ROS 2 Humble Hawksbill 启动文件 launch

分别有CLI-launch，Python+XML+YAML对比，launch教程三部分：

docs.ros.org/en/humble/Tutorials/Launch/CLI-Intro.html

docs.ros.org/en/humble/How-To-Guides/Launch-file-different-formats.html

docs.ros.org/en/humble/Tutorials/Launch/Launch-Main.html

ros2 launch -h

usage: ros2 launch [-h] [-n] [-d] [-p | -s] [-a] [--launch-prefix LAUNCH_PREFIX]
                   [--launch-prefix-filter LAUNCH_PREFIX_FILTER]
                   package_name [launch_file_name] [launch_arguments ...]

Run a launch file

positional arguments:
  package_name          Name of the ROS package which contains the launch file
  launch_file_name      Name of the launch file
  launch_arguments      Arguments to the launch file; '<name>:=<value>' (for duplicates, last one wins)

options:
  -h, --help            show this help message and exit
  -n, --noninteractive  Run the launch system non-interactively, with no terminal associated
  -d, --debug           Put the launch system in debug mode, provides more verbose output.
  -p, --print, --print-description
                        Print the launch description to the console without launching it.
  -s, --show-args, --show-arguments
                        Show arguments that may be given to the launch file.
  -a, --show-all-subprocesses-output
                        Show all launched subprocesses' output by overriding their output configuration using the
                        OVERRIDE_LAUNCH_PROCESS_OUTPUT envvar.
  --launch-prefix LAUNCH_PREFIX
                        Prefix command, which should go before all executables. Command must be wrapped in quotes if
                        it contains spaces (e.g. --launch-prefix 'xterm -e gdb -ex run --args').
  --launch-prefix-filter LAUNCH_PREFIX_FILTER
                        Regex pattern for filtering which executables the --launch-prefix is applied to by matching
                        the executable name.

例如：

ros2 launch turtlesim multisim.launch.py

其中，multisim.launch.py 如下：

# turtlesim/launch/multisim.launch.py

from launch import LaunchDescription
import launch_ros.actions

def generate_launch_description():
    return LaunchDescription([
        launch_ros.actions.Node(
            namespace= "turtlesim1", package='turtlesim', executable='turtlesim_node', output='screen'),
        launch_ros.actions.Node(
            namespace= "turtlesim2", package='turtlesim', executable='turtlesim_node', output='screen'),
    ])

控制其中的小乌龟用rqt工具或如下指令：

1：ros2 topic pub  /turtlesim1/turtle1/cmd_vel geometry_msgs/msg/Twist “{linear: {x: 2.0, y: 0.0, z: 0.0}, angular: {x: 0.0, y: 0.0, z: 1.8}}”

2：ros2 topic pub  /turtlesim2/turtle1/cmd_vel geometry_msgs/msg/Twist “{linear: {x: 2.0, y: 0.0, z: 0.0}, angular: {x: 0.0, y: 0.0, z: -1.8}}”

启动launch教程
ROS 2 Launch 文件允许同时启动和配置多个包含 ROS 2 节点的可执行文件。

学习启动
1 创建 ROS 2 启动文件。

本教程将展示如何创建一个启动文件，该文件将同时启动节点及其配置。

from launch import LaunchDescription
from launch_ros.actions import Node

def generate_launch_description():
    return LaunchDescription([
        Node(
            package='turtlesim',
            namespace='turtlesim1',
            executable='turtlesim_node',
            name='sim'
        ),
        Node(
            package='turtlesim',
            namespace='turtlesim2',
            executable='turtlesim_node',
            name='sim'
        ),
        Node(
            package='turtlesim',
            executable='mimic',
            name='mimic',
            remappings=[
                ('/input/pose', '/turtlesim1/turtle1/pose'),
                ('/output/cmd_vel', '/turtlesim2/turtle1/cmd_vel'),
            ]
        )
    ])

