Writing a Composable Node (C++) Tutorial (#4106) (#4172)

* Writing a Composable Node (C++) Tutorial

* Add Package.xml note

* Add links to demos

Signed-off-by: David V. Lu!! <[email protected]>
Co-authored-by: Chris Lalancette <[email protected]>
(cherry picked from commit 0c79b0f)

Co-authored-by: David V. Lu!! <[email protected]>

source/Concepts/Intermediate/About-Composition.rst

@@ -54,6 +54,8 @@ Additionally, once a component is created, it must be registered with the index
    # To register multiple components in the same shared library, use multiple calls
    # rclcpp_components_register_nodes(talker_component "composition::Talker2")
 
+For an example, :doc:`check out this tutorial <../../Tutorials/Intermediate/Writing-a-Composable-Node>`
+
 .. note::
 
    In order for the component_container to be able to find desired components, it must be executed or launched from a shell that has sourced the corresponding workspace.

source/Releases/Release-Dashing-Diademata.rst

@@ -499,7 +499,7 @@ If not present, registration macros must be added to the project's CMake.
   add_library(listener src/listener.cpp)
   rclcpp_components_register_nodes(listener "composition::Listener")
 
-For more information on composition, see `the tutorial <https://index.ros.org/doc/ros2/Tutorials/Composition/>`__
+For more information on composition, see `the tutorial <../Tutorials/Intermediate/Writing-a-Composable-Node>`
 
 rclpy
 ^^^^^

source/Tutorials/Intermediate.rst

@@ -8,6 +8,7 @@ Intermediate
    Intermediate/Creating-an-Action
    Intermediate/Writing-an-Action-Server-Client/Cpp
    Intermediate/Writing-an-Action-Server-Client/Py
+   Intermediate/Writing-a-Composable-Node
    Intermediate/Composition
    Intermediate/Monitoring-For-Parameter-Changes-CPP
    Intermediate/Launch/Launch-Main

source/Tutorials/Intermediate/Composition.rst

@@ -21,6 +21,8 @@ Background
 
 See the :doc:`conceptual article <../../Concepts/Intermediate/About-Composition>`.
 
+For information on how to write a composable node, :doc:`check out this tutorial <Writing-a-Composable-Node>`.
+
 Run the demos
 -------------
 

source/Tutorials/Intermediate/Writing-a-Composable-Node.rst

@@ -0,0 +1,170 @@
+Writing a Composable Node (C++)
+===============================
+
+.. contents:: Table of Contents
+   :depth: 2
+   :local:
+
+Starting Place
+--------------
+
+Let's assume that you have a regular ``rclcpp::Node`` executable that you want to run in the same process as other nodes to enable more efficient communication.
+
+We'll start from having a class that directly inherits from ``Node``, and that also has a main method defined.
+
+.. code-block:: c++
+
+    namespace palomino
+    {
+        class VincentDriver : public rclcpp::Node
+        {
+            // ...
+        };
+    }
+
+    int main(int argc, char * argv[])
+    {
+        rclcpp::init(argc, argv);
+        rclcpp::spin(std::make_shared<palomino::VincentDriver>());
+        rclcpp::shutdown();
+        return 0;
+    }
+
+This will typically be compiled as an executable in your Cmake.
+
+.. code-block:: cmake
+
+    # ...
+    add_executable(vincent_driver src/vincent_driver.cpp)
+    # ...
+    install(TARGETS vincent_driver
+        DESTINATION lib/${PROJECT_NAME}
+    )
+
+Code Updates
+------------
+
+Add the Package Dependency
+^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Your `package.xml <https://github.com/ros2/demos/tree/{REPOS_FILE_BRANCH}/composition/package.xml>`__ should have a dependency on ``rclcpp_components``, a la
+
+.. code-block:: xml
+
+    <depend>rclcpp_components</depend>
+
+Alternatively, you can independently add a ``build_depend/exec_depend``.
+
+Class Definition
+^^^^^^^^^^^^^^^^
+
+The only change to your class definition that you may have to do is ensure that `the constructor for the class <https://github.com/ros2/demos/tree/{REPOS_FILE_BRANCH}/composition/src/talker_component.cpp>`__ takes a ``NodeOptions`` argument.
+
+.. code-block:: c++
+
+    VincentDriver(const rclcpp::NodeOptions & options) : Node("vincent_driver", options)
+    {
+      // ...
+    }
+
+No More Main Method
+^^^^^^^^^^^^^^^^^^^
+
+Replace your main method with a ``pluginlib``-style macro invocation.
+
+.. code-block:: c++
+
+    #include <rclcpp_components/register_node_macro.hpp>
+    RCLCPP_COMPONENTS_REGISTER_NODE(palomino::VincentDriver)
+
+.. caution::
+    If the main method you are replacing contains a ``MultiThreadedExecutor``, be sure to make note and ensure that your container node is multithreaded.
+    See section below.
+
+CMake Changes
+^^^^^^^^^^^^^
+First, add ``rclcpp_components`` as a dependency in your CMakeLists.txt with:
+
+.. code-block:: cmake
+
+    find_package(rclcpp_components REQUIRED)
+
+Second, we're going to replace our ``add_executable`` with a ``add_library`` with a new target name.
+
+.. code-block:: cmake
+
+    add_library(vincent_driver_component src/vincent_driver.cpp)
+
+Third, replace other build commands that used the old target to act on the new target.
+i.e. ``ament_target_dependencies(vincent_driver ...)`` becomes ``ament_target_dependencies(vincent_driver_component ...)``
+
+Fourth, add a new command to declare your component.
+
+.. code-block:: cmake
+
+    rclcpp_components_register_node(
+        vincent_driver_component
+        PLUGIN "palomino::VincentDriver"
+        EXECUTABLE vincent_driver
+    )
+
+Fifth and finally, change any installation commands in the CMake that operated on the old target to install the library version instead.
+For instance, do not install either target into ``lib/${PROJECT_NAME}``.
+Replace with the library installation.
+
+.. code-block:: cmake
+
+    ament_export_targets(export_vincent_driver_component)
+    install(TARGETS vincent_driver_component
+            EXPORT export_vincent_driver_component
+            ARCHIVE DESTINATION lib
+            LIBRARY DESTINATION lib
+            RUNTIME DESTINATION bin
+    )
+
+
+Running Your Node
+-----------------
+
+See the :doc:`Composition tutorial <Composition>` for an in-depth look at composing nodes.
+The quick and dirty version is that if you had the following in your Python launch file,
+
+.. code-block:: python
+
+    from launch_ros.actions import Node
+
+    # ..
+
+    ld.add_action(Node(
+        package='palomino',
+        executable='vincent_driver',
+        # ..
+    ))
+
+you can replace it with
+
+.. code-block:: python
+
+    from launch_ros.actions import ComposableNodeContainer
+    from launch_ros.descriptions import ComposableNode
+
+    # ..
+    ld.add_action(ComposableNodeContainer(
+        name='a_buncha_nodes',
+        namespace='',
+        package='rclcpp_components',
+        executable='component_container',
+        composable_node_descriptions=[
+            ComposableNode(
+                package='palomino',
+                plugin='palomino::VincentDriver',
+                name='vincent_driver',
+                # ..
+                extra_arguments=[{'use_intra_process_comms': True}],
+            ),
+        ]
+    ))
+
+.. caution::
+
+    If you need multi-threading, instead of setting your executable to ``component_container``, set it to ``component_container_mt``