Action client with feedback in ROS - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What does this ROS action client print when receiving feedback?

Consider a ROS action client that sends a goal and prints feedback messages received from the server. What will be printed if the server sends feedback with progress 50% and then 100%?

ROS

import actionlib
from some_msgs.msg import SomeAction, SomeGoal

def feedback_cb(feedback):
    print(f"Progress: {feedback.progress}%")

client = actionlib.SimpleActionClient('some_action', SomeAction)
client.wait_for_server()
goal = SomeGoal()
client.send_goal(goal, feedback_cb=feedback_cb)
client.wait_for_result()

A Progress: 50%\nProgress: 100%

B Progress: 100%\nProgress: 50%

C No output because feedback_cb is not called

D Progress: 0%\nProgress: 50%

💡 Hint

Feedback callbacks are called each time the server sends feedback.

The feedback callback prints the progress value each time feedback is received. The server sends 50% then 100%, so both are printed in order.

❓ state_output

intermediate

What is the final state of the action client after goal completion?

Given a ROS action client that sends a goal and waits for the result, what will be the state of the client after the server completes the goal successfully?

ROS

import actionlib
from some_msgs.msg import SomeAction, SomeGoal

client = actionlib.SimpleActionClient('some_action', SomeAction)
client.wait_for_server()
goal = SomeGoal()
client.send_goal(goal)
client.wait_for_result()
state = client.get_state()

A actionlib.GoalStatus.PREEMPTED (2)

B actionlib.GoalStatus.SUCCEEDED (3)

C actionlib.GoalStatus.ABORTED (4)

D actionlib.GoalStatus.PENDING (0)

💡 Hint

Check the meaning of the state codes in actionlib.GoalStatus.

When the server completes the goal successfully, the client state is SUCCEEDED, which corresponds to 3.

📝 Syntax

advanced

Which option correctly sets up a feedback callback in a ROS action client?

Select the code snippet that correctly passes a feedback callback function to the send_goal method of a ROS action client.

A client.send_goal(goal, feedbackCallback=feedback_cb())

B client.send_goal(goal, feedback=feedback_cb)

C client.send_goal(goal, feedbackCallback=feedback_cb)

D client.send_goal(goal, feedback_cb=feedback_cb)

💡 Hint

Check the parameter name expected by send_goal for feedback callbacks.

The correct parameter name is feedback_cb. Passing feedback_cb=feedback_cb correctly assigns the callback function.

🔧 Debug

advanced

Why does this ROS action client not print feedback messages?

This ROS action client code does not print any feedback messages even though the server sends them. What is the cause?

ROS

import actionlib
from some_msgs.msg import SomeAction, SomeGoal

def feedback_cb(feedback):
    print(f"Progress: {feedback.progress}%")

client = actionlib.SimpleActionClient('some_action', SomeAction)
client.wait_for_server()
goal = SomeGoal()
client.send_goal(goal)
client.wait_for_result()

A The feedback callback is not passed to send_goal, so it is never called.

B The feedback callback function has a syntax error and is ignored.

C The goal message is empty, so no feedback is sent.

D The client never waits for the server, so feedback is lost.

💡 Hint

Check how the feedback callback is connected to the client.

The feedback callback must be passed as feedback_cb=feedback_cb in send_goal to be called on feedback.

🧠 Conceptual

expert

What is the role of feedback in a ROS action client-server interaction?

In ROS action communication, what is the main purpose of feedback messages sent from the action server to the client?

A To send the final result of the goal execution to the client.

B To confirm that the goal has been accepted by the server.

C To inform the client about the ongoing progress or partial results before the goal completes.

D To cancel the goal execution on the server side.

💡 Hint

Think about what feedback means in everyday tasks.

Feedback provides updates during execution, like progress reports, so the client knows how the goal is advancing.

Practice

1. What is the main purpose of an action client in ROS when using feedback?

easy

A. To directly control robot hardware without feedback

B. To send goals to an action server and receive progress updates

C. To replace service calls with simpler requests

D. To only receive the final result without intermediate updates

Solution

1. Step 1: Understand the role of an action client

   An action client sends goals to an action server and can receive feedback during execution.

2. Step 2: Identify feedback purpose

   Feedback callbacks provide real-time progress updates while the task runs.

