Error handling for network calls in iOS Swift - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ ui_behavior

intermediate

Handling network error UI update

You make a network call that can fail. Which option correctly updates the UI to show an error message when the call fails?

iOS Swift

func fetchData() {
  networkCall { result in
    switch result {
    case .success(let data):
      self.updateUI(with: data)
    case .failure(let error):
      // What to do here?
    }
  }
}

A DispatchQueue.global().async { self.errorLabel.text = error.localizedDescription }

B DispatchQueue.main.async { self.errorLabel.text = error.localizedDescription }

C print(error.localizedDescription)

D self.errorLabel.text = error.localizedDescription

💡 Hint

UI updates must happen on the main thread.

UI changes in iOS must be done on the main thread. Option B uses DispatchQueue.main.async to update the error label text safely. Option B updates UI on a background thread, which can cause issues. Option B updates UI on a global background thread, which is incorrect. Option B only prints the error and does not update the UI.

🧠 Conceptual

intermediate

Understanding URLSession error handling

What does the 'error' parameter in URLSession dataTask completion handler represent?

A The HTTP status code of the response

B The data returned from the server

C A client-side error such as network failure or request timeout

D An error returned by the server in the HTTP response body

💡 Hint

Think about what kind of errors happen before getting a response.

The 'error' parameter represents client-side errors like no internet connection or timeout. Server errors come as HTTP status codes in the response, not as this error parameter.

❓ lifecycle

advanced

Properly cancelling a network call on view disappear

You start a URLSession dataTask in a view controller. What is the best way to cancel the task when the view disappears to avoid memory leaks or unwanted callbacks?

A Do nothing; the system cancels tasks automatically

B Set task to nil inside viewDidDisappear method

C Call task.resume() inside viewWillDisappear method

D Call task.cancel() inside viewWillDisappear method

💡 Hint

Think about when you want to stop the network call cleanly.

Calling task.cancel() inside viewWillDisappear stops the network call and prevents callbacks after the view is gone. Setting task to nil does not cancel the task. The system does not cancel tasks automatically. Calling resume() starts or resumes the task, which is the opposite of cancel.

📝 Syntax

advanced

Correct error handling syntax in async network call

Which option correctly uses Swift's async/await syntax with error handling for a network call?

iOS Swift

func loadData() async {
  do {
    let data = try await fetchData()
    process(data)
  } catch {
    handle(error)
  }
}

A do { let data = try await fetchData(); process(data) } catch { handle(error) }

B let data = await fetchData() catch { handle(error) }

C try { let data = await fetchData() } catch { handle(error) }

D let data = try fetchData() await process(data)

💡 Hint

Remember the correct placement of try, await, and do-catch blocks.

Option A correctly wraps the try await call inside a do block and catches errors. Option A has invalid syntax mixing await and catch. Option A misuses try without await and wrong do-catch syntax. Option A misorders try and await and misses async context.

🔧 Debug

expert

Diagnosing error message on successful network calls

You notice your app shows an error message even when the network call succeeds. Which option is the most likely cause?

iOS Swift

URLSession.shared.dataTask(with: url) { data, response, error in
  if error != nil {
    print("Error occurred")
  }
  DispatchQueue.main.async {
    self.errorLabel.text = "Failed to load"
  }
}.resume()

A The errorLabel update is outside the error check, so it always shows failure even if no error

B The UI update runs regardless of error presence, so errorLabel always shows failure even on success

C The error check is incorrect; it should be if error == nil

D The network call is missing .resume() to start the task

💡 Hint

Look where the UI update code is placed relative to the error check.

The errorLabel.text update is outside the if error != nil block, so it runs always, showing failure even on success. This can confuse the user. The correct approach is to update UI only inside the error check. Option A describes the symptom but B pinpoints the cause. Option A is wrong logic. Option A is false because .resume() is present.

Practice

1. What is the main purpose of the error parameter in a network call's completion handler in Swift?

easy

A. To update the UI directly from the background thread

B. To store the data returned from the server

C. To specify the URL of the network request

D. To check if the network request failed and handle the failure

Solution

1. Step 1: Understand the role of the error parameter

   The error parameter indicates if something went wrong during the network request. Developers check this parameter to detect failures and respond appropriately, like showing an alert.

