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Monitoring for parameter changes (C++)

Goal: Learn to use the ParameterEventHandler class to monitor and respond to parameter changes.

Tutorial level: Intermediate

Time: 20 minutes

Minimum Platform: Galactic

Background

Often a node needs to respond to changes to its own parameters or another node’s parameters. The ParameterEventHandler class makes it easy to listen for parameter changes so that your code can respond to them. This tutorial will show you how to use the C++ version of the ParameterEventHandler class to monitor for changes to a node’s own parameters as well as changes to another node’s parameters.

Prerequisites

Before starting this tutorial, you should first complete the following tutorials:

In addition, you must be running the Galactic distribution of ROS 2.

Tasks

In this tutorial, you will create a new package to contain some sample code, write some C++ code to use the ParameterEventHandler class, and test the resulting code.

1 Create a package

First, open a new terminal and source your ROS 2 installation so that ros2 commands will work.

Follow these instructions to create a new workspace named ros2_ws.

Recall that packages should be created in the src directory, not the root of the workspace. So, navigate into ros2_ws/src and then create a new package there:

$ ros2 pkg create --build-type ament_cmake --license Apache-2.0 cpp_parameter_event_handler --dependencies rclcpp

Your terminal will return a message verifying the creation of your package cpp_parameter_event_handler and all its necessary files and folders.

The --dependencies argument will automatically add the necessary dependency lines to package.xml and CMakeLists.txt.

1.1 Update package.xml

Because you used the --dependencies option during package creation, you don’t have to manually add dependencies to package.xml or CMakeLists.txt. As always, though, make sure to add the description, maintainer email and name, and license information to package.xml.

<description>C++ parameter events client tutorial</description>
<maintainer email="[email protected]">Your Name</maintainer>
<license>Apache License 2.0</license>

2 Write the C++ node

Inside the ros2_ws/src/cpp_parameter_event_handler/src directory, create a new file called parameter_event_handler.cpp and paste the following code within:

#include <memory>

#include "rclcpp/rclcpp.hpp"

class SampleNodeWithParameters : public rclcpp::Node
{
public:
  SampleNodeWithParameters()
  : Node("node_with_parameters")
  {
    this->declare_parameter("an_int_param", 0);

    // Create a parameter subscriber that can be used to monitor parameter changes
    // (for this node's parameters as well as other nodes' parameters)
    param_subscriber_ = std::make_shared<rclcpp::ParameterEventHandler>(this);

    // Set a callback for this node's integer parameter, "an_int_param"
    auto cb = [this](const rclcpp::Parameter & p) {
        RCLCPP_INFO(
          this->get_logger(), "cb: Received an update to parameter \"%s\" of type %s: \"%ld\"",
          p.get_name().c_str(),
          p.get_type_name().c_str(),
          p.as_int());
      };
    cb_handle_ = param_subscriber_->add_parameter_callback("an_int_param", cb);
  }

private:
  std::shared_ptr<rclcpp::ParameterEventHandler> param_subscriber_;
  std::shared_ptr<rclcpp::ParameterCallbackHandle> cb_handle_;
};

int main(int argc, char ** argv)
{
  rclcpp::init(argc, argv);
  rclcpp::spin(std::make_shared<SampleNodeWithParameters>());
  rclcpp::shutdown();

  return 0;
}

2.1 Examine the code

The first statement, #include <memory> is included so that the code can utilize the std::make_shared template. The next, #include "rclcpp/rclcpp.hpp" is included to allow the code to reference the various functionality provided by the rclcpp interface, including the ParameterEventHandler class.

After the class declaration, the code defines a class, SampleNodeWithParameters. The constructor for the class declares an integer parameter an_int_param, with a default value of 0. Next, the code creates a ParameterEventHandler that will be used to monitor changes to parameters. Finally, the code creates a lambda function and sets it as the callback to invoke whenever an_int_param is updated.

Note

It is very important to save the handle that is returned by add_parameter_callback; otherwise, the callback will not be properly registered.

