Components
Launch Actions
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from launch import LaunchDescription
from launch.actions import DeclareLaunchArgument, OpaqueFunction
from launch.substitutions import LaunchConfiguration
from launch_ros.actions import Node
def launch_setup(context, *args, **kwargs):
# Grab the value of a declared argument at runtime:
mode = LaunchConfiguration('mode').perform(context)
if mode == 'foo':
return [ Node(package='pkg', executable='foo_node', output='screen') ]
else:
return [ Node(package='pkg', executable='bar_node', output='screen') ]
def generate_launch_description():
return LaunchDescription([
DeclareLaunchArgument('mode', default_value='foo'),
OpaqueFunction(function=launch_setup),
])
DeclareLaunchArgument('mode', default_value='foo')defines an argument that could be overriden on commandline. E.g.,ros2 launch your_pkg your_launch.py mode:=bar- You can see launchfile args like:
ros2 launch your_pkg your_launch.py --show-args
- You can see launchfile args like:
LaunchConfiguration('mode').perform(context)is one true way to pull in a CLI arg at runtime.OpaqueFunctionis basically a hook for calling your custom function. It’s “opaque” because it’s a blackbox to the launch system - whose behavior is unknown until runtime.- This concept came from compiler theory, which means “a call whose internals are unknown to the optimizer”
- C/C++: Opaque pointers
(typedef struct Foo Foo;)hide implementation details behind an incomplete type. It’s the core of Pimpl idiom
