Built-in property wrappers (@State, @Published) in Swift - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this SwiftUI @State example?

Consider this SwiftUI view code snippet. What will be printed to the console when the button is tapped once?

Swift

import SwiftUI

struct ContentView: View {
    @State private var count = 0

    var body: some View {
        Button("Tap me") {
            count += 1
            print("Count is now \(count)")
        }
    }
}

A Count is now 0

B No output printed

C Count is now 1

D Count is now 2

💡 Hint

Remember @State stores a value that updates when changed.

When the button is tapped, the count increments from 0 to 1, so the print shows "Count is now 1".

❓ Predict Output

intermediate

What happens when @Published property changes in this ObservableObject?

Given this Swift class, what will be printed when model.name = "Alice" is executed?

Swift

import Foundation
import Combine

class UserModel: ObservableObject {
    @Published var name: String = ""
}

let model = UserModel()
let cancellable = model.$name.sink { newName in
    print("Name changed to: \(newName)")
}

model.name = "Alice"

A Name changed to: Alice

B No output

C Name changed to:

D Runtime error

💡 Hint

@Published triggers updates to subscribers when the property changes.

Assigning "Alice" to name triggers the sink closure, printing the new value.

🔧 Debug

advanced

Why does this SwiftUI view not update when @Published changes?

Look at this code. The view does not update when model.name changes. What is the cause?

Swift

import SwiftUI

class UserModel: ObservableObject {
    @Published var name = ""
}

struct ContentView: View {
    @ObservedObject var model = UserModel()

    var body: some View {
        Text(model.name)
    }
}

let model = UserModel()
model.name = "Bob"

A The Text view cannot display @Published properties

B The @Published property must be private

C The model instance is not initialized

D The model property in ContentView should be marked with @ObservedObject

💡 Hint

SwiftUI views must observe ObservableObject with @ObservedObject to update.

Without @ObservedObject, the view does not watch for changes and won't update.

📝 Syntax

advanced

Which option correctly declares a @State property in SwiftUI?

Choose the correct syntax to declare a @State property named isOn initialized to false.

A @State private var isOn = false

B var @State isOn = false

C @State var isOn: Bool = false

D @State let isOn = false

💡 Hint

@State properties are usually private vars.

Option A uses correct syntax: @State private var isOn = false

🚀 Application

expert

How to combine @Published and @State for shared data in SwiftUI?

You want a SwiftUI view to update when a shared model's property changes. The model uses @Published. Which approach correctly connects the model to the view's @State?

A Use @Published in the view instead of @State

B Use @ObservedObject for the model in the view and bind the property directly, not @State

C Use @StateObject for the model and ignore @Published properties

D Declare the model as @State and access the @Published property inside

💡 Hint

Think about which property wrapper watches ObservableObject changes.

@ObservedObject watches the model's @Published properties and updates the view. @State is for local state, not shared models.

Practice

1. What is the main purpose of the @State property wrapper in SwiftUI?

easy

A. To mark properties in a class that notify other objects when changed

B. To create a constant value that never changes

C. To store local, simple data inside a view and update the UI when it changes

D. To handle network requests asynchronously

Solution

1. Step 1: Understand what @State does

   @State is used to keep simple, local data inside a SwiftUI view.

2. Step 2: Recognize its effect on UI

   When the @State data changes, SwiftUI automatically updates the view to reflect the new data.

3. Final Answer:

   To store local, simple data inside a view and update the UI when it changes -> Option C

4. Quick Check:

   @State manages local view data = B [OK]

Hint: Remember: @State is for local view data that changes [OK]

Common Mistakes:

• Confusing @State with @Published
• Thinking @State is for class properties
• Assuming @State handles network or async tasks

2. Which of the following is the correct way to declare a @Published property inside a Swift class?

easy

A. struct MyStruct { @State var count: Int = 0 }

B. struct MyStruct { @Published var count: Int = 0 }

C. class MyClass { @State var count: Int = 0 }

D. class MyClass { @Published var count: Int = 0 }

Solution

1. Step 1: Identify where @Published can be used

   @Published is used inside classes that conform to ObservableObject, not structs.

2. Step 2: Check syntax correctness

   class MyClass { @Published var count: Int = 0 } correctly declares a class with a @Published property and initializes it.

3. Final Answer:

   class MyClass { @Published var count: Int = 0 } -> Option D

4. Quick Check:

   @Published used in classes = A [OK]

Hint: Use @Published only inside classes, not structs [OK]

Common Mistakes:

• Using @Published in structs
• Confusing @State with @Published
• Missing var keyword or initialization

3. Consider the following SwiftUI code snippet:

class Counter: ObservableObject {
    @Published var value = 0
}

struct ContentView: View {
    @StateObject var counter = Counter()
    var body: some View {
        VStack {
            Text("Count: \(counter.value)")
            Button("Increment") {
                counter.value += 1
            }
        }
    }
}

What will be displayed after tapping the "Increment" button twice?

medium

A. Count: 0

B. Count: 2

C. Count: 1

D. Count: 3

Solution

1. Step 1: Understand the initial state

   The Counter class starts with value = 0. The view shows "Count: 0" initially.

2. Step 2: Analyze button action and updates

   Each tap on "Increment" increases counter.value by 1. After two taps, value becomes 2, and the UI updates automatically.

3. Final Answer:

   Count: 2 -> Option B

4. Quick Check:

   2 increments update value to 2 [OK]

Hint: Each button tap adds 1; count updates automatically [OK]

Common Mistakes:

• Forgetting @Published triggers UI update
• Assuming @StateObject resets value
• Counting taps incorrectly

4. Identify the error in this SwiftUI code snippet:

class Model: ObservableObject {
    @Published var text: String
    init() {}
}

struct ViewContent: View {
    @StateObject var model = Model()
    var body: some View {
        Text(model.text)
    }
}

medium

A. Missing initial value for @Published property 'text' in Model

B. Cannot use @StateObject with ObservableObject classes

C. Text view cannot display a String property

D. Model class should be a struct

Solution

1. Step 1: Check property initialization

   The @Published property text is declared but not initialized, and the init() is empty.

2. Step 2: Understand Swift requirement for properties

   All non-optional properties must have an initial value or be initialized in the constructor. Here, text lacks both.

3. Final Answer:

   Missing initial value for @Published property 'text' in Model -> Option A

4. Quick Check:

   Uninitialized property causes error = A [OK]

Hint: Always initialize @Published properties or provide init values [OK]

Common Mistakes:

• Assuming @StateObject can't be used with ObservableObject
• Thinking Text can't display String
• Believing Model must be a struct

5. You want to create a SwiftUI app where a view shows a user's name that can be edited in a separate view. Which combination of property wrappers should you use to ensure the UI updates correctly when the name changes?

hard

A. Use @State in the main view and @Published in a shared ObservableObject class

B. Use @Published in both views

C. Use @State in both views

D. Use @StateObject in the editing view only

Solution

1. Step 1: Understand data sharing between views

   When multiple views need to share and update data, use an ObservableObject class with @Published properties.

2. Step 2: Use @State for local view data and @Published for shared data

   The main view can use @State for local state, but the shared user name should be in a class marked with @Published to notify changes.

3. Final Answer:

   Use @State in the main view and @Published in a shared ObservableObject class -> Option A

4. Quick Check:

   Shared data uses @Published; local uses @State [OK]

Hint: Shared data needs @Published; local view data uses @State [OK]

Common Mistakes:

• Using @State only for shared data
• Using @Published in structs or views directly
• Not using ObservableObject for shared data