---
title: "multithreading a gui"
date: "28/05/2025"
---

# the problem

On the [Cavalier Autonomous Racing](https://autonomousracing.dev) team at my school, we have a pretty ~~useless~~ cool basestation acting as a real-time telemetry visualization tool. We want to expand this GUI significantly, including tabulated windows for each sub-team.

Leveraging concurrency is vital for keeping things efficient and data up-to-date but this is easier said than done. Consider the original design:

# original architecture

Originally, the GUI followed the traditional QtC++ ROS single-threaded pattern. The GUI event loop ran on one thread, the main one:

```cpp

void spin_node(basestation::Gui* gui, ...) {
    ...

    rclcpp::executors::SingleThreadedExecutor executor;
    rclcpp::Node::SharedPtr node = std::make_shared<basestation::GuiNode>(init_gui);

    executor.add_node(node);
    while (init_gui->is_running) {
        executor.spin_once(timeout);
    }
}
```

All of 30+ topic callbacks were registered on this thread:

```cpp
GuiNode::GuiNode(basestation::Gui* init_gui) : Node("uva_gui") {
    this->gui = init_gui;

    rclcpp::QoS best_effort_qos = rclcpp::QoS(rclcpp::QoSInitialization(RMW_QOS_POLICY_HISTORY_KEEP_LAST, 1));
    best_effort_qos.best_effort();

    this->pubDesVel_ = create_publisher<std_msgs::msg::Float32>("vehicle/set_desired_velocity", 1);
    ...
    this->subAutonomy_ = create_subscription<deep_orange_msgs::msg::Autonomy>("/telemetry/autonomy", 1, std::bind(&GuiNode::receiveAutonomy, this, std::placeholders::_1));
}
```

The Qt Framework is quite complex and beyond the scope of this post. Big picture, the GUI controls a bunch of data-independent visualizations that are handled on this GUI thread.

How do we optimize this? It is necessary to take a step back and observe the structure of the entire application:

![single-threaded-design](/public/posts/multithreading-a-gui/single-threaded-design.webp)

Many flaws are now clear:

- Single, high-frequency topics can flood the GUI, causing stale data for other Widgets
- Data races in which multiple callbacks modify shared data are especially difficult to handle
- Poor separation of responsibility: the `MainWindow` is responsible for too much&mdash;it should not need first-hand knowledge of every Widget's API

Luckily, we're far from the first (and the last) to encounter a problem such as this. Enter ROS's exhaustive suite of concurrency tools.

# multi-threaded architecture

First, let's pin down what exactly can be parallelized.

1. Subscriptions: group subscription callbacks[^1] according to the displayed widgets. For the CAR, this was three `Vehicle`, `Trajectory`, and `Perception` groups. Now, widgets can be updated as dependent data is received.
2. Visualizations: now that data is handled independently, GUI widgets can be updated (with care) as well.

> Just kidding. Because of ROS, all GUI widgets can only be updated on the main GUI thread.

This lead me to the follow structure:

![multi-threaded-design](/public/posts/multithreading-a-gui/multi-threaded-implementation.webp)

- Three callback groups are triggered at differing intervals according to their urgency on the GUI node
    - A thread-safe queue[^2] processes all ingested data for each callback group
- Every 10ms, the GUI is updated, highest to lowest urgency messages first
- The `MainWindow` houses the visualization widgets as before&mdash;however, the GUI thread actually performs the update logic
- GUI Widgets were re-implemented to be thread-safe with basic locking, a small amount of overhead for safe memory access

## \*actual\* multi-threaded implementation

Sounds good, right? Well, I should've done my research first. The Qt framework has *already internalized* the logic for this entire paradigm of multithreaded code. Turns out all I need are:

- [Signals/slots](https://doc.qt.io/qt-6/signalsandslots.html) and a `Qt::QueuedConnection`
- Running the GUI with `MultithreadedExecutor`

As it turns out, signals and slots *automatically* leverage ROS's internal thread-safe message queue, ensuring deserialization one at a time.

The following (final) design employs two threads:

1. Main GUI Thread (Qt Event Loop): handles UI rendering + forward signals/slots to executor
2. Executor Thread: runs callbacks and publishes messages

The executor is simply spawned in the main thread:

```cpp
std::thread ros_thread([this]() {
    executor.spin();
});
```

Data flows from a called subscription &rightarrow; queued signal &rightarrow; signal connected to a slot &rightarrow; slot runs the GUI]widget when scheduled.

```cpp
// 1. Subscribe to a topic
this->subAutonomy_ = create_subscription<...>("/telemetry/autonomy", 1, std::bind(&GuiNode::receiveAutonomy, ...));

// 2. Queue a signal to the emitter
void GuiNode::receiveAutonomy(deep_orange_msgs::msg::Autonomy::SharedPtr msg) {
    this->des_vel = msg->desired_velocity_readout;
    signal_emitter->autonomyReceived(msg);
}

// 3. Signal connects to slot (registered in initialization)
QObject::connect(signal_emitter, &GuiSignalEmitter::autonomyReceived,
                 window, &MainWindow::receiveAutonomy, Qt::QueuedConnection);

// 4. Slot-performed logic when ROS runs thread
void MainWindow::receiveAutonomy(
    const deep_orange_msgs::msg::Autonomy::SharedPtr msg) {
  ...
}
```

Elegantly, registering a signal/slot with `Qt::QueuedConnection` does *all of the hard work*:

- Queueing messages in the target thread's loop
- Actual slot execution happens in the GUI thread
- Prevents cross-thread memory access/critical sections
- Qt event-level synchronization

# retrospective

Looking back, this GUI should've been implemented with a modern web framework such as [React](https://react.dev/) with [react-ros](https://github.com/flynneva/react-ros?tab=readme-ov-file). CAR needs high-speed, reactive data, and a QtC++ front-end is simply not meant for this level of complexity. I made it a lot harder than it needed to be with my lack of due diligence, but the single-threaded GUI event loop in ROS is more harm than help.


[^1]: See [the ROS documentation](https://docs.ros.org/en/foxy/How-To-Guides/Using-callback-groups.onhtml) to learn more. The CAR publishes various topic-related data at set rates, so I'm looking to run various groups of mutually exclusive callbacks at a set interval (i.e. `MutuallyExclusive`)
[^2]: The simplest implementation did the job:

    ```cpp
    ...
    template <typename T>
    class ThreadSafeQueue {
       public:
        void push(const T& item) {
            std::lock_guard<std::mutex> lock(mutex_);
            queue_.push(item);
            condition_.notify_one();
        }

        bool pop(T& item, std::chrono::milliseconds timeout = std::chrono::milliseconds(0)) {
            std::unique_lock<std::mutex> lock(mutex_);
            if (timeout.count() > 0) {
                if (!condition_.wait_for(lock, timeout, [this] { return !queue_.empty(); })) {
                    return false;
                }
            } else if (queue_.empty()) {
                return false;
            }

            item = queue_.front();
            queue_.pop();
            return true;
        }

        bool empty() const {
            std::lock_guard<std::mutex> lock(mutex_);
            return queue_.empty();
        }

        size_t size() const {
            std::lock_guard<std::mutex> lock(mutex_);
            return queue_.size();
        }

        void clear() {
            std::lock_guard<std::mutex> lock(mutex_);
            std::queue<T> empty;
            std::swap(queue_, empty);
        }
    ...
    };
