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Chapter-13-Creating-A-Custom-Service-Interface-(C++).md

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Chapter 13. Creating A Custom Service Interface (C++)

In this lesson, we will create a simple custom ROS2 service interface of our own - which will contain an integer named number as a request message and a string named decision about whether the integer is “Odd” or “Even” as the response message.

Service Interface

A service interface is a request-response architecture interface, which allows two nodes (service client & service server) to communicate with each other through a request-response communication model.

Service Client

The node which sends a request message to the service server node.

Service Server

The node which processes the request message sent from the service client and gives a response message back to the service client.

  1. ROS2 also comes with some default service interfaces. To see them, run the ros2 interface list command from a terminal.

  2. To see the details/content of any interface, run the following bash command:

    ros2 interface show <complete-interface-name>

    Example: Lets us check the details/content of std_srvs/srv/SetBool service interface.

    Untitled

    In the above picture,

    bool data : is the request message of std_srvs/srv/SetBool service interface.

    bool success & string message : are the response messages of std_srvs/srv/SetBool service interface.

    The three dashed line --- marks the division between the request message & the response messages of the interface.

  3. Open your VS Code in Workspace Directory.

  4. Under the udemy_ros2_pkg package folder, create a new folder named srv. Inside this srv folder, we are going to create our custom ROS2 service interface files.

  5. Create a new file inside the srv folder named OddEvenCheck.srv .

    💡 Note that the custom .srv files follow the PascalCasing Convention for naming.

  6. Add the following code to OddEvenCheck.srv and save the file.

    # A Service Interface is like a contract or protocol of communication between the Service-Client Node and the Service-Server Node.

# Request Message Type & Variable Name
# The Service-Client Node will be feeding an "int64" type value named "number" as a "Request Message" to the Service-Server Node.
int64 number   # Number to check if odd or even

---
# The three dashes signify the division between the Request Message and Response Message in the Service Interface.

# Response Mesage Type & Variable Name
# The Service-Client will be recieving a "string" named "decision" as a "Response Message" - back from the Service-Server.
string decision  # Either "Odd" or "Even"

  7. Next, head over to the package.xml file of the udemy_ros2_pkg folder, where we will add in some new dependencies that will help us to use the newly created custom service interface file.

    Add the following code to the package.xml file and save the file.

    <?xml version="1.0"?>
<?xml-model href="http://download.ros.org/schema/package_format3.xsd" schematypens="http://www.w3.org/2001/XMLSchema"?>
<package format="3">
  <name>udemy_ros2_pkg</name>
  <version>0.0.0</version>
  <description>TODO: Package description</description>
  <maintainer email="pritam@todo.todo">pritam</maintainer>
  <license>TODO: License declaration</license>

  <buildtool_depend>ament_cmake</buildtool_depend>

  <depend>rclcpp</depend>
  <depend>std_msgs</depend>
  <build_depend>rosidl_default_generators</build_depend> 
  <!-- Indicates that the package requires the rosidl_default_generators package during the build process. rosidl_default_generators contains the default code generators for the ROS 2 Interface Definition Language (IDL). -->
  <exec_depend>rosidl_default_runtime</exec_depend>
  <!-- Indicates that the package requires the rosidl_default_runtime package during execution so that the Interface Definition Language (IDL) created can be used during node runtime. -->
  <member_of_group>rosidl_interface_packages</member_of_group>
  <!-- To include the newly created service interface to the group of other ROS2 interfaces.  -->

  <test_depend>ament_lint_auto</test_depend>
  <test_depend>ament_lint_common</test_depend>

  <export>
    <build_type>ament_cmake</build_type>
  </export>

</package>

  8. Next, head over to the CMakeLists.txt file of the udemy_ros2_pkg folder and add the following lines of code to it.

    cmake_minimum_required(VERSION 3.8)
project(udemy_ros2_pkg)

if(CMAKE_COMPILER_IS_GNUCXX OR CMAKE_CXX_COMPILER_ID MATCHES "Clang")
  add_compile_options(-Wall -Wextra -Wpedantic)
endif()

# find dependencies
find_package(ament_cmake REQUIRED)
find_package(rclcpp REQUIRED)
find_package(std_msgs REQUIRED)
# Necessary import for using Custom Service Interfaces
find_package(rosidl_default_generators REQUIRED)

if(BUILD_TESTING)
  find_package(ament_lint_auto REQUIRED)
  set(ament_cmake_copyright_FOUND TRUE)
  set(ament_cmake_cpplint_FOUND TRUE)
  ament_lint_auto_find_test_dependencies()
endif()

# We need to tell our ros2 compiler - the exact specifics of the newly created custom service interface file - that it needs to have the IDL Code generated for.
# This line of code should always come before the add_excutable blocks, if you are planning to use the generated custom interface in these executables.
rosidl_generate_interfaces(${PROJECT_NAME} "srv/OddEvenCheck.srv" ADD_LINTER_TESTS)                                                             
 
# Set support for using custom interfaces in C++ from this package
# This line should always be below the "rosidl_generate_interfaces()" code - otherwise it will produce compilation error.
rosidl_get_typesupport_target(cpp_typesupport_target "${PROJECT_NAME}" "rosidl_typesupport_cpp")

add_executable(publisher src/publisher.cpp) 
ament_target_dependencies(publisher rclcpp std_msgs)

add_executable(subscriber src/subscriber.cpp) 
ament_target_dependencies(subscriber rclcpp std_msgs)

add_executable(rpm_publisher src/rpm_publisher.cpp)
ament_target_dependencies(rpm_publisher rclcpp std_msgs)

add_executable(rpm_subscriber src/rpm_subscriber.cpp)
ament_target_dependencies(rpm_subscriber rclcpp std_msgs)

install(TARGETS 
        publisher 
        subscriber
        rpm_publisher
        rpm_subscriber
        DESTINATION lib/${PROJECT_NAME}
)

install(
  DIRECTORY
  launch
  DESTINATION share/${PROJECT_NAME}
)

ament_package()

  9. Save all the files after making the changes and build the workspace.

Just now when we compiled our workspace, ROS2 generated some new C++ header files for us to be able to use our newly created custom interface OddEvenCheck.srv in our ROS2 C++ codes.

We can see these header files at ros2_cpp_udemy_tutorial/install/udemy_ros2_pkg/include/udemy_ros2_pkg/udemy_ros2_pkg/srv directory.

Untitled

Now we have successfully created our OddEvenCheck.srv custom interface file and integrated it with our workspace.

To see the name of our newly created OddEvenCheck.srv custom interface in ros2 interface list , open a new terminal and run the following commands:

cd Ros2_Workspaces/ros2_cpp_udemy_tutorial 
# Moving to the workspace directory

source install/setup.bash
# Sourcing the workspace

ros2 interface list

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We can also see the contents of the OddEvenCheck.srv interface using the ros2 interface show udemy_ros2_pkg/srv/OddEvenCheck command:

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