3. Final Answer:

   To send goals to an action server and receive progress updates -> Option B

4. Quick Check:

   Action client = send goals + get feedback [OK]

Hint: Action clients send goals and get feedback updates [OK]

Common Mistakes:

• Thinking action clients only get final results
• Confusing action clients with service clients
• Ignoring feedback callbacks

2. Which of the following is the correct way to create an action client with feedback in ROS Python?

easy

A. client = actionlib.SimpleActionClient('move', MoveAction, feedback_cb=feedback_cb)

B. client = actionlib.ActionClient('move', MoveAction, feedback_cb=feedback_cb)

C. client = actionlib.SimpleActionClient('move', MoveAction)

D. client = actionlib.ActionClient('move', MoveAction)

Solution

1. Step 1: Recall ROS Python action client syntax

   The standard way is to use SimpleActionClient with server name and action type.

2. Step 2: Feedback callback is set separately

   Feedback callbacks are registered when sending goals, not in constructor.

3. Final Answer:

   client = actionlib.SimpleActionClient('move', MoveAction) -> Option C

4. Quick Check:

   SimpleActionClient + feedback_cb separate [OK]

Hint: Feedback callback is passed when sending goal, not client creation [OK]

Common Mistakes:

• Passing feedback_cb in constructor (wrong)
• Using ActionClient instead of SimpleActionClient
• Missing action type argument

3. Given this ROS Python snippet, what will be printed if the feedback callback receives progress=50?

def feedback_cb(feedback):
    print(f"Progress: {feedback.progress}%")

client = actionlib.SimpleActionClient('task', TaskAction)
client.wait_for_server()
goal = TaskGoal()
client.send_goal(goal, feedback_cb=feedback_cb)

medium

A. Progress: 50%

B. Progress: feedback.progress%

C. Progress: None%

D. No output because feedback_cb is not called

Solution

1. Step 1: Understand feedback callback behavior

   The feedback_cb prints the progress attribute from feedback with formatting.

2. Step 2: Substitute progress value

   If feedback.progress is 50, the print outputs "Progress: 50%" exactly.

3. Final Answer:

   Progress: 50% -> Option A

4. Quick Check:

   Formatted string with feedback.progress = output [OK]

Hint: Feedback callback prints formatted progress value [OK]

Common Mistakes:

• Printing variable name instead of value
• Assuming feedback_cb is not called
• Ignoring string formatting

4. What is wrong with this ROS Python action client code snippet?

client = actionlib.SimpleActionClient('task', TaskAction)
client.send_goal(TaskGoal(), feedback_cb=feedback_cb)
client.wait_for_server()

medium

A. wait_for_server() should be called before send_goal()

B. feedback_cb cannot be passed to send_goal()

C. TaskGoal() must be assigned to a variable before sending

D. SimpleActionClient requires a third argument for feedback

Solution

1. Step 1: Check action client usage order

   wait_for_server() must be called before sending a goal to ensure server is ready.

2. Step 2: Identify error in code order

   Here, send_goal() is called before wait_for_server(), which is incorrect.

3. Final Answer:

   wait_for_server() should be called before send_goal() -> Option A

4. Quick Check:

   wait_for_server() before send_goal() [OK]

Hint: Always wait for server before sending goal [OK]

Common Mistakes:

• Calling send_goal before wait_for_server
• Misunderstanding feedback_cb usage
• Assuming feedback_cb is constructor argument

5. You want to implement an action client that sends multiple goals sequentially and prints feedback for each. Which approach correctly handles feedback for each goal without mixing updates?

hard

A. Ignore feedback and only wait for final results of each goal

B. Use a single global feedback callback for all goals without distinguishing them

C. Send all goals at once without waiting and use one feedback callback

D. Create a new feedback callback function for each goal that captures its goal ID

Solution

1. Step 1: Understand feedback handling for multiple goals

   Each goal's feedback should be handled separately to avoid mixing progress updates.

2. Step 2: Use closures or separate callbacks

   Creating a unique feedback callback per goal that captures its ID ensures correct feedback association.

3. Final Answer:

   Create a new feedback callback function for each goal that captures its goal ID -> Option D

4. Quick Check:

   Separate feedback callbacks per goal prevent mix-up [OK]

Hint: Use unique feedback callbacks per goal to track progress separately [OK]

Common Mistakes:

• Using one callback for all goals causing mixed feedback
• Sending all goals simultaneously without control
• Ignoring feedback defeats purpose of action client