Passing data to destination in iOS Swift - Mini App: Build & Ship

Build: User Detail Screen

This app has a list of users. When you tap a user, it opens a detail screen showing the user's name passed from the list screen.

Target UI

User List Screen
----------------
| Alice       > |
| Bob         > |
| Charlie     > |
----------------

User Detail Screen
--------------------
| User Name: Alice  |
|                   |
| [Back]            |
--------------------

Create a User List screen with a list of user names.

When a user taps a name, navigate to User Detail screen.

Pass the selected user name to the User Detail screen.

Display the passed user name on the User Detail screen.

Add a back button to return to the User List screen.

Starter Code

iOS Swift

import SwiftUI

struct UserListView: View {
    let users = ["Alice", "Bob", "Charlie"]

    var body: some View {
        NavigationView {
            List(users, id: \.self) { user in
                // TODO: Add NavigationLink to UserDetailView passing user
                Text(user)
            }
            .navigationTitle("User List")
        }
    }
}

struct UserDetailView: View {
    let userName: String

    var body: some View {
        VStack {
            // TODO: Display userName here
            Spacer()
        }
        .navigationTitle("User Detail")
        .navigationBarTitleDisplayMode(.inline)
    }
}

struct ContentView: View {
    var body: some View {
        UserListView()
    }
}

@main
struct PassingDataApp: App {
    var body: some Scene {
        WindowGroup {
            ContentView()
        }
    }
}

Task 1

Task 2

Solution

iOS Swift

import SwiftUI

struct UserListView: View {
    let users = ["Alice", "Bob", "Charlie"]

    var body: some View {
        NavigationView {
            List(users, id: \.self) { user in
                NavigationLink(destination: UserDetailView(userName: user)) {
                    Text(user)
                }
            }
            .navigationTitle("User List")
        }
    }
}

struct UserDetailView: View {
    let userName: String

    var body: some View {
        VStack {
            Text("User Name: \(userName)")
                .font(.title)
                .padding()
            Spacer()
        }
        .navigationTitle("User Detail")
        .navigationBarTitleDisplayMode(.inline)
    }
}

struct ContentView: View {
    var body: some View {
        UserListView()
    }
}

@main
struct PassingDataApp: App {
    var body: some Scene {
        WindowGroup {
            ContentView()
        }
    }
}

We use NavigationLink in the list to navigate to UserDetailView. We pass the selected user name as a parameter to UserDetailView. Inside UserDetailView, we display the passed user name in a Text view. The navigation view automatically provides a back button to return to the list.

This is a simple way to pass data between screens in SwiftUI using navigation.

Final Result

Completed Screen

User List Screen
----------------
| Alice       > |
| Bob         > |
| Charlie     > |
----------------

User Detail Screen (after tapping Alice)
--------------------
| User Name: Alice  |
|                   |
| [Back]            |
--------------------

Tap a user name in the list to open the detail screen showing that user's name.

Tap the back button to return to the user list.

Stretch Goal

Add a button on User Detail screen to share the user name using the iOS share sheet.

💡 Hint

Use SwiftUI's .sheet modifier with UIActivityViewController wrapped in UIViewControllerRepresentable.

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)