SampleNodeWithParameters()
: Node("node_with_parameters")
{
  this->declare_parameter("an_int_param", 0);

  // Create a parameter subscriber that can be used to monitor parameter changes
  // (for this node's parameters as well as other nodes' parameters)
  param_subscriber_ = std::make_shared<rclcpp::ParameterEventHandler>(this);

  // Set a callback for this node's integer parameter, "an_int_param"
  auto cb = [this](const rclcpp::Parameter & p) {
      RCLCPP_INFO(
        this->get_logger(), "cb: Received an update to parameter \"%s\" of type %s: \"%ld\"",
        p.get_name().c_str(),
        p.get_type_name().c_str(),
        p.as_int());
    };
  cb_handle_ = param_subscriber_->add_parameter_callback("an_int_param", cb);
}

Following the SampleNodeWithParameters is a typical main function which initializes ROS, spins the sample node so that it can send and receive messages, and then shuts down after the user enters ^C at the console.

int main(int argc, char ** argv)
{
  rclcpp::init(argc, argv);
  rclcpp::spin(std::make_shared<SampleNodeWithParameters>());
  rclcpp::shutdown();

  return 0;
}

2.2 Add executable

To build this code, first open the CMakeLists.txt file and add the following lines of code below the dependency find_package(rclcpp REQUIRED)

add_executable(parameter_event_handler src/parameter_event_handler.cpp)
ament_target_dependencies(parameter_event_handler rclcpp)

install(TARGETS
  parameter_event_handler
  DESTINATION lib/${PROJECT_NAME}
)

3 Build and run

It’s good practice to run rosdep in the root of your workspace (ros2_ws) to check for missing dependencies before building:

$ rosdep install -i --from-path src --rosdistro $ROS_DISTRO -y

Navigate back to the root of your workspace, ros2_ws, and build your new package:

$ colcon build --packages-select cpp_parameter_event_handler

Open a new terminal, navigate to ros2_ws, and source the setup files:

$ . install/setup.bash

Now run the node:

$ ros2 run cpp_parameter_event_handler parameter_event_handler

The node is now active and has a single parameter and will print a message whenever this parameter is updated. To test this, open up another terminal and source the ROS setup file as before (. install/setup.bash) and execute the following command:

$ ros2 param set node_with_parameters an_int_param 43

The terminal running the node will display a message similar to the following:

[INFO] [1606950498.422461764] [node_with_parameters]: cb: Received an update to parameter "an_int_param" of type integer: "43"

The callback we set previously in the node has been invoked and has displayed the new updated value. You can now terminate the running parameter_event_handler sample using ^C in the terminal.

Extensions

So far, we built and tested a small node that monitors a single parameter owned by the node itself. Using this node as a base, two other usecases where the ParameterEventHandler can be useful are presented below.

Monitor changes to another node’s parameters

You can also use the ParameterEventHandler to monitor parameter changes to another node’s parameters. Let’s update the SampleNodeWithParameters class to also monitor for changes to a parameter in another node. We will use the parameter_blackboard demo application to host a double parameter that we will monitor for updates.

First update the constructor to add the following code after the existing code:

// Now, add a callback to monitor any changes to the remote node's parameter. In this
// case, we supply the remote node name.
auto cb2 = [this](const rclcpp::Parameter & p) {
    RCLCPP_INFO(
      this->get_logger(), "cb2: Received an update to parameter \"%s\" of type: %s: \"%.02lf\"",
      p.get_name().c_str(),
      p.get_type_name().c_str(),
      p.as_double());
  };
auto remote_node_name = std::string("parameter_blackboard");
auto remote_param_name = std::string("a_double_param");
cb_handle2_ = param_subscriber_->add_parameter_callback(remote_param_name, cb2, remote_node_name);

Then add another member variable, cb_handle2 for the additional callback handle:

private:
  std::shared_ptr<rclcpp::ParameterEventHandler> param_subscriber_;
  std::shared_ptr<rclcpp::ParameterCallbackHandle> cb_handle_;
  std::shared_ptr<rclcpp::ParameterCallbackHandle> cb_handle2_;  // Add this
};

In a terminal, navigate back to the root of your workspace, ros2_ws, and build your updated package as before:

$ colcon build --packages-select cpp_parameter_event_handler

Then source the setup files:

