Feature request (launch_testing): Check if a topic is publishing messages, with a timeout

[adityapande-1995]

Feature request

Feature description

It would be nice to have a way to check if a topic is publishing messages (with a timeout).
THis suggestion came up in #263

Implementation considerations

One way to do that can be :

from threading import Event, Thread
import unittest

import launch
import launch.actions
import launch_ros.actions
import launch_testing.actions
import pytest
import rclpy
from rclpy.node import Node
from std_msgs.msg import String


@pytest.mark.launch_test
def generate_test_description():
    return launch.LaunchDescription([
        launch_ros.actions.Node(
            package='demo_nodes_cpp',
            executable='talker',
            name='demo_node_1',
        ),

        launch_testing.actions.ReadyToTest()
    ])


class TestFixture(unittest.TestCase):

    def test_check_if_msgs_published(self, proc_output):
        rclpy.init()
        node = MakeTestNode('test_node')
        node.start_subscriber()
        msgs_received_flag = node.msg_event_object.wait(timeout=5.0)
        assert msgs_received_flag, 'Did not receive msgs !'
        rclpy.shutdown()


class MakeTestNode(Node):

    def __init__(self, name='test_node'):
        super().__init__(name)
        self.msg_event_object = Event()

    def start_subscriber(self):
        # Create a subscriber
        self.subscription = self.create_subscription(
            String,
            'chatter',
            self.subscriber_callback,
            10
        )

        # Add a spin thread
        self.ros_spin_thread = Thread(target=lambda node: rclpy.spin(node), args=(self,))
        self.ros_spin_thread.start()

    def subscriber_callback(self, data):
        self.msg_event_object.set()

[jacobperron]

IMO, ideally syntax would look like this:

def test_foo(self):
    wait_for_topics = WaitForTopics(['foo', 'bar', 'baz'])
    assert wait_for_topics.wait(timeout=5.0)
    ...

However, we need an instance to a ROS node (and context). Maybe we can use the existing ROSAdapter class, e.g.:

class WaitForTopics:
   def __init__(self, *, topics):
       self.__ros_adapter = ROSAdapter()
       self.__ros_adapter.start()

    def __del__(self):
       self.__ros_adapter.shutdown()

@ivanpauno @wjwwood, thoughts?

[adityapande-1995]

Relevant PR : #274