Rcl.NET.Templates 1.1.0

rclnet

rclnet is a fast and easy-to-use .NET wrapper over ROS 2 client library, allowing .NET applications to interact with other ROS applications.

Supported Platforms

Supported .NET Versions:

• .NET 7

Supported ROS 2 Distributions:

• Humble Hawksbill
• Foxy Fitzroy

Getting Started

rclnet provides project templates to help you getting started quickly. You can install the templates using the following command:

dotnet new install Rcl.NET.Templates

Then, to create a node application:

mkdir MyNode
cd MyNode
dotnet new ros2-node

To create a message only library:

mkdir MyMessageLibrary
cd MyMessageLibrary
dotnet new ros2-msg

If you prefer creating the projects by yourself, or want to use rclnet in existing projects, refer the Installing and Generating Messages section.

Installing

Stable releases of rclnet are hosted on NuGet. You can install them using the following command:

dotnet add package Rcl.NET

For message only projects, you can install only Rosidl.Runtime without Rcl.NET:

dotnet add package Rosidl.Runtime

To generate message classes, you'll also need to install ros2cs utility:

dotnet tool install -g ros2cs

Generating Messages

rclnet does not ship with message definitions. In order to communicate with other ROS 2 nodes, you need to generate messages first.

Message definitions are .NET classes / structs, you can either include messages in a console app which runs as an ROS 2 node, or compile separately in another library.

Projects containing messages will have to meet the following requirements:

• Rcl.NET or Rosidl.Runtime NuGet package is installed.
• AllowUnsafeBlocks is set to true. This can be done by adding the following lines to the .csproj file:

  <PropertyGroup>
      <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
  </PropertyGroup>
  

• Runtime marshalling for the assembly is disabled. You can add the following line to somewhere in the source code of the project:

  [assembly: System.Runtime.CompilerServices.DisableRuntimeMarshalling]
  

To generate messages, you also need to add a ros2cs.spec file to somewhere in the project (usually the project root). A ros2cs.spec file contains configurations such as output directory and where to find packages, see here for detailed explanations.

Assuming you've already installed the ros2cs utility, simply run the following command to generate messages:

ros2cs /path/to/ros2cs.spec

Features

• Completely asynchronous and async/await friendly.
• Flexible asynchronous scheduling control to fit rclnet into existing applications.
• Unified message generation for POCOs and blittable structures.
• Intuitive ROS graph querying and monitoring APIs.
• Easy-to-use POCO-based APIs.
• Fast and zero managed heap allocation APIs operating directly on native message buffers.
• Single package with runtime support for different ROS 2 distros.
• Builtin support for querying topic messages and ROS graph events with Reactive Extensions.

Supported ROS Features

✅Supported ⚠️Partial support ❌Not supported ⏳In development

Asynchronous Execution Model

Unlike rclcpp and rclpy, rclnet doesn't have the concept of executors. Each RclContext runs its own event loop for waiting on signals and dispatching callbacks, which is essentialy a single-threaded executor.

Although rclnet does not provide multi-threaded executors, it doesn't mean that you can't process messages or handle service requests using multiple threads. All communication primitives in rclnet provide both synchronous and asynchronous APIs for different needs and scenarios.

Synchronous APIs are simpler and faster if the work need to be done is simple enough, e.g. neither CPU-intensive nor needs to issue blocking calls. Asynchronous APIs, in contrast, are for scenarios where you need to perform asynchronous calls or offload blocking operations into background threads.

Take subscriptions for example, you can receive messages synchronously using IRclSubscription<T>.Subscribe, or asynchronously using IRclSubscription<T>.ReadAllAsync. Synchronous subscriptions always handle messages on the event loop. While for asynchronous subscriptions, you can choose where you'd like to process the received messages:

await foreach (var msg in sub.ReadAllAsync())
{
    // Perform asynchronous operation.
    await SomeAsyncOperation(msg);

    // Perform synchronous operation and wait for its completion without blocking the event loop.
    await Task.Run(() => SomeOffloadedSyncOperation(msg));
}

In the above example, the event loop of the RclContext is used for listening to events only. Where are the messages handled depends on the SynchronizationContext currently captured.

If there's no SynchronizationContext in use, event handling happens in background threads by default. Otherwise, the events will be handled in the captured SynchronizationContext. If you are using rclnet inside a GUI application, this usually means that the events are handled on the UI thread.

RclContexts can also have their own SynchronizationContexts, which always schedule asynchronous operations on the event loop. This is extremely helpful if you want to introduce single-threaded concurrency into your application:

await using var context = new RclContext(useSynchronizationContext: true);

...

// Enforce execution on the event loop so that we can capture its SynchronizationContext.
await context.Yield();

// All following awaits will resume on the event loop by default.
await foreach (var msg in sub.ReadAllAsync())
{
    // On event loop.
    await SomeAsyncOperation(msg);
    // On event loop.
    await Task.Run(() => {
        // On thread pool.
        SomeOffloadedSyncOperation(msg);
    });
    // On event loop.
    await Task.Yield();
    // On event loop.

