Passing data to destination in iOS Swift - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ ui_behavior

intermediate

Passing data using prepare(for:sender:)

What will be the value of destinationVC.receivedText after this segue is performed?

iOS Swift

override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
  if segue.identifier == "showDetail" {
    let destinationVC = segue.destination as! DetailViewController
    destinationVC.receivedText = "Hello Swift"
  }
}

A "Hello Swift"

B nil

C "showDetail"

D Crash due to wrong cast

💡 Hint

Remember that prepare(for:sender:) is called before the segue happens and you can set properties on the destination view controller.

The code safely casts the destination view controller and sets its receivedText property to "Hello Swift" before the segue happens, so the destinationVC.receivedText will be "Hello Swift".

❓ navigation

intermediate

Passing data with programmatic navigation

What will be printed when the button is tapped?

iOS Swift

class FirstViewController: UIViewController {
  @IBAction func buttonTapped(_ sender: UIButton) {
    let secondVC = SecondViewController()
    secondVC.data = "Swift Rocks"
    navigationController?.pushViewController(secondVC, animated: true)
  }
}

class SecondViewController: UIViewController {
  var data: String?
  override func viewDidLoad() {
    super.viewDidLoad()
    print(data ?? "No data")
  }
}

A No data

B nil

C Swift Rocks

D Crash due to missing storyboard

💡 Hint

Check when viewDidLoad is called and if the data property is set before that.

The data property is set before pushing the view controller, so when viewDidLoad runs, data contains "Swift Rocks" and it prints that.

❓ lifecycle

advanced

Data availability timing in destination VC

Why might receivedText be nil inside viewDidLoad() of the destination view controller when using a segue?

iOS Swift

override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
  let destinationVC = segue.destination as! DetailViewController
  DispatchQueue.main.async {
    destinationVC.receivedText = "Delayed Data"
  }
}

class DetailViewController: UIViewController {
  var receivedText: String?
  override func viewDidLoad() {
    super.viewDidLoad()
    print(receivedText ?? "No data")
  }
}

A Because prepare(for:sender:) is called after viewDidLoad

B Because the data is set asynchronously after viewDidLoad runs

C Because receivedText is private and inaccessible

D Because the segue identifier is missing

💡 Hint

Think about when the DispatchQueue.main.async block executes relative to viewDidLoad.

The data is assigned asynchronously on the main queue, which happens after viewDidLoad runs, so receivedText is still nil during viewDidLoad.

📝 Syntax

advanced

Correct syntax for passing data with segue

Which option correctly passes a string to the destination view controller's property message in prepare(for:sender:)?

iOS Swift

override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
  if segue.identifier == "showMessage" {
    let dest = segue.destination as! MessageViewController
    // Pass data here
  }
}

A dest.message := "Hello from source"

B dest.message == "Hello from source"

C dest.message <- "Hello from source"

D dest.message = "Hello from source"

💡 Hint

Remember the assignment operator in Swift.

Option D uses the correct assignment operator '=' to set the property. The others use invalid operators causing syntax errors.

🔧 Debug

expert

Why does data not appear in destination VC?

Given this code, why does the destination view controller's label show empty text after navigation?

iOS Swift

class FirstVC: UIViewController {
  var textToSend = "Important Data"
  override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
    let dest = segue.destination as! SecondVC
    dest.receivedText = textToSend
  }
}

class SecondVC: UIViewController {
  var receivedText: String?
  @IBOutlet weak var label: UILabel!
  override func viewDidLoad() {
    super.viewDidLoad()
    label.text = receivedText
  }
}

A The label outlet is not connected in storyboard, so label is nil

B receivedText is nil because prepare(for:) is not called

C viewDidLoad runs before prepare(for:), so label.text is empty

D The segue identifier is incorrect, so prepare(for:) is skipped

💡 Hint

Check if the label outlet is properly connected in Interface Builder.

If the label outlet is not connected, label is nil and setting label.text does nothing, so the label appears empty.

Practice

1. What is the main purpose of passing data to a destination view controller in iOS Swift?

easy

A. To change the app's theme colors dynamically

B. To handle user authentication automatically

C. To improve app performance by caching data

D. To share information so the next screen can display relevant content

Solution

1. Step 1: Understand data sharing between screens

   Passing data allows one screen to send information to another so the next screen can show content based on that data.

2. Step 2: Identify the purpose of passing data

   This process is mainly to make the user experience relevant by showing specific information on the destination screen.

