[WIP] Add a tutorial for the ParameterEventHandler class

source/Tutorials.rst

@@ -48,6 +48,7 @@ Beginner: Client Libraries
    Tutorials/Single-Package-Define-And-Use-Interface
    Tutorials/Using-Parameters-In-A-Class-CPP
    Tutorials/Using-Parameters-In-A-Class-Python
+   Tutorials/Monitoring-For-Parameter-Changes-CPP
    Tutorials/Getting-Started-With-Ros2doctor
 
 Intermediate

source/Tutorials/Monitoring-For-Parameter-Changes-CPP.rst

@@ -0,0 +1,349 @@
+Monitoring for parameter changes (C++)
+======================================
+
+**Goal:** Learn to use the ParameterEventHandler class to monitor and respond to parameter changes.
+
+**Tutorial level:** Beginner
+
+**Time:** 20 minutes
+
+.. contents:: Contents
+   :depth: 2
+   :local:
+
+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: 
+
+- :ref:`ROS2Params`
+- :ref:`CppParamNode`
+
+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 :ref:`source your ROS 2 installation <ConfigROS2>` so that ``ros2`` commands will work.
+
+Navigate into the ``dev_ws`` directory created in a previous tutorial (or follow `these instructions <https://index.ros.org/doc/ros2/Tutorials/Workspace/Creating-A-Workspace/#create-a-new-directory>`_ if you no longer have the directory and need to create it again).
+
+Recall that packages should be created in the ``src`` directory, not the root of the workspace. So, navigate into ``dev_ws/src`` and then create a new package there:
+
+.. code-block:: console
+
+  ros2 pkg create --build-type ament_cmake 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``.
+
+.. code-block:: xml
+
+  <description>C++ parameter events client tutorial</description>
+  <maintainer email="you@email.com">Your Name</maintainer>
+  <license>Apache License 2.0</license>
+
+2 Write the C++ node
+^^^^^^^^^^^^^^^^^^^^
+
+Inside the ``dev_ws/src/cpp_parameter_event_handler/src`` directory, create a new file called ``parameter_event_handler.cpp`` and paste the following code within:
+
+.. code-block:: C++
+
+    #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);
+      }
+
+      ~SampleNodeWithParameters()
+      {
+        param_subscriber_->remove_parameter_callback(cb_handle_);
+      }
+
+    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. 
+
+.. code-block:: C++
+
+    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);
+    }
+
+The ``ParameterEventHandler``'s add_parameter_callback method returns a callback handle that is stored in a member variable. This handle is used in the ``~SampleNodeWithParameters`` destructor to remove the callback when the node is destroyed.
+
+.. code-block:: C++
+
+    ~SampleNodeWithParameters()
+    {
+      param_subscriber_->remove_parameter_callback(cb_handle_);
+    }
+
+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.
+
+.. code-block:: C++
+
+    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)`` 
+
+.. code-block:: console
+
+    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 (``dev_ws``) to check for missing dependencies before building:
+
+.. tabs::
+
+   .. group-tab:: Linux
+
+      .. code-block:: console
+
+        rosdep install -i --from-path src --rosdistro <distro> -y
+
+   .. group-tab:: macOS
+
+      rosdep only runs on Linux, so you can skip ahead to next step.
+
+   .. group-tab:: Windows
+
+      rosdep only runs on Linux, so you can skip ahead to next step.
+
+Navigate back to the root of your workspace, ``dev_ws``, and build your new package:
+
+.. code-block:: console
+
+    colcon build --packages-select cpp_parameter_event_handler
+
+Open a new terminal, navigate to ``dev_ws``, and source the setup files:
+
+.. tabs::
+
+  .. group-tab:: Linux
+
+    .. code-block:: console
+
+      . install/setup.bash
+
+  .. group-tab:: macOS
+
+    .. code-block:: console
+
+      . install/setup.bash
+
+  .. group-tab:: Windows
+
+    .. code-block:: console
+
+      call install/setup.bat
+
+Now run the node:
+
+.. code-block:: console
+
+     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:
+
+.. code-block:: console
+
+    ros2 param set node_with_parameters an_int_param 43
+
+The terminal running the node will display a message similar to the following:
+
+.. code-block:: console
+
+    [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.
+
+3.1 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:
+
+.. code-block:: C++
+
+    // 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);
+
+
+The destructor and member variables need to be updated as well to account for the new ``cb_handle2`` member variable:
+
+.. code-block:: C++
+
+    ~SampleNodeWithParameters()
+    {
+      param_subscriber_->remove_parameter_callback(cb_handle_);
+      param_subscriber_->remove_parameter_callback(cb_handle2_);  // Add this
+    }
+
+  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, ``dev_ws``, and build your updated package as before:
+
+.. code-block:: console
+
+    colcon build --packages-select cpp_parameter_event_handler
+
+Then source the setup files:
+
+.. tabs::
+
+  .. group-tab:: Linux
+
+    .. code-block:: console
+
+      . install/setup.bash
+
+  .. group-tab:: macOS
+
+    .. code-block:: console
+
+      . install/setup.bash
+
+  .. group-tab:: Windows
+
+    .. code-block:: console
+
+      call install/setup.bat
+
+Now, to test monitoring of remote parameters, first run the newly-built parameter_event_handler code:
+
+.. code-block:: console
+
+     ros2 run cpp_parameter_event_handler parameter_event_handler
+
+Next, from another teminal (with ROS initialized), run the parameter_blackboard demo application, as follows:
+
+.. code-block:: console
+
+     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:
+
+.. code-block:: console
+
+     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:
+
+.. code-block:: console
+
+    [INFO] [1606952588.237531933] [node_with_parameters]: cb2: Received an update to parameter "a_double_param" of type: double: "3.45"
+
+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. The ParameterEventHandler is a convenient way to monitor for parameter changes so that you can then respond to the updated values.
+
+Next steps
+----------
+
+Now that you have examined parameters and have some packages and ROS 2 systems of your own, the :ref:`next tutorial <Ros2Doctor>` will show you how to examine issues in your environment and systems in case you have problems.