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.