$ . install/setup.bash

Now, to test monitoring of remote parameters, first run the newly-built parameter_event_handler code:

$ ros2 run cpp_parameter_event_handler parameter_event_handler

Next, from another terminal (with ROS initialized), run the parameter_blackboard demo application, as follows:

$ ros2 run demo_nodes_cpp parameter_blackboard

Finally, from a third terminal (with ROS initialized), let’s set a parameter on the parameter_blackboard node:

$ ros2 param set parameter_blackboard a_double_param 3.45

Upon executing this command, you should see output in the parameter_event_handler window, indicating that the callback function was invoked upon the parameter update:

[INFO] [1606952588.237531933] [node_with_parameters]: cb2: Received an update to parameter "a_double_param" of type: double: "3.45"

Monitor all node parameters simultaneously

If you need to monitor multiple nodes or parameters at the same time, it would be cumbersome to have to call add_parameter_callback once for each of them. In this case, you can use add_parameter_event_callback to register a single callback that fires when any parameters of any nodes change.

To do this, first update the SampleNodeWithParameters constructor to add the following code:

this->declare_parameter("another_double_param", 0.0);

...

auto event_cb = [this](const rcl_interfaces::msg::ParameterEvent & parameter_event) {
    RCLCPP_INFO(
      this->get_logger(), "Received parameter event from node \"%s\"",
      parameter_event.node.c_str());

    for (const auto& p : parameter_event.changed_parameters) {
      RCLCPP_INFO(
        this->get_logger(), "Inside event: \"%s\" changed to %s",
        p.name.c_str(),
        rclcpp::Parameter::from_parameter_msg(p).value_to_string().c_str());
    };
  };
event_cb_handle_ = param_subscriber_->add_parameter_event_callback(event_cb);

This declares a new double parameter another_double_param and adds an event callback that will monitor both parameters. Note that the parameter_event is of type rcl_interfaces/msg/ParameterEvent. Although it’s not shown in this tutorial, event callbacks can also be used to monitor when parameters are added or deleted.

Finally, don’t forget to add the event callback handle as a private member:

private:
  ...
  std::shared_ptr<rclcpp::ParameterEventCallbackHandle> event_cb_handle_;

Navigate back to the root of your workspace, ros2_ws, and rebuild your updated package as before:

$ colcon build --packages-select cpp_parameter_event_handler

Then source the setup files:

$ . install/setup.bash

To test the new event callback, first run the parameter_event_handler node:

$ ros2 run cpp_parameter_event_handler parameter_event_handler

Then, from a second terminal (with ROS sourced), let’s set the original int parameter:

$ ros2 param set node_with_parameters an_int_param 44

Upon executing this command, you should see both the single-parameter callback, as well as the event callback being fired:

[INFO] [1747144403.418980063] [node_with_parameters]: cb: Received an update to parameter "an_int_param" of type integer: "44"
[INFO] [1747144403.419086611] [node_with_parameters]: Received parameter event from node "/node_with_parameters"
[INFO] [1747144403.419114103] [node_with_parameters]: Inside event: "an_int_param" changed to 44

Now set the new double parameter:

$ ros2 param set node_with_parameters another_double_param 4.4

Since no single-parameter callback was added (via add_parameter_callback) for the double parameter, we should see only the event callback fire:

[INFO] [1747144452.917437113] [node_with_parameters]: Received parameter event from node "/node_with_parameters"
[INFO] [1747144452.917591649] [node_with_parameters]: Inside event: "another_double_param" changed to 4.400000

Note

When setting multiple parameters at once, it’s best to use set_parameters_atomically, explained in Parameters. This way, the event callback is only fired once.

Summary

You created a node with a parameter and used the ParameterEventHandler class to set a callback to monitor changes to that parameter. You also used the same class to monitor changes to a remote node, and to monitor all parameters in a single event callback. The ParameterEventHandler is a convenient way to monitor for parameter changes so that you can then respond to the updated values.

Related content

To learn how to adapt ROS 1 parameter files for ROS 2, see the Migrating YAML parameter files from ROS 1 to ROS2 tutorial.