Error handling for network calls in iOS Swift

Introduction

Network calls can fail for many reasons like no internet or server problems. Handling errors helps your app stay friendly and avoid crashes.

When fetching data from the internet like loading user info or images.

When sending data to a server, like submitting a form or uploading a photo.

When your app depends on online services and you want to show messages if something goes wrong.

Syntax

iOS Swift

URLSession.shared.dataTask(with: url) { data, response, error in
  if let error = error {
    // Handle error here
    print("Error: \(error.localizedDescription)")
    return
  }
  // Use data if no error
}.resume()

Use the error parameter to check if the network call failed.

Always call .resume() to start the network task.

Examples

This example prints an error message if the call fails, or prints the size of data received if successful.

iOS Swift

URLSession.shared.dataTask(with: url) { data, response, error in
  if let error = error {
    print("Network error: \(error.localizedDescription)")
    return
  }
  if let data = data {
    print("Data received: \(data.count) bytes")
  }
}.resume()

This uses guard statements to handle errors and missing data clearly.

iOS Swift

URLSession.shared.dataTask(with: url) { data, response, error in
  guard error == nil else {
    print("Failed with error: \(error!.localizedDescription)")
    return
  }
  guard let data = data else {
    print("No data received")
    return
  }
  print("Success with data size: \(data.count)")
}.resume()

Sample App

This simple app fetches a post from the internet. It prints an error message if the call fails or prints the size of the data received. It uses DispatchQueue.main.async to update UI or print on the main thread.

iOS Swift

import UIKit

class ViewController: UIViewController {
  override func viewDidLoad() {
    super.viewDidLoad()
    fetchData()
  }

  func fetchData() {
    guard let url = URL(string: "https://jsonplaceholder.typicode.com/posts/1") else { return }
    URLSession.shared.dataTask(with: url) { data, response, error in
      if let error = error {
        DispatchQueue.main.async {
          print("Error fetching data: \(error.localizedDescription)")
        }
        return
      }
      guard let data = data else {
        DispatchQueue.main.async {
          print("No data received")
        }
        return
      }
      DispatchQueue.main.async {
        print("Data received: \(data.count) bytes")
      }
    }.resume()
  }
}

Important Notes

Always check the error parameter first to catch network problems.

Use DispatchQueue.main.async to update UI or print from network callbacks.

Remember to call .resume() to start the network request.

Summary

Network calls can fail; always check for errors.

Use the error parameter in the completion handler to detect problems.

Update UI on the main thread after network responses.

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