2 使用 Launch 启动和监控多个节点。

关于启动文件如何工作以及可用的资源和文档的更高级概述。

import platform

import launch
import launch.actions
import launch.substitutions
import launch_ros.actions

env_var_name = 'USER' if platform.system() != 'Windows' else 'USERNAME'

def generate_launch_description():
    return launch.LaunchDescription([
        launch.actions.DeclareLaunchArgument(
            'node_prefix',
            default_value=[launch.substitutions.EnvironmentVariable(env_var_name), '_'],
            description='Prefix for node names'),
        launch_ros.actions.Node(
            package='demo_nodes_cpp', executable='talker', output='screen',
            name=[launch.substitutions.LaunchConfiguration('node_prefix'), 'talker']),
    ])

3 使用替代品。

在描述可重用的启动文件时，可以使用替换来提供更大的灵活性。

from launch_ros.actions import Node

from launch import LaunchDescription
from launch.actions import DeclareLaunchArgument, ExecuteProcess, TimerAction
from launch.conditions import IfCondition
from launch.substitutions import LaunchConfiguration, PythonExpression


def generate_launch_description():
    turtlesim_ns = LaunchConfiguration('turtlesim_ns')
    use_provided_red = LaunchConfiguration('use_provided_red')
    new_background_r = LaunchConfiguration('new_background_r')

    turtlesim_ns_launch_arg = DeclareLaunchArgument(
        'turtlesim_ns',
        default_value='turtlesim1'
    )
    use_provided_red_launch_arg = DeclareLaunchArgument(
        'use_provided_red',
        default_value='False'
    )
    new_background_r_launch_arg = DeclareLaunchArgument(
        'new_background_r',
        default_value='200'
    )

    turtlesim_node = Node(
        package='turtlesim',
        namespace=turtlesim_ns,
        executable='turtlesim_node',
        name='sim'
    )
    spawn_turtle = ExecuteProcess(
        cmd=[[
            'ros2 service call ',
            turtlesim_ns,
            '/spawn ',
            'turtlesim/srv/Spawn ',
            '"{x: 2, y: 2, theta: 0.2}"'
        ]],
        shell=True
    )
    change_background_r = ExecuteProcess(
        cmd=[[
            'ros2 param set ',
            turtlesim_ns,
            '/sim background_r ',
            '120'
        ]],
        shell=True
    )
    change_background_r_conditioned = ExecuteProcess(
        condition=IfCondition(
            PythonExpression([
                new_background_r,
                ' == 200',
                ' and ',
                use_provided_red
            ])
        ),
        cmd=[[
            'ros2 param set ',
            turtlesim_ns,
            '/sim background_r ',
            new_background_r
        ]],
        shell=True
    )

    return LaunchDescription([
        turtlesim_ns_launch_arg,
        use_provided_red_launch_arg,
        new_background_r_launch_arg,
        turtlesim_node,
        spawn_turtle,
        change_background_r,
        TimerAction(
            period=2.0,
            actions=[change_background_r_conditioned],
        )
    ])

4 使用事件处理程序。

事件处理程序可用于监视进程的状态或定义一组复杂的规则，这些规则可用于动态修改启动文件。

ROS 2 中的 Launch 是一个执行和管理用户定义流程的系统。 它负责监控它启动的进程的状态，以及报告和响应这些进程的状态变化。 这些更改称为事件，可以通过向启动系统注册事件处理程序来处理。 可以为特定事件注册事件处理程序，并可用于监视进程的状态。 此外，它们可用于定义一组复杂的规则，可用于动态修改启动文件。

