Conditional launching in ROS - Mini Project: Build & Apply

Conditional Launching in ROS

📖 Scenario: You are working on a robot project using ROS (Robot Operating System). You want to create a launch file that starts different nodes based on a condition, such as whether the robot is in simulation mode or real hardware mode.

🎯 Goal: Create a ROS launch file that conditionally launches either a simulator_node or a hardware_node based on a boolean launch argument called use_simulator.

📋 What You'll Learn

Create a launch argument named use_simulator with a default value of true.

Define two nodes: simulator_node and hardware_node.

Use a conditional statement to launch simulator_node only if use_simulator is true.

Use a conditional statement to launch hardware_node only if use_simulator is false.

💡 Why This Matters

🌍 Real World

Robots often need to run different software depending on whether they are in simulation or real hardware. Conditional launching helps manage this easily.

💼 Career

ROS developers use conditional launching to create flexible and reusable launch files for various robot configurations and environments.

1

Create the launch argument

Create a launch argument called use_simulator with a default value of true in the launch file.

ROS

from launch import LaunchDescription
from launch.actions import DeclareLaunchArgument

# Your code here

Hint

Use DeclareLaunchArgument to create the argument with the name use_simulator and set default_value='true'.

2

Define the simulator and hardware nodes

Define two nodes named simulator_node and hardware_node using Node from launch_ros.actions. Use package my_robot and executable names simulator_node and hardware_node respectively.

ROS

from launch import LaunchDescription
from launch.actions import DeclareLaunchArgument
from launch_ros.actions import Node

use_simulator_arg = DeclareLaunchArgument('use_simulator', default_value='true')

ld = LaunchDescription()
ld.add_action(use_simulator_arg)

# Define simulator_node and hardware_node below
# Your code here

Hint

Import Node from launch_ros.actions and create two nodes with the exact package and executable names.

3

Add conditional logic to launch nodes

Use launch.conditions.IfCondition to add simulator_node to the launch description only if use_simulator is true, and add hardware_node only if use_simulator is false. Use LaunchConfiguration('use_simulator') to access the argument value.

ROS

from launch import LaunchDescription
from launch.actions import DeclareLaunchArgument
from launch_ros.actions import Node
from launch.conditions import IfCondition
from launch.substitutions import LaunchConfiguration

use_simulator_arg = DeclareLaunchArgument('use_simulator', default_value='true')

simulator_node = Node(package='my_robot', executable='simulator_node')
hardware_node = Node(package='my_robot', executable='hardware_node')

ld = LaunchDescription()
ld.add_action(use_simulator_arg)

# Add conditional launch of nodes below
# Your code here

Hint

Use IfCondition with LaunchConfiguration('use_simulator') to conditionally launch nodes. Remember to add both nodes to the launch description.

4

Complete the launch description function

Wrap the launch description in a function called generate_launch_description() that returns the LaunchDescription object ld.

ROS

from launch import LaunchDescription
from launch.actions import DeclareLaunchArgument
from launch_ros.actions import Node
from launch.conditions import IfCondition
from launch.substitutions import LaunchConfiguration

use_simulator_arg = DeclareLaunchArgument('use_simulator', default_value='true')

simulator_node = Node(package='my_robot', executable='simulator_node', condition=IfCondition(LaunchConfiguration('use_simulator')))
hardware_node = Node(package='my_robot', executable='hardware_node', condition=IfCondition(LaunchConfiguration('use_simulator').perform(None) == 'false'))

ld = LaunchDescription()
ld.add_action(use_simulator_arg)
ld.add_action(simulator_node)
ld.add_action(hardware_node)

# Define generate_launch_description function below
# Your code here

Hint

Define a function named generate_launch_description that returns the launch description ld.

Practice

1. What is the main purpose of using conditional launching in ROS launch files?

easy

A. To update ROS to the latest version

B. To speed up the robot's movement

C. To change the robot's hardware automatically

D. To start nodes only when certain conditions are met

Solution

1. Step 1: Understand conditional launching concept

   Conditional launching allows starting nodes only if specific conditions are true, saving resources.

2. Step 2: Identify the main purpose

   The main goal is to control which nodes run based on conditions, not to affect hardware or speed directly.

3. Final Answer:

   To start nodes only when certain conditions are met -> Option D

4. Quick Check:

   Conditional launching = start nodes conditionally [OK]

Hint: Think: conditional launching controls node start based on conditions [OK]

Common Mistakes:

• Confusing conditional launching with robot speed control
• Thinking it updates hardware automatically
• Assuming it updates ROS version

2. Which syntax correctly uses IfCondition in a ROS 2 Python launch file to conditionally start a node?

easy

A. Node(if='launch_arg == true', package='pkg', executable='node')

B. Node(condition=IfCondition(launch_arg == true), package='pkg', executable='node')

C. Node(condition=IfCondition('launch_arg == "true"'), package='pkg', executable='node')

D. Node(condition=IfCondition('launch_arg = true'), package='pkg', executable='node')

Solution

1. Step 1: Recall correct IfCondition usage

   IfCondition expects a string expression comparing launch arguments, e.g., 'launch_arg == "true"'.

2. Step 2: Check each option's syntax

   Node(condition=IfCondition('launch_arg == "true"'), package='pkg', executable='node') uses correct string comparison and quotes. Others have syntax errors or wrong operators.

