cascade_lifecycle

Managed nodes (or lifecycle nodes, LN) are an extremely useful concept in ROS2. It provides a mechanism to define states in a node so that its life cycle can be better controlled.

When an application is made up of multiple LNs, it is common to use a node to orchestrate the transitions of each one. This occurs, for example, in Navigation2 or in ROS2 Planning System.

cascade_lifecycle provides a mechanism that can make managing LNs easier. This idea is based on my developments with BICA. This mechanism allows defining dependencies between LNs. When an LN A establishes an LN B as a dependency, when an A enters a state, B automatically enters this state. This allows creating configuration/activation/deactivation trees.

The class rclcpp_cascade_lifecycle::CascadeLifecycleNode extends the rclcpp_lifecycle::LifecycleNode API with next operations:

void add_activation (const std::string & node_name);
void remove_activation (const std::string & node_name);
void clear_activation ();

Using rclcpp_cascade_lifecycle in the next example, node_b makes the same state transitions as node_a:

auto node_a = std::make_shared<rclcpp_cascade_lifecycle::CascadeLifecycleNode>("node_A");
auto node_b = std::make_shared<rclcpp_cascade_lifecycle::CascadeLifecycleNode>("node_B");

rclcpp::experimental::executors::EventsExecutor executor;
executor.add_node(node_a->get_node_base_interface());
executor.add_node(node_b->get_node_base_interface());

node_a->add_activation("node_B");
node_a->trigger_transition(lifecycle_msgs::msg::Transition::TRANSITION_CONFIGURE);
{
  rclcpp::Rate rate(10);
  auto start = node_a->now();
  while ((node_a->now() - start).seconds() < 0.5) {
    executor.spin_some();
    rate.sleep();
  }
}

ASSERT_EQ(node_a->get_current_state().id(), lifecycle_msgs::msg::State::PRIMARY_STATE_INACTIVE);
ASSERT_EQ(node_b->get_current_state().id(), lifecycle_msgs::msg::State::PRIMARY_STATE_INACTIVE);

Hope it helps!!!