Polling Subscriber

Agnocast provides agnocast::PollingSubscriber, a pull-based subscription that lets you fetch the latest message on demand rather than receiving it via a callback. This corresponds to the subscription->take() pattern in rclcpp, but returns a zero-copy shared pointer from shared memory instead of copying data into a provided message.

PollingSubscriber is available at both Stage 1 (rclcpp::Node) and Stage 2 (agnocast::Node).

Migration from rclcpp take()

Before (rclcpp)

class MyPollingNode : public rclcpp::Node
{
  rclcpp::Subscription<std_msgs::msg::String>::SharedPtr sub_;
  rclcpp::TimerBase::SharedPtr timer_;

  void timer_callback()
  {
    std_msgs::msg::String msg;
    rclcpp::MessageInfo msg_info;
    if (sub_->take(msg, msg_info)) {
      RCLCPP_INFO(get_logger(), "Polled: %s", msg.data.c_str());
    }
  }

public:
  MyPollingNode() : Node("my_polling_node")
  {
    sub_ = create_subscription<std_msgs::msg::String>(
      "/topic", rclcpp::QoS(rclcpp::KeepLast(1)),
      [](std_msgs::msg::String::SharedPtr) {});  // dummy callback required by rclcpp

    timer_ = create_wall_timer(1s,
      std::bind(&MyPollingNode::timer_callback, this));
  }
};

After (Agnocast Stage 1)

class MyPollingNode : public rclcpp::Node
{
  agnocast::PollingSubscriber<std_msgs::msg::String>::SharedPtr sub_;  // (1)
  rclcpp::TimerBase::SharedPtr timer_;

  void timer_callback()
  {
    auto msg = sub_->take_data();                                      // (2)
    if (msg) {
      RCLCPP_INFO(get_logger(), "Polled: %s", msg->data.c_str());
    }
  }

public:
  MyPollingNode() : Node("my_polling_node")
  {
    sub_ = agnocast::create_subscription<std_msgs::msg::String>(       // (3)
      this, "/topic", rclcpp::QoS(rclcpp::KeepLast(1)));

    timer_ = create_wall_timer(1s,
      std::bind(&MyPollingNode::timer_callback, this));
  }
};

Key changes:

1. rclcpp::Subscription → agnocast::PollingSubscriber (no dummy callback needed)
2. sub_->take(msg, msg_info) → sub_->take_data() which returns an agnocast::ipc_shared_ptr<const T> (zero-copy)
3. Use agnocast::create_subscription free function (no callback argument)

After (Agnocast Stage 2)

class MyPollingNode : public agnocast::Node                            // (1)
{
  agnocast::PollingSubscriber<std_msgs::msg::String>::SharedPtr sub_;
  agnocast::TimerBase::SharedPtr timer_;                               // (2)

  void timer_callback()
  {
    auto msg = sub_->take_data();
    if (msg) {
      RCLCPP_INFO(get_logger(), "Polled: %s", msg->data.c_str());
    }
  }

public:
  MyPollingNode() : Node("my_polling_node")
  {
    sub_ = this->create_subscription<std_msgs::msg::String>(           // (3)
      "/topic", rclcpp::QoS(rclcpp::KeepLast(1)));

    timer_ = this->create_wall_timer(1s,
      std::bind(&MyPollingNode::timer_callback, this));
  }
};

Additional changes from Stage 1:

1. Base class changes to agnocast::Node
2. Timer type changes to agnocast::TimerBase::SharedPtr
3. Subscription creation uses member function instead of free function