3. Final Answer:

   Node(condition=IfCondition('launch_arg == "true"'), package='pkg', executable='node') -> Option C

4. Quick Check:

   IfCondition needs string expression with == [OK]

Hint: Use quotes around condition string and double equals for comparison [OK]

Common Mistakes:

• Using single equals (=) instead of double equals (==)
• Not quoting the condition expression
• Passing boolean directly instead of string

3. Given this launch snippet, what nodes will start if the launch argument use_camera is set to false?

from launch import LaunchDescription
from launch.actions import DeclareLaunchArgument
from launch.conditions import IfCondition
from launch_ros.actions import Node

def generate_launch_description():
    return LaunchDescription([
        DeclareLaunchArgument('use_camera', default_value='false'),
        Node(
            package='camera_pkg',
            executable='camera_node',
            condition=IfCondition('use_camera == "true"')
        ),
        Node(
            package='lidar_pkg',
            executable='lidar_node'
        )
    ])

medium

A. Only lidar_node starts

B. Both camera_node and lidar_node start

C. Only camera_node starts

D. No nodes start

Solution

1. Step 1: Analyze the condition for camera_node

   camera_node starts only if 'use_camera' equals 'true'. Here, 'use_camera' is 'false', so it won't start.

2. Step 2: Check lidar_node launch condition

   lidar_node has no condition, so it always starts.

3. Final Answer:

   Only lidar_node starts -> Option A

4. Quick Check:

   Condition false skips camera_node, lidar_node always runs [OK]

Hint: Nodes without conditions always start; check condition expressions carefully [OK]

Common Mistakes:

• Assuming camera_node starts when use_camera is false
• Thinking no nodes start if condition fails
• Ignoring unconditional nodes

4. Identify the error in this ROS 2 launch file snippet that prevents conditional launching from working:

Node(
    package='my_pkg',
    executable='my_node',
    condition=IfCondition(use_sensor == 'true')
)

medium

A. IfCondition cannot be used with Node

B. use_sensor should be a string inside quotes

C. Missing package name

D. Executable name must be capitalized

Solution

1. Step 1: Check IfCondition argument type

   IfCondition expects a string expression, but here use_sensor is used as a variable without quotes.

2. Step 2: Confirm correct syntax

   The condition should be written as IfCondition('use_sensor == "true"') with quotes around the expression.

3. Final Answer:

   use_sensor should be a string inside quotes -> Option B

4. Quick Check:

   IfCondition needs string condition expression [OK]

Hint: Always quote condition expressions in IfCondition [OK]

Common Mistakes:

• Passing variable without quotes to IfCondition
• Thinking IfCondition is invalid with Node
• Ignoring case sensitivity in executable

5. You want to launch either sim_node or real_node based on a launch argument use_sim. Which launch snippet correctly implements this conditional launching in ROS 2 Python launch file?

hard

A. Node(package='robot_pkg', executable='sim_node', condition=IfCondition('use_sim == "true"'))
Node(package='robot_pkg', executable='real_node', condition=IfCondition('use_sim == "false"'))

B. Node(package='robot_pkg', executable='sim_node', condition=IfCondition(use_sim == true))
Node(package='robot_pkg', executable='real_node', condition=IfCondition(use_sim == false))

C. Node(package='robot_pkg', executable='sim_node')
Node(package='robot_pkg', executable='real_node')

D. Node(package='robot_pkg', executable='sim_node', condition=IfCondition('use_sim = true'))
Node(package='robot_pkg', executable='real_node', condition=IfCondition('use_sim = false'))

Solution

1. Step 1: Check condition syntax for sim_node

   Node(package='robot_pkg', executable='sim_node', condition=IfCondition('use_sim == "true"')) Node(package='robot_pkg', executable='real_node', condition=IfCondition('use_sim == "false"')) uses correct string expressions with double equals and quotes for 'use_sim == "true"'.

2. Step 2: Check condition syntax for real_node

   Node(package='robot_pkg', executable='sim_node', condition=IfCondition('use_sim == "true"')) Node(package='robot_pkg', executable='real_node', condition=IfCondition('use_sim == "false"')) also correctly uses 'use_sim == "false"' for real_node, ensuring only one node launches.

3. Step 3: Verify other options

   Node(package='robot_pkg', executable='sim_node', condition=IfCondition(use_sim == true)) Node(package='robot_pkg', executable='real_node', condition=IfCondition(use_sim == false)) lacks quotes and uses booleans incorrectly; Node(package='robot_pkg', executable='sim_node') Node(package='robot_pkg', executable='real_node') launches both nodes unconditionally; Node(package='robot_pkg', executable='sim_node', condition=IfCondition('use_sim = true')) Node(package='robot_pkg', executable='real_node', condition=IfCondition('use_sim = false')) uses single equals (=) which is invalid.

4. Final Answer:

   Option A correctly uses IfCondition with string expressions and double equals -> Option A

5. Quick Check:

   Use quotes and == for conditions to launch nodes selectively [OK]

Hint: Use '==' inside quotes for conditions; avoid single '=' [OK]

Common Mistakes:

• Using single equals (=) instead of double equals (==)
• Not quoting the condition expression
• Launching both nodes without conditions