Allow users to configure the executor for executables in `demo_nodes_…

…cpp` (#666)

* Allow users to build demo executables to run with a different executor

Signed-off-by: Yadunund <yadunund@openrobotics.org>
Signed-off-by: Yadu <yadunund@gmail.com>
Co-authored-by: Michael Carroll <michael@openrobotics.org>
Co-authored-by: Tomoya Fujita <Tomoya.Fujita@sony.com>

demo_nodes_cpp/CMakeLists.txt

@@ -92,9 +92,12 @@ function(create_demo_library plugin executable)
     ARCHIVE DESTINATION lib
     LIBRARY DESTINATION lib
     RUNTIME DESTINATION bin)
+  set(DEMO_EXECUTOR "rclcpp::executors::SingleThreadedExecutor" CACHE STRING "The executor for the demo nodes")
+  message(STATUS "Setting executor of ${executable} to ${DEMO_EXECUTOR}")
   rclcpp_components_register_node(${library}
     PLUGIN ${plugin}
-    EXECUTABLE ${executable})
+    EXECUTABLE ${executable}
+    EXECUTOR ${DEMO_EXECUTOR})
 endfunction()
 
 # Timers

demo_nodes_cpp/README.md

@@ -37,6 +37,14 @@ Run the command below to compile the `demo_nodes_cpp` ROS 2 package:
 colcon build --packages-up-to demo_nodes_cpp
 ```
 
+**Note**: By default, the demo executables will spin up the [SingleThreaded executor](https://docs.ros.org/en/rolling/Concepts/Intermediate/About-Executors.html#executors) if run in separate processes, i.e., not composed in the same component container.
+To configure the demo executables to run with a different executor, build the package with the custom `DEMO_EXECUTOR` flag set to the fully qualified name of the executor.
+For example, to run with the experimental `EventsExecutor`,
+
+```bash
+colcon build --packages-select demo_nodes_cpp --cmake-args -DDEMO_EXECUTOR:STRING=rclcpp::experimental::executors::EventsExecutor
+```
+
 ## **Run**
 
 ### Basic Talker & Listener