How to receive subscription callbacks while a loop is running?

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Software: python, ros2 (foxy)

I have a main loop in my node, and I would also still like to receive updates from a subscription - however, I can't quite figure out how to make it work. I tried to play around with the executor but that didn't go quite right... Any help or pointers in the right direction would be greatly appreciated.

Simple example, starting with the simple subscriber:

  # subscriber
  import rclpy
  from rclpy.node import Node
  from std_msgs.msg import String

  class MinimalSubscriber(Node):
    def __init__(self):
      super().__init__('minimal_subscriber')
      self.subscription = self.create_subscription(
          String,
          'topic',
          self.listener_callback,
          10)

      self.run_loop()

    def run_loop(self):
      while True:
        pass


    def listener_callback(self, msg):
      self.get_logger().info('I heard: "%s"' % msg.data)


  def main(args=None):
    rclpy.init(args=args)

    minimal_subscriber = MinimalSubscriber()

    rclpy.spin(minimal_subscriber)

    minimal_subscriber.destroy_node()
    rclpy.shutdown()


  if __name__ == '__main__':
    main()

The publisher:

  # publisher
  import rclpy
  from rclpy.node import Node
  from std_msgs.msg import String


  class MinimalPublisher(Node):
    def __init__(self):
      super().__init__('minimal_publisher')
      self.publisher_ = self.create_publisher(String, 'topic', 10) 
      timer_period = 0.5  # seconds
      self.timer = self.create_timer(timer_period, self.timer_callback)
      self.i = 0 

    def timer_callback(self):
      msg = String()
      msg.data = 'Hello World: %d' % self.i
      self.publisher_.publish(msg)
      self.get_logger().info('Publishing: "%s"' % msg.data)
      self.i += 1

  def main(args=None):
    rclpy.init(args=args)

    minimal_publisher = MinimalPublisher()

    rclpy.spin(minimal_publisher)

    minimal_publisher.destroy_node()
    rclpy.shutdown()


  if __name__ == '__main__':
    main()