Passing data to destination in iOS Swift

Introduction

Passing data to destination means sending information from one screen to another in your app. This helps screens show the right details based on what the user did before.

When you tap on a list item and want to show details about that item on a new screen.

When you fill a form and want to send the entered data to the next screen for confirmation.

When you select a setting and want the next screen to know which option was chosen.

When you want to pass user preferences from one screen to another to customize the display.

Syntax

iOS Swift

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

This method runs before the screen changes, so you can set data on the new screen.

Make sure to set the segue identifier in your storyboard to match the code.

Examples

Passing a username string to the Profile screen before it appears.

iOS Swift

override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
  if segue.identifier == "showProfile" {
    let profileVC = segue.destination as! ProfileViewController
    profileVC.username = "Alice"
  }
}

Passing a selected item object to the Item screen using the sender parameter.

iOS Swift

override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
  if segue.identifier == "showItem" {
    let itemVC = segue.destination as! ItemViewController
    if let selectedItem = sender as? Item {
      itemVC.item = selectedItem
    }
  }
}

Sample App

This example shows a Master screen that passes a color name to a Detail screen. When the button is tapped, the app moves to the Detail screen and shows the selected color.

iOS Swift

import UIKit

class MasterViewController: UIViewController {
  var selectedColor: String = "Blue"

  override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
    if segue.identifier == "showColorDetail" {
      let detailVC = segue.destination as! DetailViewController
      detailVC.colorName = selectedColor
    }
  }

  @IBAction func showDetailTapped(_ sender: UIButton) {
    performSegue(withIdentifier: "showColorDetail", sender: nil)
  }
}

class DetailViewController: UIViewController {
  var colorName: String?

  override func viewDidLoad() {
    super.viewDidLoad()
    view.backgroundColor = .white
    let label = UILabel()
    label.text = "Selected color: \(colorName ?? \"None\")"
    label.textAlignment = .center
    label.frame = view.bounds
    view.addSubview(label)
  }
}

Important Notes

Always check the segue identifier to avoid passing data to the wrong screen.

Use optional binding to safely unwrap data when needed.

Remember to set the destination screen's properties before the transition happens.

Summary

Passing data lets screens share information to show relevant content.

Use prepare(for:sender:) to set data on the destination screen before navigation.

Check segue identifiers and safely unwrap data to avoid crashes.

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)