    // We can also spin up multiple coroutines to run concurrently on the event loop.
    Task task1 = Coroutine1Async(msg),
         task2 = Coroutine2Async(msg);

    // Or asynchronously wait for all coroutines to complete.
    await Task.WhenAll(task1, task2);

    ...

    // The execution of current async method will stay on the event
    // loop unless we break out of the SynchronizationContext using
    // ConfigureAwait(false), or RclContext.YieldBackground().

    await AnotherAsyncOperation(msg).ConfigureAwait(false);
    // On thread pool thread.

    // We can still transition back to the event loop with context.Yield().

    await context.Yield();
    // On event loop.

    await RclContext.YieldBackground();
    // On thread pool thread.
}

As shown in the above example, besides of SynchronizationContext, you can also use RclContext.Yield, RclContext.YieldBackground and ConfigureAwait(false) to perform fine-grained control over the asynchronous exection flow.

Additional Notes about IRclWaitObject.WaitOneAsync

Timers and guard conditions created by RclContext implements IRclWaitObject interface, which allow the caller to asynchronously wait for the signal.

IRclWaitObject interface exposes the following two overloads of WaitOneAsync:

ValueTask WaitOneAsync(bool runContinuationAsynchronously, CancellationToken cancellationToken = default);
ValueTask WaitOneAsync(CancellationToken cancellationToken = default);

The latter overload simply calls another one with runContinuationAsynchronously set to true.

WaitOneAsync allows the caller to explicitly control the execution of the continuation via runContinuationAsynchronously parameter. Assuming there's no captured SynchronizationContext or TaskScheduler, when runContinuationAsynchronously is set to true, the continuation will be scheduled to execute in thread pool. And if runContinuationAsynchronously is set to false, the continuation is guaranteed to execute on the event loop.

However, when a SynchronizationContext or TaskScheduler is captured, the continuation of the call to WaitOneAsync will always execute in the captured context, regardless of the value of runContinuationAsynchronously.

Since context capture can be suppressed by calling ConfigureAwait with continueOnCapturedContext set to false, execution of the continuation can be precisely controlled using runContinuationAsynchronously in conjunction with continueOnCapturedContext.

Building and Running Examples

Install dependencies

The following instruction assumes that you've already installed ROS 2 foxy or humble in your system.

You'll need .NET 7.0 SDK to build and run the examples, see instructions here.

Make sure you have all dependencies installed by running:

rosdep install -i --from-paths examples

Run with dotnet run

Now you can run example projects using dotnet run, e.g.

dotnet run examples/turtle_rotate

Run with ros2 run

Or you can build and install examples as colcon packages:

colcon build --executor sequential --merge-install --paths examples/*
source install/setup.bash

To run an example node, use ros2 run, e.g.

ros2 run graph_monitor graph_monitor

Showcase

Subscribing

await using var ctx = new RclContext(args);
using var node = ctx.CreateNode("hellow_world");
using var sub = node.CreateSubscription<Twist>("/cmd_vel");
await foreach (Twist msg in sub.ReadAllAsync())
{
    ...
}

Publishing

using var pub = node.CreatePublisher<Vector3>("/vec");
pub.Publish(new Vector3(x: 1, y: 2, z: 3));

Handling Service Calls

using var server = node.CreateService<
    EmptyService,
    EmptyServiceRequest,
    EmptyServiceResponse>("/vec",
        (request, state) =>
        {
            return new EmptyServiceResponse();
        });
await Task.Delay(-1);

Calling Services

using var client = node.CreateClient<
    EmptyService,
    EmptyServiceRequest,
    EmptyServiceResponse>("/vec");
await client.InvokeAsync(new EmptyServiceRequest());

Monitoring ROS Graph Changes

node.Graph
    .OfType<NodeAppearedEvent>()
    .Subscribe(x =>
    {
        Console.WriteLine($"Node {x.Node.Name} is online.");
    });

await node.Graph.WaitForServiceServerAsync("/my/service");

Calling Action Servers

using var client = node.CreateActionClient<
    SpinAction,
    SpinActionGoal,
    SpinActionResult,
    SpinActionFeedback>("/spin");

using var goal = await client.SendGoalAsync(
        new SpinActionGoal(targetYaw: Math.PI));

await foreach (var feedback in goal.ReadFeedbacksAsync())
{
    Console.WriteLine(feedback.AngularDistanceTraveled);
}

var result = await goal.GetResultAsync();

Zero (Managed Heap) Allocation APIs

using var sub = node.CreateNativeSubscription<Twist>("/cmd_vel");
await foreach (RosMessageBuffer msg in sub.ReadAllAsync())
{
    using (msg) ProcessMessage(msg);

    static void ProcessMessage(RosMessageBuffer buffer)
    {
        ref var twist = ref buffer.AsRef<Twist.Priv>();
        ...
    }
}