3. Final Answer:

   To share information so the next screen can display relevant content -> Option D

4. Quick Check:

   Passing data = share info for relevant display [OK]

Hint: Passing data means sharing info for next screen display [OK]

Common Mistakes:

• Confusing data passing with UI styling
• Thinking it improves performance directly
• Assuming it handles authentication

2. Which method is commonly overridden to pass data to the destination view controller before a segue?

easy

A. prepare(for:sender:)

B. applicationDidBecomeActive()

C. viewWillAppear()

D. viewDidLoad()

Solution

1. Step 1: Identify the method for segue preparation

   The method prepare(for:sender:) is called before a segue happens and is used to set data on the destination controller.

2. Step 2: Confirm other methods are unrelated

   viewDidLoad() and viewWillAppear() are lifecycle methods but not used for passing data during segue. applicationDidBecomeActive() is an app lifecycle method.

3. Final Answer:

   prepare(for:sender:) -> Option A

4. Quick Check:

   Data passing before segue = prepare(for:sender:) [OK]

Hint: Use prepare(for:sender:) to pass data before segue [OK]

Common Mistakes:

• Using viewDidLoad() to pass data
• Confusing lifecycle methods with segue methods
• Trying to pass data after segue completes

3. Given the code snippet below, what will be the value of destinationVC.userName after the segue?

override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
  if segue.identifier == "showDetail" {
    let destinationVC = segue.destination as! DetailViewController
    destinationVC.userName = "Alice"
  }
}

medium

A. "Alice"

B. nil

C. An empty string ""

D. Causes a runtime crash

Solution

1. Step 1: Check the segue identifier and casting

   The code checks if the segue identifier is "showDetail" and casts the destination to DetailViewController safely using forced cast.

2. Step 2: Assign the userName property

   It sets destinationVC.userName to "Alice" before the segue completes.

3. Final Answer:

   "Alice" -> Option A

4. Quick Check:

   prepare sets userName = "Alice" [OK]

Hint: Check identifier and cast before assigning data [OK]

Common Mistakes:

• Forgetting to check segue.identifier
• Not casting destinationVC correctly
• Assuming userName remains nil

4. What is the error in the following code that causes a crash when performing the segue?

override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
  let destinationVC = segue.destination as! DetailViewController
  destinationVC.data = "Hello"
}

medium

A. Assigning data after the segue completes

B. Using forced cast instead of optional cast

C. Missing check for segue.identifier before casting

D. Not calling super.prepare(for:sender:)

Solution

1. Step 1: Identify missing segue identifier check

   The code casts destinationVC without checking if the segue.identifier matches the expected one, risking wrong destination type.

2. Step 2: Understand crash cause

   If the segue is not to DetailViewController, forced cast fails and crashes the app.

3. Final Answer:

   Missing check for segue.identifier before casting -> Option C

4. Quick Check:

   Always check segue.identifier before casting [OK]

Hint: Always check segue.identifier before casting destination [OK]

Common Mistakes:

• Skipping identifier check causes crash
• Assuming forced cast is always safe
• Ignoring optional cast or guard statements

5. You want to pass a user's score (an Int) from GameViewController to ResultViewController using a segue with identifier "showResult". Which code snippet correctly passes the score safely?

hard

A. override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
if segue.identifier == "showResult" {
let dest = segue.destination as! ResultViewController
dest.score = userScore
}
}

B. override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
if segue.identifier == "showResult" {
guard let dest = segue.destination as? ResultViewController else { return }
dest.score = userScore
}
}

C. override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
let dest = segue.destination as? ResultViewController
dest?.score = userScore
}

D. override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
if segue.identifier == "showResult" {
let dest = segue.destination
dest.score = userScore
}
}

Solution

1. Step 1: Check segue identifier and safely cast destination

   override func prepare(for segue: UIStoryboardSegue, sender: Any?) { if segue.identifier == "showResult" { guard let dest = segue.destination as? ResultViewController else { return } dest.score = userScore } } uses a guard let to safely cast segue.destination to ResultViewController only if the identifier matches "showResult".

2. Step 2: Assign the score property safely

   Inside the guard, dest.score is assigned userScore safely, avoiding crashes if cast fails.

3. Final Answer:

   Checks identifier and uses guard let for safe casting -> Option B

4. Quick Check:

   Safe cast + identifier check = override func prepare(for segue: UIStoryboardSegue, sender: Any?) { if segue.identifier == "showResult" { guard let dest = segue.destination as? ResultViewController else { return } dest.score = userScore } } [OK]

Hint: Use guard let with identifier check for safe data passing [OK]

Common Mistakes:

• Forcing cast without identifier check (Option A)
• Not checking identifier before casting (Option B)
• Not casting destination at all (Option D)