2. Final Answer:

   To check if the network request failed and handle the failure -> Option D

3. Quick Check:

   Error parameter = check failure [OK]

Hint: Error parameter signals failure in network calls [OK]

Common Mistakes:

• Confusing error with data parameter
• Trying to update UI directly from background thread
• Ignoring error and assuming success

2. Which of the following is the correct syntax to check for an error in a Swift network call completion handler?

easy

A. if error == nil { print("Error occurred") }

B. if error != nil { print("Error occurred") }

C. if error = nil { print("Error occurred") }

D. if error != false { print("Error occurred") }

Solution

1. Step 1: Recall error checking syntax in Swift

   To check if an error exists, use if error != nil. Using != nil means an error was found, so handle it.

2. Final Answer:

   if error != nil { print("Error occurred") } -> Option B

3. Quick Check:

   Check error with != nil [OK]

Hint: Use 'if error != nil' to detect errors [OK]

Common Mistakes:

• Using assignment '=' instead of comparison '=='
• Checking error == nil to detect error (wrong logic)
• Comparing error to false instead of nil

3. Consider this Swift code snippet for a network call:

URLSession.shared.dataTask(with: url) { data, response, error in
  if let error = error {
    print("Error: \(error.localizedDescription)")
  } else {
    print("Success")
  }
}.resume()

What will be printed if the network request fails due to no internet connection?

medium

A. Error: The Internet connection appears to be offline.

B. No output

C. Success

D. Crash due to unwrapped nil

Solution

1. Step 1: Trace the code on network failure

   The code uses optional binding if let error = error which succeeds when error is not nil, printing the localized description like "The Internet connection appears to be offline." for no internet.

2. Final Answer:

   Error: The Internet connection appears to be offline. -> Option A

3. Quick Check:

   Error prints localizedDescription [OK]

Hint: Error prints message; success prints 'Success' [OK]

Common Mistakes:

• Assuming success prints even on error
• Expecting crash from optional binding
• Ignoring error and printing nothing

4. This Swift code attempts to handle a network error but has a logic error:

URLSession.shared.dataTask(with: url) { data, response, error in
  if error == nil {
    print("No error")
  } else {
    print(error!.localizedDescription)
  }
}.resume()

What is the likely cause of the logic error?

medium

A. Not calling resume() on the data task

B. Force unwrapping error when it is nil

C. Using error == nil instead of error != nil

D. Printing error.localizedDescription without checking error

Solution

1. Step 1: Analyze the logic and best practice

   The code checks if error == nil for success first, then handles error in else (safe unwrap). While functional, best practice is to check if error != nil first to explicitly handle failures before success.

2. Final Answer:

   Using error == nil instead of error != nil -> Option C

3. Quick Check:

   Error check logic reversed [OK]

Hint: Check error != nil to handle errors safely [OK]

Common Mistakes:

• Reversing error check logic
• Force unwrapping error without check
• Not calling resume() on data task

5. You want to update a label's text after a network call completes. Which is the correct way to handle errors and update the UI safely in Swift?

URLSession.shared.dataTask(with: url) { data, response, error in
  // Handle error and update label
}

hard

A. Check if error != nil, then update label text inside DispatchQueue.main.async block

B. Update label text directly inside the completion handler without dispatching

C. Ignore error and update label text on background thread

D. Use try-catch inside the completion handler to update label

Solution

1. Step 1: Handle threading and errors for UI updates

   Completion handlers run on background threads, so dispatch UI updates to main. First check if error != nil to handle failure (e.g., update label with error message), using DispatchQueue.main.async.

2. Final Answer:

   Check if error != nil, then update label text inside DispatchQueue.main.async block -> Option A

3. Quick Check:

   UI updates on main thread after error check [OK]

Hint: Always update UI on main thread after error check [OK]

Common Mistakes:

• Updating UI on background thread causing crashes
• Ignoring error and updating UI blindly
• Using try-catch incorrectly for async callbacks