Action client with feedback in ROS - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to import the ROS action client library.

ROS

from actionlib import [1]

Drag options to blanks, or click blank then click option'

AActionClient

BSimpleActionServer

CSimpleActionClient

DActionServer

Attempts:

3 left

💡 Hint

Common Mistakes

Importing SimpleActionServer instead of SimpleActionClient

Using ActionClient which is not the correct class name

The SimpleActionClient class is used to create an action client in ROS.

2fill in blank

medium

Complete the code to create an action client for the 'fibonacci' action.

ROS

client = SimpleActionClient('[1]', FibonacciAction)

Drag options to blanks, or click blank then click option'

Afibonacci

Bmove_base

Cnavigate

Dfollow_path

Attempts:

3 left

💡 Hint

Common Mistakes

Using unrelated action names like 'move_base'

Using capitalized or incorrect action names

The action client must connect to the 'fibonacci' action server by specifying its name.

3fill in blank

hard

Fix the error in the feedback callback function definition.

ROS

def feedback_cb([1]):
    rospy.loginfo('Feedback received: %s' % feedback.sequence)

Drag options to blanks, or click blank then click option'

Agoal_handle

Bresult

Cstatus

Dfeedback

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'result' or 'status' which are not feedback parameters

Using 'goal_handle' which is for servers

The feedback callback receives a feedback parameter containing the feedback message.

4fill in blank

hard

Fill both blanks to send a goal and register the feedback callback.

ROS

goal = FibonacciGoal(order=[1])
client.[2](goal, feedback_cb=feedback_cb)

Drag options to blanks, or click blank then click option'

A10

Bsend_goal

Csend_goal_async

D5

Attempts:

3 left

💡 Hint

Common Mistakes

Using send_goal_async which does not exist

Setting order to an unrelated number

The goal order is set to 10, and send_goal is used to send the goal with a feedback callback.

5fill in blank

hard

Fill all three blanks to wait for the result and print it.

ROS

client.[1]()
result = client.[2]()
print('Result sequence:', result.[3])

Drag options to blanks, or click blank then click option'

Await_for_result

Bget_result

Csequence

Dcancel_goal

Attempts:

3 left

💡 Hint

Common Mistakes

Using cancel_goal instead of wait_for_result

Trying to print the whole result object instead of its sequence

First wait for the result, then get it, and finally print the sequence attribute from the result.

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