Why Action definition (.action files) in ROS? - Purpose & Use Cases

The Big Idea

Discover how a simple file can make your robot's complex tasks run smoothly without messy code!

The Scenario

Imagine you need a robot to perform a task like moving to a location and reporting progress. You try to write separate code pieces for starting, updating, and finishing the task manually.

The Problem

Manually managing task states and communication is confusing and error-prone. You might miss updates or mix messages, causing the robot to behave unpredictably.

The Solution

Action definition files (.action) let you clearly define goals, feedback, and results in one place. ROS uses this to handle communication smoothly and reliably.

Before vs After

✗ Before

Publish goal message; listen for feedback; check result topic; manage states manually

✓ After

Define .action file with goal, feedback, result; ROS auto-generates code to handle communication.

What It Enables

It enables seamless, structured communication for complex robot tasks with automatic handling of progress and results.

Real Life Example

A delivery robot uses an action to move to a destination, sending progress updates and final success or failure status without extra manual messaging code.

Key Takeaways

Manual task communication is complex and fragile.

.action files define goals, feedback, and results clearly.

ROS automates messaging, making robot tasks reliable and easier to manage.

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