So, you're diving into ROS2 with Python and want to use your own custom message types? Awesome! Creating custom messages is a fundamental part of ROS2 development when the standard message types don't quite cut it for your application. This article will walk you through the process step-by-step, ensuring you can seamlessly integrate your custom messages into your ROS2 Python projects. Let's get started, guys!

Defining Your Custom Message

Before you can import a custom message, you need to define it. ROS2 uses .msg files to define message structures. These files are simple text files that specify the fields of your message, including their data types and names. Understanding this foundational element is crucial for successfully using custom messages in ROS2 Python. Think of it as creating a blueprint for your data.

To begin, create a msg directory inside your package if it doesn't already exist. This is where you'll store all your .msg files. Inside this directory, create a new file with a .msg extension, for example, MyCustomMessage.msg. Open this file in a text editor, and define your message fields. Each field is specified by its data type and name, separated by a space. You can use standard ROS2 data types like int32, float64, string, bool, and arrays of these types. For example, if you wanted to define a message with an integer ID, a floating-point value, and a string description, your MyCustomMessage.msg file might look like this:

int32 id
float64 value
string description

Each line in the .msg file represents a field in your custom message. The int32 represents a 32-bit integer, float64 is a double-precision floating-point number, and string is a sequence of characters. The names id, value, and description are identifiers that you will use in your code to access these fields. Keep the names descriptive and relevant to the data they represent. This will make your code easier to read and maintain.

Arrays are also supported in .msg files. To define an array, use the [] notation after the data type. For example, int32[] ids defines an array of 32-bit integers. You can also define fixed-size arrays by specifying the size within the brackets, like float64[3] coordinates, which defines an array of three 64-bit floating-point numbers. Arrays are useful for representing collections of data, such as sensor readings or coordinate lists. Be mindful of the size and type of arrays to avoid memory issues and ensure efficient data handling.

After defining your message fields, save the .msg file. This file serves as the template for creating instances of your custom message in your ROS2 code. You can define as many fields as you need, combining different data types and arrays to represent complex data structures. Remember to keep your message definitions clear and concise, focusing on the essential information that your ROS2 nodes need to exchange. By carefully designing your .msg files, you can ensure that your ROS2 system communicates effectively and efficiently.

Configuring CMakeLists.txt and package.xml

Alright, now that you've defined your custom message, you need to tell ROS2 how to build it. This involves modifying two crucial files in your package: CMakeLists.txt and package.xml. These files contain the instructions and metadata that ROS2 uses to compile and manage your package, including the custom message definitions. Without properly configuring these files, ROS2 won't be able to generate the necessary code for your custom message, and you won't be able to import it into your Python scripts. So, pay close attention to these steps!

First, let's modify package.xml. Open your package.xml file, and ensure that you have the following lines within the <export> section:

  <build_type>ament_python</build_type>

This line tells ROS2 that your package uses the ament_python build type, which is necessary for building Python-based ROS2 packages. Next, you need to add dependencies for rosidl_default_generators and rosidl_runtime. These dependencies are required to generate the code for your custom messages. Add the following lines outside the <export> section:

  <buildtool_depend>rosidl_default_generators</buildtool_depend>

  <exec_depend>rosidl_runtime</exec_depend>
  <member_of_group>rosidl_interface_packages</member_of_group>

Now, let's move on to CMakeLists.txt. Open your CMakeLists.txt file. First, find the find_package calls and add rosidl_default_generators to the list of components for find_package(ament_cmake REQUIRED) and create a new find package call for find_package(rosidl_default_generators REQUIRED) like so:

find_package(ament_cmake REQUIRED)
find_package(rosidl_default_generators REQUIRED)

Next, add the following lines to generate the message interfaces:

rosidl_generate_interfaces(${PROJECT_NAME}
  "msg/MyCustomMessage.msg"
  DEPENDENCIES std_msgs
)

Replace MyCustomMessage.msg with the actual name of your .msg file. The DEPENDENCIES argument specifies any other message types that your custom message depends on. In this case, we're assuming it depends on std_msgs, which is a common dependency for ROS2 messages. If your message doesn't depend on any other messages, you can omit the DEPENDENCIES argument. This step is crucial because it tells ROS2 to generate the necessary code for your custom message interface, allowing you to use it in your Python nodes. This generated code includes the Python classes that represent your message and the functions needed to serialize and deserialize it.

Finally, you need to add the ament_package() call at the end of your CMakeLists.txt file. This function configures the package for installation and use. It ensures that the necessary files are installed in the correct locations and that the package can be found by other ROS2 packages. This step is essential for making your custom message available to other ROS2 nodes in your system. Without it, your package won't be properly installed, and you won't be able to use your custom message in other projects.

