support multiple fields in ros2topic echo

[sangteak601]

Added support for multiple fields in ros2 topic echo. Fixes #951

[fujitatomoya]

the implementation looks good to me.

@sangteak601 can you add the multiple fields test aligned with

ros2cli/ros2topic/test/test_cli.py

Lines 579 to 617 in f3fc36c

	@launch_testing.markers.retry_on_failure(times=5, delay=1)
	def test_topic_echo_field(self):
	with self.launch_topic_command(
	arguments=['echo', '/arrays', '--field', 'alignment_check'],
	) as topic_command:
	assert topic_command.wait_for_output(functools.partial(
	launch_testing.tools.expect_output, expected_lines=[
	'0',
	'---',
	], strict=True
	), timeout=10)
	assert topic_command.wait_for_shutdown(timeout=10)
	@launch_testing.markers.retry_on_failure(times=5, delay=1)
	def test_topic_echo_field_nested(self):
	with self.launch_topic_command(
	arguments=['echo', '/cmd_vel', '--field', 'twist.angular'],
	) as topic_command:
	assert topic_command.wait_for_output(functools.partial(
	launch_testing.tools.expect_output, expected_lines=[
	'x: 0.0',
	'y: 0.0',
	'z: 0.0',
	'---',
	], strict=True
	), timeout=10)
	assert topic_command.wait_for_shutdown(timeout=10)
	@launch_testing.markers.retry_on_failure(times=5, delay=1)
	def test_topic_echo_field_not_a_member(self):
	with self.launch_topic_command(
	arguments=['echo', '/arrays', '--field', 'not_member'],
	) as topic_command:
	assert topic_command.wait_for_output(functools.partial(
	launch_testing.tools.expect_output, expected_lines=[
	"Invalid field 'not_member': 'Arrays' object has no attribute 'not_member'",
	], strict=True
	), timeout=10)
	assert topic_command.wait_for_shutdown(timeout=10)

[CWAbhi]

@sangteak601, you need to add the test files in test_cli.py for the tests to pass. Once you add them, it should work. I faced the same issue, but it got fixed after doing this."
Currently i was issuing some new unknow issue so i don't think you will get that
Let me know if you want further refinements!
For reference you can check my drafted PR .
#958

[fujitatomoya]

lgtm with green CI.

[fujitatomoya]

Pulls: #964
Gist: https://gist.githubusercontent.com/fujitatomoya/1f9e28f0651634736fd6085e208718e9/raw/b4b3a3940ab75725d4b9cce82b27a1c1fabb39ef/ros2.repos
BUILD args: --packages-above-and-dependencies ros2topic
TEST args: --packages-above ros2topic
ROS Distro: rolling
Job: ci_launcher
ci_launcher ran: https://ci.ros2.org/job/ci_launcher/15091

• Linux
• Linux-aarch64
• Linux-rhel
• Windows

[fujitatomoya]

@sangteak601 thank you very much for your contribution, PR has been merged 👍