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The example creates two nodes:MinimalTimer and test_node, but spins only one of them rclcpp::spin(minimal_timer);. rclcpp::spin() adds a node to an executor. The executor calls callbacks, like the parameter service callbacks. test_node must be added to an executor so the parameter service callbacks can send a response.

See the manual composition demo for an example.

rclcpp::executors::SingleThreadedExecutor exec;
exec.add_node(minimal_timer);
exec.add_node(minimal_timer->node_);
exec.spin();