URLSession basics in iOS Swift - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ ui_behavior

intermediate

What happens when you call resume() on a URLSessionDataTask?

You create a URLSessionDataTask but forget to call resume(). What will happen?

A The task will never start and no data will be fetched.

B The app will crash with a runtime error.

C The task will start immediately and fetch data.

D The task will start but complete with an error.

💡 Hint

Think about what resume() does for a URLSessionDataTask.

URLSessionDataTask starts in a suspended state. You must call resume() to start it. Without resume(), the task stays suspended and does nothing.

🧠 Conceptual

intermediate

What is the role of the completion handler in URLSession dataTask?

Why do we provide a completion handler when creating a dataTask with URLSession?

A To configure the request headers before sending.

B To automatically retry the request on failure.

C To cancel the task if it takes too long.

D To handle the response data, response, and error after the request finishes.

💡 Hint

Think about what happens after the network call completes.

The completion handler runs after the network request finishes. It provides the data received, the server response, and any error that occurred.

📝 Syntax

advanced

Which code snippet correctly creates and starts a URLSession data task?

Select the code that correctly creates a data task to fetch data from a URL and starts it.

iOS Swift

let url = URL(string: "https://example.com")!
let task = URLSession.shared.dataTask(with: url) { data, response, error in
  // handle response
}
// What is missing here?

A task.run()

B task.resume()

C task.begin()

D task.start()

💡 Hint

Look for the method that starts the task.

The correct method to start a URLSessionDataTask is resume(). Other methods do not exist and cause errors.

❓ lifecycle

advanced

What happens if you call resume() twice on the same URLSessionDataTask?

Consider this code: let task = URLSession.shared.dataTask(with: url) { data, response, error in // handle response } task.resume() task.resume() What is the effect of calling resume() twice?

A The task runs twice and fetches data two times.

B The app crashes with an exception.

C The second resume() call has no effect; the task runs only once.

D The task cancels after the second resume() call.

💡 Hint

Think about the task state after the first resume().

Calling resume() on a running task does nothing. The task runs once and ignores subsequent resume() calls.

🔧 Debug

expert

Why does this URLSession dataTask completion handler never run?

Look at this code: let url = URL(string: "https://example.com")! let task = URLSession.shared.dataTask(with: url) { data, response, error in print("Response received") } // Missing line here Why does "Response received" never print?

A Because the task was never started with resume().

B Because the URL is invalid and the task fails silently.

C Because the completion handler is not called on the main thread.

D Because the URLSession.shared is nil.

💡 Hint

Check if the task is started after creation.

URLSessionDataTask starts suspended. Without calling resume(), the completion handler never runs.

Practice

1. What is the main purpose of URLSession in Swift?

easy

A. To create user interface elements

B. To handle network requests and communicate with the internet

C. To manage local file storage

D. To perform mathematical calculations

Solution

1. Step 1: Understand URLSession role

   URLSession is designed to manage network tasks like downloading or uploading data.

2. Step 2: Compare with other options

   Creating UI, managing files, or math are unrelated to URLSession.

3. Final Answer:

   To handle network requests and communicate with the internet -> Option B

4. Quick Check:

   URLSession = network communication [OK]

Hint: Remember URLSession is for internet tasks only [OK]

Common Mistakes:

• Confusing URLSession with UI components
• Thinking URLSession manages local files
• Assuming URLSession does math operations

2. Which of the following is the correct way to create a data task with URLSession?

easy

A. let task = URLSession.dataTask(url) { data, error in }

B. let task = URLSession.shared.dataTask(url)

C. let task = URLSession.shared.downloadTask(url)

D. let task = URLSession.shared.dataTask(with: url) { data, response, error in }

Solution

1. Step 1: Check correct method signature

   The correct method is dataTask(with:completionHandler:) on URLSession.shared.

2. Step 2: Validate options

   let task = URLSession.shared.dataTask(with: url) { data, response, error in } matches the correct syntax with closure parameters data, response, error. Others have wrong method names or missing parameters.

