Feature Request: Dynamic Type Handling for Generic Subscriptions about rclcpp HOT 10 OPEN

I stumbled upon https://github.com/facontidavide/rosx_introspection and was able to make a prototype! Here is the pseudo-code:

#include "rosx_introspection/ros_parser.hpp"
#include "rosx_introspection/ros_utils/ros2_helpers.hpp"

using namespace RosMsgParser;

ParsersCollection<ROS2_Deserializer> parser_;

void print_fields(const std::string& topic_name, const std::string& topic_type, std::shared_ptr<const rclcpp::SerializedMessage> serialized_message) {
    parser_.registerParser(topic_name, ROSType(topic_type),
                            GetMessageDefinition(topic_type));
    std::vector<uint8_t> buffer(serialized_message->get_rcl_serialized_message().buffer_length);
    memcpy(buffer.data(), serialized_message->get_rcl_serialized_message().buffer, serialized_message->get_rcl_serialized_message().buffer_length);

    const FlatMessage* flat_container = parser_.deserialize(topic_name, Span<uint8_t>(buffer));

    for (auto& it : flat_container->value)
    {
        std::cout << it.first << " >> " << it.second.convert<double>() << std::endl;
    }
    for (auto& it : flat_container->name)
    {
        std::cout << it.first << " >>>" << it.second << std::endl;
    }
}

@facontidavide thanks for that project. As there is no real guide, I got inspired from the tests mostly, am I using your library correctly? Any recommendations?

@Barry-Xu-2018 @fujitatomoya maybe this could be a good starting point to implement something similar in rclcpp?

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@tonynajjar

This is a simple sample (The code is a bit ugly. I haven't had time to check out the implementation of rosx_introspection yet.). It tries to get items of sensor_msgs::msg::Temperature based on generic subscription.
https://github.com/Barry-Xu-2018/msg_introspection_demo

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I found this simillar unanswered question: https://answers.ros.org/question/405796/deserialize-message-with-topic-name-ros2/

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@Barry-Xu-2018 do you have any thoughts?

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I think there are two scenarios.

• The message type is one of a few known types
• The message type is completely unknown (many types).

In the first scenario, you should be able to easily cast the pointer to the message type.

In the second scenario, you can use type support to parse each type in the message one by one.
e.g.
https://github.com/ros2/rosbag2/blob/fa84cfeeb0b640d53c23d25634ec5b0177ae1efe/rosbag2_transport/src/rosbag2_transport/player_service_client.cpp#L163-L168
If you want this way, I can provide a simple example to demonstrate how to do.

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Thanks @Barry-Xu-2018 for your answer. Mine is the second scenario, I'd be grateful if you can provide an example. I would try it out and give feedback in a couple of hours

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@Barry-Xu-2018 @tonynajjar

what about having either demos or examples for this? i believe that would be useful for ROS community?

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what about having either demos or examples for this? i believe that would be useful for ROS community?

It would definitely be useful to a handful of people given that it's been asked a couple of times already here and there

Also the reason why we would need a tutorial for this is because it's (imo) a reasonable thing for high-level users of ROS to want to do but the way to do it seems pretty low-level (with type support and other things I've never heard of with 5 years of experience with ROS). So I'm wondering if it would make sense to expose this functionality on a higher-level? Some function in client libraries?

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Just found #2260 which is also similar

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@tonynajjar

I'd be grateful if you can provide an example. I would try it out and give feedback in a couple of hours

Apologies for the delayed reply. I will prepare a simple example for you.

I'll look into https://github.com/facontidavide/rosx_introspection.

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