本教程展示了 ROS 2 启动文件中事件处理程序的使用示例。

from launch_ros.actions import Node

from launch import LaunchDescription
from launch.actions import (DeclareLaunchArgument, EmitEvent, ExecuteProcess,
                            LogInfo, RegisterEventHandler, TimerAction)
from launch.conditions import IfCondition
from launch.event_handlers import (OnExecutionComplete, OnProcessExit,
                                OnProcessIO, OnProcessStart, OnShutdown)
from launch.events import Shutdown
from launch.substitutions import (EnvironmentVariable, FindExecutable,
                                LaunchConfiguration, LocalSubstitution,
                                PythonExpression)


def generate_launch_description():
    turtlesim_ns = LaunchConfiguration('turtlesim_ns')
    use_provided_red = LaunchConfiguration('use_provided_red')
    new_background_r = LaunchConfiguration('new_background_r')

    turtlesim_ns_launch_arg = DeclareLaunchArgument(
        'turtlesim_ns',
        default_value='turtlesim1'
    )
    use_provided_red_launch_arg = DeclareLaunchArgument(
        'use_provided_red',
        default_value='False'
    )
    new_background_r_launch_arg = DeclareLaunchArgument(
        'new_background_r',
        default_value='200'
    )

    turtlesim_node = Node(
        package='turtlesim',
        namespace=turtlesim_ns,
        executable='turtlesim_node',
        name='sim'
    )
    spawn_turtle = ExecuteProcess(
        cmd=[[
            FindExecutable(name='ros2'),
            ' service call ',
            turtlesim_ns,
            '/spawn ',
            'turtlesim/srv/Spawn ',
            '"{x: 2, y: 2, theta: 0.2}"'
        ]],
        shell=True
    )
    change_background_r = ExecuteProcess(
        cmd=[[
            FindExecutable(name='ros2'),
            ' param set ',
            turtlesim_ns,
            '/sim background_r ',
            '120'
        ]],
        shell=True
    )
    change_background_r_conditioned = ExecuteProcess(
        condition=IfCondition(
            PythonExpression([
                new_background_r,
                ' == 200',
                ' and ',
                use_provided_red
            ])
        ),
        cmd=[[
            FindExecutable(name='ros2'),
            ' param set ',
            turtlesim_ns,
            '/sim background_r ',
            new_background_r
        ]],
        shell=True
    )

    return LaunchDescription([
        turtlesim_ns_launch_arg,
        use_provided_red_launch_arg,
        new_background_r_launch_arg,
        turtlesim_node,
        RegisterEventHandler(
            OnProcessStart(
                target_action=turtlesim_node,
                on_start=[
                    LogInfo(msg='Turtlesim started, spawning turtle'),
                    spawn_turtle
                ]
            )
        ),
        RegisterEventHandler(
            OnProcessIO(
                target_action=spawn_turtle,
                on_stdout=lambda event: LogInfo(
                    msg='Spawn request says "{}"'.format(
                        event.text.decode().strip())
                )
            )
        ),
        RegisterEventHandler(
            OnExecutionComplete(
                target_action=spawn_turtle,
                on_completion=[
                    LogInfo(msg='Spawn finished'),
                    change_background_r,
                    TimerAction(
                        period=2.0,
                        actions=[change_background_r_conditioned],
                    )
                ]
            )
        ),
        RegisterEventHandler(
            OnProcessExit(
                target_action=turtlesim_node,
                on_exit=[
                    LogInfo(msg=(EnvironmentVariable(name='USER'),
                            ' closed the turtlesim window')),
                    EmitEvent(event=Shutdown(
                        reason='Window closed'))
                ]
            )
        ),
        RegisterEventHandler(
            OnShutdown(
                on_shutdown=[LogInfo(
                    msg=['Launch was asked to shutdown: ',
                        LocalSubstitution('event.reason')]
                )]
            )
        ),
    ])

5 使用 ROS 2 启动大型项目。

本教程描述了为大型项目编写启动文件的一些技巧。重点是如何构建启动文件，以便在不同情况下尽可能多地重用它们。此外，它还涵盖了不同 ROS 2 启动工具的使用示例，例如参数、YAML 文件、重新映射、命名空间、默认参数和 RViz 配置。

