Hi @Benja !
Sure, I will share the whole launch file, even though it is quite a mess.
Explanation :
I have several ROS2 Nodes that gets launched by my launch file, one of the nodes launch the Gazebo server and outputs a particular string when the server is completely initialized (with std::cout, not the ROS2 logging system). ROS2 launch system is able to detect this output with an OnProcessIO event handler, created as follow (in LaunchDescription) :
RegisterEventHandler(
OnProcessIO(
target_action=ns3_simulator,
on_stdout=lambda info: spawn_px4 if(info.text.decode().strip() == 'Network simulator ready')
else LogInfo(
msg='ns-3 says "{}"'.format(info.text.decode().strip())
)
)
),
Note: My setup involves a network simulator on top of Gazebo, and I need to make sure the network simulator is also running before starting the px4, which is why the names refer to a network simulator. It would work the same with any other program outputing a string, just make sure to write on the standard output !
The full launch file (I didn’t manage to attach a .py or .zip file 
) :
# ROS2 Launch program that starts a co-simulation of Gazebo and NS-3 along with the Gazebo GUI.
# It sets-up a virtual network infrastructure based on the file setup_network.bash and launches PX4-Autopilot instances.
import os
from launch import LaunchDescription
from launch.actions import ExecuteProcess, DeclareLaunchArgument, LogInfo, RegisterEventHandler, SetEnvironmentVariable
from launch.event_handlers import OnProcessStart, OnProcessIO, OnExecutionComplete, OnProcessExit, OnShutdown
from launch_ros.actions import Node
from launch.substitutions import LaunchConfiguration, LocalSubstitution
from launch.conditions import IfCondition
import yaml
def generate_launch_description():
ws_path = '/home/theotime/simulation_ws'
config_path = ws_path + '/src/config/example_config.yaml'
# copy all the environment variables of the user
node_env = os.environ.copy()
# read the config file (Need pyYaml >= 5.1)
with open(config_path) as f:
config = yaml.load(f, Loader=yaml.FullLoader)
# Boolean parameter to launch the Gazebo GUI or not
use_gz_gui = LaunchConfiguration('use_gz_gui', default=True)
use_gz_gui_arg = DeclareLaunchArgument(
'use_gz_gui',
default_value=use_gz_gui,
description='If true, launch the Gazebo GUI')
# ROS2 Action: run a bash script as sudo to configure the virtual network (require sudo)
setup_network = ExecuteProcess(
cmd=['sudo', ws_path + '/src/launch/setup_network.bash', str(config['robots_number'])],
name='setup_network'
)
# Sets the environment variable GZ_IP, needed to communicate with Gazebo server from different namespaces (Gazebo's transport library)
# [config] bridge_ip: IP of the bridge providing direct communication with the network namespaces
set_GZ_IP = SetEnvironmentVariable(
name='GZ_IP',
value=[str(config['bridge_ip'])]
)
# ROS2 Node: Launch the network coordinator node.
# This python file is a modified version of the network coordinator from the ROS-NetSim framework
# Require sudo mode to access the network namespaces
network_coordinator = Node(
package='network_coordinator',
executable='network_coordinator',
name='net_coord',
arguments=[config_path],
prefix=["sudo -E env \"PATH=$PATH\" \"PYTHONPATH=$PYTHONPATH\" python3 "],
shell=True
)
# ROS2 Node: Launch the robotics coordinator node.
# This c++ file is a modified version of the robotics coordinator from the ROS-NetSim framework
# The Gazebo server (i.e. the robotics simulator) is started by this Node, thus the name "gazebo"
robotic_coordinator = Node(
package='robotics_coordinator',
executable='robotics_coord',
name='gazebo',
parameters=[
{"config_file": config_path},
{"verbose": False}
]
)
# ROS2 Node: Launch the network simulator node.
ns3_simulator = Node(
package='ns-3_sim_ros',
executable='ns-3_sim_ros',
name='ns3',
parameters=[
{"config_file": config_path},
{"verbose": False}
]
)
# ROS2 Action: Runs a command to launch an empty Gazebo GUI. It connects automatically to a running Gazebo Server
gazebo_gui = ExecuteProcess(
cmd=['gz', 'sim', '-g', '>', '/dev/null'],
name='gazebo_gui',
condition=IfCondition(LaunchConfiguration('use_gz_gui'))
)
# ROS2 Action: Runs a bash script to launch N PX4-Autopilot instances from the ~/PX4-Autopilot directory
# [config] robots_number: Number of px4_Autopilot to launch
# [config] robots_model: name of the model that will be spawned in Gazebo (a model with this name must exist in PX4-Autopilot/Tools/simulation/gz/models)
spawn_px4 = ExecuteProcess(
cmd=['/home/theotime/simulation_ws/src/launch/spawn_px4.bash', str(config['robots_number']), str(config['robots_model'])],
name='spawn_px4',
env=node_env
)
# ROS2 Node: Launch the tf2 broadcaster node
# [config] robot_model: name of the robot model used in Gazebo
def tf2_broadcaster(i):
return Node(
package='tf2_cpp',
executable='uav_tf2_broadcaster',
name='uav_tf2_broadcaster_'+str(i),
parameters=[
{'robot_name': config['robots_model']+'_'+str(i)}
]
)
# ROS2 Action: Runs a script that destroys the virtual network infrastructure
# Require sudo mode
# Note: we wrap this script in a function in a clumsy way because of a bug in ROS2 launch system:
# it appears the OnShutDown event handler doesn't work with any action other than LogInfo()
# see https://robotics.stackexchange.com/questions/24908/how-to-execute-a-script-at-shutdown-of-a-launch-process
def remove_network(event, context):
os.system('sudo '+ws_path+'/src/launch/remove_network.bash '+str(config['robots_number']))
return [
LogInfo(msg=['Launch was asked to shutdown: ', LocalSubstitution('event.reason')])
]
# The main ROS2 LaunchDescription object
ld = LaunchDescription([
use_gz_gui_arg,
setup_network,
set_GZ_IP,
RegisterEventHandler(
OnProcessExit(
target_action=setup_network,
on_exit=[
network_coordinator,
ns3_simulator,
robotic_coordinator,
gazebo_gui
]
)
),
RegisterEventHandler(
OnProcessIO(
target_action=ns3_simulator,
on_stdout=lambda info: spawn_px4 if(info.text.decode().strip() == 'Network simulator ready')
else LogInfo(
msg='ns-3 says "{}"'.format(info.text.decode().strip())
)
)
),
RegisterEventHandler(
OnShutdown(
on_shutdown=remove_network
)
)
])
# Add N tf2_broadcaster nodes to the LaunchDescription
for i in range(1, config['robots_number']+1):
ld.add_action(tf2_broadcaster(i))
return ld
