Error handling for network calls in iOS Swift - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a function that fetches data from a URL.

iOS Swift

func fetchData(from url: URL, completion: @escaping (Result<Data, Error>) -> Void) [1]

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

Aasync {

B-> Void {

Cthrows {

D{

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'throws' without marking the function as throwing.

Using 'async' without async context.

Adding '-> Void {' which is redundant here.

The function body starts with an opening brace '{'.

2fill in blank

medium

Complete the code to create a URLSession data task with error handling.

iOS Swift

let task = URLSession.shared.dataTask(with: url) { data, response, error in
    if let [1] = error {
        completion(.failure(error))
        return
    }
}

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

Aerror

Bdata

Cresponse

Durl

Attempts:

3 left

💡 Hint

Common Mistakes

Checking 'data' or 'response' instead of 'error'.

Not unwrapping the optional error properly.

We check if 'error' is not nil to handle errors from the network call.

3fill in blank

hard

Fix the error in the code to safely unwrap data and call completion with success.

iOS Swift

guard let [1] = data else {
    completion(.failure(NSError(domain: "", code: -1, userInfo: nil)))
    return
}
completion(.success(data))

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

Aerror

Bresponse

Cdata

Durl

Attempts:

3 left

💡 Hint

Common Mistakes

Unwrapping 'response' or 'error' instead of 'data'.

Passing the optional 'data' directly without unwrapping.

We unwrap 'data' to ensure it is not nil before using it.

4fill in blank

hard

Fill both blanks to start the task and resume it.

iOS Swift

let task = URLSession.shared.dataTask(with: url) { data, response, error in
    // handle response
}
task[1][2]

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

A.

Bstart()

Cresume()

Dbegin()

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'start()' which does not exist.

Forgetting the dot before the method call.

The task is started by calling 'task.resume()'.

5fill in blank

hard

Fill all three blanks to complete a function that fetches data and handles errors properly.

iOS Swift

func fetchData(from url: URL, completion: @escaping (Result<Data, Error>) -> Void) {
    let task = URLSession.shared.dataTask(with: url) { data, response, error in
        if let [1] = error {
            completion(.failure(error))
            return
        }
        guard let [2] = data else {
            completion(.failure(NSError(domain: "", code: -1, userInfo: nil)))
            return
        }
        completion(.success([3]))
    }
    task.resume()
}

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

Aerror

Bdata

Dresponse

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong variable names for unwrapping.

Passing optional 'data' without unwrapping.

We unwrap 'error' to check for errors, unwrap 'data' to ensure data exists, and pass 'data' to success.

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