3. Final Answer:

   let task = URLSession.shared.dataTask(with: url) { data, response, error in } -> Option D

4. Quick Check:

   Correct dataTask syntax = let task = URLSession.shared.dataTask(with: url) { data, response, error in } [OK]

Hint: Look for shared session and closure with three parameters [OK]

Common Mistakes:

• Using dataTask without shared session
• Missing response parameter in closure
• Using downloadTask instead of dataTask

3. What will the following code print if the network request succeeds and returns data?

let url = URL(string: "https://example.com")!
let task = URLSession.shared.dataTask(with: url) { data, response, error in
  if let data = data {
    print("Data received: \(data.count) bytes")
  } else {
    print("No data")
  }
}
task.resume()

medium

A. Data received: [number] bytes

B. Syntax error

C. No data

D. Data received: 0 bytes

Solution

1. Step 1: Analyze dataTask completion handler

   If the request succeeds, data contains the downloaded bytes, so data.count is a positive number.

2. Step 2: Check print statements

   The code prints "Data received: \(data.count) bytes" if data exists, otherwise "No data".

3. Final Answer:

   Data received: [number] bytes -> Option A

4. Quick Check:

   Successful dataTask prints data size [OK]

Hint: If data exists, print shows byte count, not zero [OK]

Common Mistakes:

• Assuming data is always nil
• Confusing data.count with zero
• Thinking code has syntax errors

4. Identify the error in this code snippet:

let url = URL(string: "https://example.com")!
let task = URLSession.shared.dataTask(with: url) { data, response, error in
  if let data = data {
    DispatchQueue.main.async {
      print("Data length: \(data.count)")
    }
  }
}
// Missing task.resume()

medium

A. Missing call to task.resume() to start the request

B. Incorrect URL string format

C. DispatchQueue.main.async should not be used here

D. Closure parameters are incorrect

Solution

1. Step 1: Check for starting the task

   URLSession tasks are created suspended. You must call task.resume() to start the network request.

2. Step 2: Validate other parts

   URL string is valid, closure parameters are correct, and updating UI on main thread is good practice.

3. Final Answer:

   Missing call to task.resume() to start the request -> Option A

4. Quick Check:

   Always call resume() to run URLSession tasks [OK]

Hint: Remember to call resume() after creating a data task [OK]

Common Mistakes:

• Forgetting to call task.resume()
• Misusing DispatchQueue for UI updates
• Incorrect closure parameters

5. You want to fetch JSON data from a URL and update a label with the result. Which approach correctly ensures the UI updates happen on the main thread?

let url = URL(string: "https://api.example.com/data")!
let task = URLSession.shared.dataTask(with: url) { data, response, error in
  guard let data = data else { return }
  let text = String(data: data, encoding: .utf8) ?? "No data"
  // Update label here
}
task.resume()

Options:

hard

A. Call label.text = text before task.resume()

B. Update label.text directly inside the closure without DispatchQueue

C. Use DispatchQueue.main.async { label.text = text } inside the closure

D. Use a background thread to update label.text

Solution

1. Step 1: Understand threading in URLSession

   The completion handler runs on a background thread, so UI updates must be dispatched to the main thread.

2. Step 2: Check each option

   Use DispatchQueue.main.async { label.text = text } inside the closure correctly uses DispatchQueue.main.async to update the label safely. Update label.text directly inside the closure without DispatchQueue risks UI bugs. Call label.text = text before task.resume() updates UI before data is fetched. Use a background thread to update label.text wrongly uses background thread for UI.

3. Final Answer:

   Use DispatchQueue.main.async { label.text = text } inside the closure -> Option C

4. Quick Check:

   UI updates on main thread = Use DispatchQueue.main.async { label.text = text } inside the closure [OK]

Hint: Always update UI on main thread with DispatchQueue.main.async [OK]

Common Mistakes:

• Updating UI on background thread
• Updating UI before data arrives
• Ignoring thread safety for UI changes