Action definition (.action files) in ROS - Mini Project: Build & Apply

Create a ROS Action Definition with .action File

📖 Scenario: You are building a robot control system that needs to perform a task which takes time and can provide feedback during execution. You will create a ROS action definition file to specify the goal, feedback, and result messages for this task.

🎯 Goal: Create a ROS .action file that defines the structure for a robot's task including goal, feedback, and result sections.

📋 What You'll Learn

Create a RobotTask.action file with three sections separated by ---

Define a goal message with a string task_name and an int32 duration

Define a feedback message with a float32 progress field

Define a result message with a bool success and a string message

💡 Why This Matters

🌍 Real World

Robots often perform tasks that take time and need progress updates. ROS actions let you define these tasks clearly so the robot and other software can communicate effectively.

💼 Career

Understanding ROS action definitions is essential for robotics developers working on task execution, control systems, and robot communication.

1

Create the Goal section in the .action file

Create a file named RobotTask.action and write the Goal section with these exact fields: string task_name and int32 duration. Do NOT add any other sections yet.

ROS

string task_name
int32 duration
# Your code here

Hint

The Goal section is the first part of the .action file before the first --- separator.

2

Add the Feedback section separated by ---

In the RobotTask.action file, add the separator --- after the Goal section, then add the Feedback section with the exact field: float32 progress.

ROS

string task_name
int32 duration
---
float32 progress
# Your code here

Hint

The Feedback section comes after the first --- separator.

3

Add the Result section separated by ---

In the RobotTask.action file, add another separator --- after the Feedback section, then add the Result section with these exact fields: bool success and string message.

ROS

string task_name
int32 duration
---
float32 progress
---
bool success
string message
# Your code here

Hint

The Result section is the last part of the .action file after the second --- separator.

4

Complete and verify the full .action file structure

Ensure the RobotTask.action file has exactly three sections separated by --- in this order: Goal with string task_name and int32 duration, Feedback with float32 progress, and Result with bool success and string message. The file should be ready to use in a ROS action server.

ROS

string task_name
int32 duration
---
float32 progress
---
bool success
string message
# Your code here

Hint

Check that the separators and fields are exactly as specified for ROS to recognize the action.

Practice

1. What is the main purpose of an .action file in ROS?

easy

A. To define goals, results, and feedback for long-running tasks

B. To list all the nodes in a ROS system

C. To configure robot sensors

D. To specify message types for topics

Solution

1. Step 1: Understand the role of .action files

   .action files are designed to define the structure of actions, which include goals, feedback, and results for tasks that take time.

2. Step 2: Compare with other ROS files

   Unlike message or service files, .action files specifically handle long-running tasks with progress updates.

3. Final Answer:

   To define goals, results, and feedback for long-running tasks -> Option A

4. Quick Check:

   Action files = goals + feedback + results [OK]

Hint: Remember: .action files split into goal, result, feedback [OK]

Common Mistakes:

• Confusing .action files with .msg or .srv files
• Thinking .action files configure hardware
• Assuming .action files list nodes

2. Which of the following is the correct structure of a ROS .action file?

easy

A. goal fields --- feedback fields --- result fields

B. goal fields --- result fields --- feedback fields

C. result fields --- feedback fields --- goal fields

D. feedback fields --- goal fields --- result fields

Solution

1. Step 1: Recall the .action file format

   The .action file has three parts separated by lines with three dashes (---): first goal, then result, then feedback.

2. Step 2: Match the order

   The correct order is goal fields first, result fields second, and feedback fields last.

3. Final Answer:

   goal fields --- result fields --- feedback fields -> Option B

4. Quick Check:

   Order = goal, result, feedback [OK]

Hint: Remember order: Goal first, Result second, Feedback last [OK]

Common Mistakes:

• Mixing up the order of feedback and result
• Using only two sections instead of three
• Forgetting the --- separators

3. Given this .action file snippet:

int32 goal_value
---
int32 result_value
---
float32 progress

What is the type of the feedback field?

medium

A. int32

B. bool

C. string

D. float32

Solution

1. Step 1: Identify the feedback section

   The .action file is split by --- lines. The third section after the second --- is the feedback part.

2. Step 2: Check the feedback field type

   The third section contains float32 progress, so the feedback field type is float32.

3. Final Answer:

   float32 -> Option D

4. Quick Check:

   Feedback type = float32 [OK]

Hint: Feedback is always after second --- line [OK]

Common Mistakes:

• Confusing result and feedback sections
• Choosing goal field type instead
• Ignoring the --- separators

4. What is wrong with this .action file?

int32 goal_value
---
float32 feedback_value
---
int32 result_value

medium

A. Result section must come before feedback

B. The order of sections is correct; no error

C. Feedback section cannot have fields

D. Feedback section is in the wrong place

Solution

1. Step 1: Check the order of sections

   The file has goal fields first, then feedback fields, then result fields, separated by --- lines.

2. Step 2: Verify field types in each section

   Goal has int32, feedback has float32, result has int32. However, the correct order is goal, result, feedback.

3. Final Answer:

   Result section must come before feedback -> Option A

4. Quick Check:

   Correct order is goal, result, feedback [OK]

Hint: Check section order and field types carefully [OK]

Common Mistakes:

• Thinking feedback must come last
• Assuming feedback cannot have fields
• Mixing up result and feedback order

5. You want to define an action for a robot to move to a position with feedback on distance left and a result indicating success. Which .action file snippet correctly defines this?

hard

A. float64 target_x
float64 target_y
---
bool success
---
float64 distance_left

B. bool success
---
float64 target_x
float64 target_y
---
float64 distance_left

C. float64 target_x
float64 target_y
---
float64 distance_left
---
bool success

D. float64 distance_left
---
float64 target_x
float64 target_y
---
bool success

Solution

1. Step 1: Identify goal, feedback, and result roles

   The goal should specify the target position (target_x, target_y). Feedback should provide ongoing info (distance_left). Result should indicate success (bool success).

2. Step 2: Match the correct order in the .action file

   The order is goal fields first, then feedback fields, then result fields. float64 target_x float64 target_y --- float64 distance_left --- bool success matches this order correctly.

3. Final Answer:

   float64 target_x float64 target_y --- float64 distance_left --- bool success -> Option C

4. Quick Check:

   Goal=target, Feedback=distance, Result=success [OK]

Hint: Goal first, feedback second, result last with correct fields [OK]

Common Mistakes:

• Swapping feedback and result sections
• Putting result fields in goal section
• Mislabeling feedback as result