrary message at runtime: Uhrendoktor/tf2_remapper@main

Just for clear, I can parse message with its string type and without its actualy type, but I still have to know the actual message type in order to use it, right? As you wrote here:
(https://github.com/Uhrendoktor/tf2_remapper/blob/c8b15a174e558bdf2fe44e607e77e5ffd9ea30b4/src/Tf2Remapper.cpp#L282C60-L282C95)