机器人上的大型应用程序通常涉及多个相互连接的节点，每个节点都可以有许多参数。 在海龟模拟器中模拟多只海龟就是一个很好的例子。 海龟模拟由多个海龟节点、世界配置以及 TF 广播器和侦听器节点组成。 在所有节点之间，存在大量影响这些节点行为和外观的 ROS 参数。 ROS 2 启动文件允许我们在一个地方启动所有节点并设置相应的参数。 在教程结束时，将在 launch_tutorial 包中构建 launch_turtlesim.launch.py 启动文件。 这个启动文件会调出不同的节点，负责模拟两个turtlesim模拟，启动TF广播器和监听器，加载参数，启动一个RViz配置。 在本教程中，将介绍这个启动文件和使用的所有相关功能。 

Python、XML 或 YAML：应该使用哪个？

提示：ROS 1 中的启动文件是用 XML 编写的，因此来自 ROS 1 的人可能最熟悉 XML。要查看发生了什么变化，可以访问将启动文件从 ROS 1 迁移到 ROS 2。

对于大多数应用程序，选择哪种 ROS 2 启动格式取决于开发人员的偏好。 但是，如果启动文件需要 XML 或 YAML 无法实现的灵活性，可以使用 Python 编写启动文件。 由于以下两个原因，使用 Python 启动 ROS 2 更加灵活：

1. Python 是一种脚本语言，因此可以在启动文件中利用该语言及其库。
2. ros2/launch（一般启动功能）和 ros2/launch_ros（ROS 2 特定启动功能）是用 Python 编写的，因此对 XML 和 YAML 可能未公开的启动功能具有较低级别的访问权限。

话虽如此，用 Python 编写的启动文件可能比用 XML 或 YAML 编写的启动文件更复杂和冗长。

同一功能对比如下：

Python

# example.launch.py

import os

from ament_index_python import get_package_share_directory

from launch import LaunchDescription
from launch.actions import DeclareLaunchArgument
from launch.actions import IncludeLaunchDescription
from launch.actions import GroupAction
from launch.launch_description_sources import PythonLaunchDescriptionSource
from launch.substitutions import LaunchConfiguration
from launch.substitutions import TextSubstitution
from launch_ros.actions import Node
from launch_ros.actions import PushRosNamespace


def generate_launch_description():

    # args that can be set from the command line or a default will be used
    background_r_launch_arg = DeclareLaunchArgument(
        "background_r", default_value=TextSubstitution(text="0")
    )
    background_g_launch_arg = DeclareLaunchArgument(
        "background_g", default_value=TextSubstitution(text="255")
    )
    background_b_launch_arg = DeclareLaunchArgument(
        "background_b", default_value=TextSubstitution(text="0")
    )
    chatter_ns_launch_arg = DeclareLaunchArgument(
        "chatter_ns", default_value=TextSubstitution(text="my/chatter/ns")
    )

    # include another launch file
    launch_include = IncludeLaunchDescription(
        PythonLaunchDescriptionSource(
            os.path.join(
                get_package_share_directory('demo_nodes_cpp'),
                'launch/topics/talker_listener.launch.py'))
    )
    # include another launch file in the chatter_ns namespace
    launch_include_with_namespace = GroupAction(
        actions=[
            # push_ros_namespace to set namespace of included nodes
            PushRosNamespace(LaunchConfiguration('chatter_ns')),
            IncludeLaunchDescription(
                PythonLaunchDescriptionSource(
                    os.path.join(
                        get_package_share_directory('demo_nodes_cpp'),
                        'launch/topics/talker_listener.launch.py'))
            ),
        ]
    )

    # start a turtlesim_node in the turtlesim1 namespace
    turtlesim_node = Node(
            package='turtlesim',
            namespace='turtlesim1',
            executable='turtlesim_node',
            name='sim'
        )

    # start another turtlesim_node in the turtlesim2 namespace
    # and use args to set parameters
    turtlesim_node_with_parameters = Node(
            package='turtlesim',
            namespace='turtlesim2',
            executable='turtlesim_node',
            name='sim',
            parameters=[{
                "background_r": LaunchConfiguration('background_r'),
                "background_g": LaunchConfiguration('background_g'),
                "background_b": LaunchConfiguration('background_b'),
            }]
        )

