How is the launch of ros2 different?

​ One 、 summary

stay launch Multi node execution ,ROS2 And ROS1 There's a big difference . First ,ros2 And ros1 The difference , lie in ROS2 No more *.launch Script files , In its place  3 Three formats are implemented ： ​

1. python Script
2. xml file
3. yaml file

Two 、 start-up launch file

2.1  launch Use of

      adopt launch file ,ROS2 Many nodes can be started at the same time , This simplifies using the command line to start different programs multiple times Node. ​

2.2 launch Start command  ​

1）xml file yaml file

ros2 launch <package_name> <launch_file_name>

​2） about py file , It can also be started directly launch file , like this : ​

ros2 launch turtlesim_mimic_launch.py

​

3、 ... and 、launch The content of the document

​

3.1 launch File content description

because launch file After all, files are multi node execution scripts , therefore , And “ ros2 run ” The contents of the instructions are roughly the same , Such as package name 、 The node name . Of course , To enrich some . Such as ：

<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>

3.2 About include

     include Can be in a launch file Contains additional launch file ​

3.3 About group

group You can put more than one node Put together ​

Four 、 Practice writing a launch file

4.1 Necessary routine

Whether it's python、xml still yaml, To write launch file The steps are almost the same . ​

1. Set the default value of command line parameters ,
2. Set up launch file The inclusive relationship of , adopt label
3. Set up Node Information , Include name、namespace、parameter
4. If you need to set remmaping Is set remapping Relationship

​4.2 for instance

Here's the official document An example , It's for turtles turtlesim Example ：

from launch import LaunchDescriptionfrom 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'),
            ])

    ])

The above code is very clear , Is to start two turtle nodes , One mimic node ; Some of the above concepts that need to be explained are ： What is the function of namespaces ？ What's the point of remapping ？（ After execution , Analyze with command line ）

4.3  remap What's the effect

remap Is a very useful technique , It's usually used to make Topic Mapping . A node doesn't know this Topic, But by renaming , Or pseudonym , Or re disguise （ Be careful , Subscribers here only accept certain names Topic）, Make some receive this Topic.

<remap from="/different_topic" to="/needed_topic"/>

hold from Medium topic, convert to to designated topic ​

Yes 2 Uses ：

1. Put one node Originally released topic, Map to another name
2. Put the others node Published original topic, Map to the desired topic

​

 4.4 Execute with parameters launch

We noticed the above launch file There is args, such as background_r, When the command line is started, the data can be transmitted through , adopt key:=value form . Such as ：

ros2 launch <package_name> <launch_file_name> background_r:=255

5、 ... and 、 Reference resources ： Three ways of writing

The following code is for reference , Are three file implementation methods of the same content , Because it's longer , I hope you can read it slowly .

5.1 be based on python The case of  

# 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,
    ])

This is the use of python Written , The last thing to come back is LaunchDescription Information , This is equivalent to xml in Content in Tags

5.2 be based on xml Writing

<!-- 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>

5.3 be based on yaml The file of

# 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"

Reference article ：

ROS2 Medium launch File entry 6 A question - You know (zhihu.com)

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