Building Your Package

Alright, after tweaking those configuration files, it's time to build your package! This process compiles your custom message definitions and generates the Python modules that you'll import into your ROS2 nodes. Building the package is a crucial step because it transforms your .msg files into usable code. Open a terminal, navigate to the root of your ROS2 workspace (where the src directory is located), and run the following commands:

colcon build

The colcon build command compiles all the packages in your workspace, including yours. It automatically detects the changes you made to CMakeLists.txt and package.xml and builds the necessary code for your custom message. As the build process progresses, you'll see output from the compiler and other build tools. This output can provide valuable information about the build process, including any errors or warnings that might occur. Keep an eye on the output to identify and resolve any issues that arise during the build.

If the build is successful, you should see a message indicating that all packages were built without errors. If there are errors, carefully examine the output to identify the cause. Common errors include syntax errors in your .msg files, incorrect dependencies in package.xml, or misconfigurations in CMakeLists.txt. Correct these errors and try building again until the build completes successfully. A successful build is essential for ensuring that your custom message is properly integrated into your ROS2 environment and can be used by your ROS2 nodes.

After building, you need to source the setup script to make your package and custom message available to your ROS2 environment. Sourcing the setup script sets the necessary environment variables, such as ROS_PACKAGE_PATH, which tells ROS2 where to find your package and its associated resources. To source the setup script, run the following command:

. install/setup.bash

This command executes the setup.bash script located in the install directory of your workspace. The . at the beginning of the command is shorthand for the source command, which executes the script in the current shell environment. After sourcing the setup script, your ROS2 environment is updated, and you can now import and use your custom message in your Python nodes. You'll need to source the setup script every time you open a new terminal to work with your ROS2 workspace.

Importing and Using Your Custom Message in Python

Now comes the exciting part: using your custom message in your Python code! First, make sure you've sourced your ROS2 environment (as mentioned in the previous step). Then, in your Python script, you can import your custom message like this:

from your_package_name.msg import MyCustomMessage

Replace your_package_name with the name of your ROS2 package, and MyCustomMessage with the name of your custom message. This import statement makes the MyCustomMessage class available in your Python script, allowing you to create instances of the message and access its fields. Now, you can create an instance of your custom message, populate its fields, and use it in your ROS2 nodes. For example, you might want to publish the message to a topic or receive it from a subscriber.

To create an instance of your custom message, simply call the constructor of the MyCustomMessage class:

message = MyCustomMessage()

This creates a new MyCustomMessage object with all its fields initialized to their default values. You can then set the values of the fields using the dot notation:

message.id = 123
message.value = 3.14
message.description = "Hello, ROS2!"

In this example, we're setting the id field to 123, the value field to 3.14, and the description field to "Hello, ROS2!". You can set the fields to any valid values that match their data types. After populating the fields, you can then use the message in your ROS2 code. For example, you might want to publish the message to a topic:

publisher = self.create_publisher(MyCustomMessage, 'my_topic', 10)

publisher.publish(message)

In this example, we're creating a publisher that publishes MyCustomMessage objects to the my_topic topic. The publish method sends the message to all subscribers of the topic. You can also receive custom messages from subscribers in a similar way. Define a callback function that receives the message as an argument:

def message_callback(msg):
    self.get_logger().info('Received message: id={}, value={}, description={}'.format(msg.id, msg.value, msg.description))

self.create_subscription(MyCustomMessage, 'my_topic', message_callback, 10)

In this example, we're creating a subscriber that subscribes to the my_topic topic and calls the message_callback function whenever a new message is received. The message_callback function receives the message as an argument and can then access its fields using the dot notation. This allows you to process the data contained in the message and use it in your ROS2 node. By following these steps, you can seamlessly integrate your custom messages into your ROS2 Python projects, enabling you to exchange complex data structures between your ROS2 nodes and build sophisticated robotic applications.

Troubleshooting Common Issues

Even with careful setup, sometimes things don't go as planned. Here are a few common issues you might encounter and how to troubleshoot them:

• ModuleNotFoundError: This usually means your package isn't being found. Double-check that you've sourced your ROS2 environment after building. Also, ensure your package name is correct in the import statement.
• AttributeError: This often indicates a mismatch between the field names in your Python code and the .msg file. Verify that you're using the correct field names and data types.
• Build Errors: Carefully examine the error messages during the build process. They often point to syntax errors in your .msg files or misconfigurations in CMakeLists.txt or package.xml.

By following these steps and troubleshooting any issues that arise, you can successfully import and use custom message types in your ROS2 Python projects. Custom messages are a powerful tool for building complex robotic systems, allowing you to exchange rich data structures between your ROS2 nodes. So, go ahead and start creating your own custom messages to enhance your ROS2 applications!