    # perform remap so both turtles listen to the same command topic
    forward_turtlesim_commands_to_second_turtlesim_node = Node(
            package='turtlesim',
            executable='mimic',
            name='mimic',
            remappings=[
                ('/input/pose', '/turtlesim1/turtle1/pose'),
                ('/output/cmd_vel', '/turtlesim2/turtle1/cmd_vel'),
            ]
        )

    return LaunchDescription([
        background_r_launch_arg,
        background_g_launch_arg,
        background_b_launch_arg,
        chatter_ns_launch_arg,
        launch_include,
        launch_include_with_namespace,
        turtlesim_node,
        turtlesim_node_with_parameters,
        forward_turtlesim_commands_to_second_turtlesim_node,
    ])

xml

<!-- example.launch.xml -->

<launch>

  <!-- args that can be set from the command line or a default will be used -->
  <arg name="background_r" default="0"/>
  <arg name="background_g" default="255"/>
  <arg name="background_b" default="0"/>
  <arg name="chatter_ns" default="my/chatter/ns"/>

  <!-- include another launch file -->
  <include file="$(find-pkg-share demo_nodes_cpp)/launch/topics/talker_listener.launch.py"/>
  <!-- include another launch file in the chatter_ns namespace-->
  <group>
    <!-- push_ros_namespace to set namespace of included nodes -->
    <push_ros_namespace namespace="$(var chatter_ns)"/>
    <include file="$(find-pkg-share demo_nodes_cpp)/launch/topics/talker_listener.launch.py"/>
  </group>

  <!-- start a turtlesim_node in the turtlesim1 namespace -->
  <node pkg="turtlesim" exec="turtlesim_node" name="sim" namespace="turtlesim1"/>
  <!-- start another turtlesim_node in the turtlesim2 namespace
      and use args to set parameters -->
  <node pkg="turtlesim" exec="turtlesim_node" name="sim" namespace="turtlesim2">
    <param name="background_r" value="$(var background_r)"/>
    <param name="background_g" value="$(var background_g)"/>
    <param name="background_b" value="$(var background_b)"/>
  </node>
  <!-- perform remap so both turtles listen to the same command topic -->
  <node pkg="turtlesim" exec="mimic" name="mimic">
    <remap from="/input/pose" to="/turtlesim1/turtle1/pose"/>
    <remap from="/output/cmd_vel" to="/turtlesim2/turtle1/cmd_vel"/>
  </node>
</launch>

yaml

# example.launch.yaml

launch:

# args that can be set from the command line or a default will be used
- arg:
    name: "background_r"
    default: "0"
- arg:
    name: "background_g"
    default: "255"
- arg:
    name: "background_b"
    default: "0"
- arg:
    name: "chatter_ns"
    default: "my/chatter/ns"


# include another launch file
- include:
    file: "$(find-pkg-share demo_nodes_cpp)/launch/topics/talker_listener.launch.py"

# include another launch file in the chatter_ns namespace
- group:
    - push_ros_namespace:
        namespace: "$(var chatter_ns)"
    - include:
        file: "$(find-pkg-share demo_nodes_cpp)/launch/topics/talker_listener.launch.py"

# start a turtlesim_node in the turtlesim1 namespace
- node:
    pkg: "turtlesim"
    exec: "turtlesim_node"
    name: "sim"
    namespace: "turtlesim1"

# start another turtlesim_node in the turtlesim2 namespace and use args to set parameters
- node:
    pkg: "turtlesim"
    exec: "turtlesim_node"
    name: "sim"
    namespace: "turtlesim2"
    param:
    -
      name: "background_r"
      value: "$(var background_r)"
    -
      name: "background_g"
      value: "$(var background_g)"
    -
      name: "background_b"
      value: "$(var background_b)"

# perform remap so both turtles listen to the same command topic
- node:
    pkg: "turtlesim"
    exec: "mimic"
    name: "mimic"
    remap:
    -
        from: "/input/pose"
        to: "/turtlesim1/turtle1/pose"
    -
        from: "/output/cmd_vel"
        to: "/turtlesim2/turtle1/cmd_vel"