@Published properties in iOS Swift - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a property that automatically notifies views when it changes.

iOS Swift

class ViewModel: ObservableObject {
  [1] var count = 0
}

Drag options to blanks, or click blank then click option'

A@State

B@ObservedObject

C@Published

D@Binding

Attempts:

3 left

💡 Hint

Common Mistakes

Using @State instead of @Published inside ObservableObject.

Forgetting to mark the class as ObservableObject.

The @Published attribute marks a property so that changes to it notify any observing views.

2fill in blank

medium

Complete the code to make the ViewModel class notify views about changes.

iOS Swift

class ViewModel: [1] {
  @Published var name = ""
}

Drag options to blanks, or click blank then click option'

AIdentifiable

BView

CStateObject

DObservableObject

Attempts:

3 left

💡 Hint

Common Mistakes

Using View or StateObject instead of ObservableObject for the class.

Not conforming to any protocol.

ObservableObject is the protocol that allows a class to notify views about changes in @Published properties.

3fill in blank

hard

Fix the error in the code by completing the property declaration to notify views.

iOS Swift

class Settings: ObservableObject {
  [1] var isOn = false
  func toggle() {
    isOn.toggle()
  }
}

Drag options to blanks, or click blank then click option'

AAdd @Published before isOn

BAdd @State before isOn

CAdd @Binding before isOn

DAdd @ObservedObject before isOn

Attempts:

3 left

💡 Hint

Common Mistakes

Using @State or @Binding inside ObservableObject classes.

Not marking the property at all.

Only properties marked with @Published notify views when they change inside ObservableObject classes.

4fill in blank

hard

Fill both blanks to declare a published property and initialize it.

iOS Swift

class UserSettings: ObservableObject {
  [1] var username: String = [2]
}

Drag options to blanks, or click blank then click option'

A@Published

B"Guest"

C""

Dvar

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting @Published.

Initializing with empty quotes instead of a meaningful default.

The property must be marked @Published and initialized with a string value like "Guest".

5fill in blank

hard

Fill all three blanks to create a published integer property with a default value and a method to increment it.

iOS Swift

class Counter: ObservableObject {
  [1] var count: Int = [2]
  func increment() {
    count [3] 1
  }
}

Drag options to blanks, or click blank then click option'

A@Published

B0

C+=

D-=

Attempts:

3 left

💡 Hint

Common Mistakes

Using -= instead of += to increment.

Not marking count as @Published.

The count property is marked @Published, starts at 0, and increments by 1 using +=.

Practice

1. What is the main purpose of marking a property with @Published in SwiftUI?

easy

A. To make the property constant

B. To make the property private

C. To automatically notify views when the property changes

D. To prevent the property from being saved

Solution

1. Step 1: Understand @Published role

   @Published marks a property to send updates when its value changes.

2. Step 2: Connect to UI updates

   This automatic notification helps SwiftUI views update when data changes.

3. Final Answer:

   To automatically notify views when the property changes -> Option C

4. Quick Check:

   @Published = automatic update notification [OK]

Hint: Remember: @Published means auto-update UI on change [OK]

Common Mistakes:

• Thinking @Published makes property private
• Confusing @Published with constant declaration
• Assuming @Published saves data permanently

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

easy

A. struct Model: ObservableObject { @Published var count: Int = 0 }

B. class Model: ObservableObject { @Published var count: Int = 0 }

C. class Model { @Published var count: Int = 0 }

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

Solution

1. Step 1: Check class and ObservableObject conformance

   @Published must be inside a class conforming to ObservableObject, so class Model: ObservableObject is correct.

2. Step 2: Verify property declaration syntax

   The correct syntax is @Published var count: Int = 0, not var count: @Published Int.

3. Final Answer:

   class Model: ObservableObject { @Published var count: Int = 0 } -> Option B

4. Quick Check:

   Correct syntax = class Model: ObservableObject { @Published var count: Int = 0 } [OK]

Hint: Remember: @Published goes before var inside ObservableObject class [OK]

Common Mistakes:

• Using struct instead of class for ObservableObject
• Placing @Published after the type
• Not conforming class to ObservableObject

3. Given the code below, what will be printed when model.count = 5 is executed?

class Model: ObservableObject {
  @Published var count: Int = 0
}

let model = Model()
model.$count.sink { print("Count changed to \($0)") }

medium

A. Count changed to 5

B. Count changed to 0

C. No output

D. Compile error

Solution

1. Step 1: Understand @Published publisher

   @Published creates a publisher accessible via $property, here $count, which emits new values on change.

2. Step 2: Analyze sink subscription and assignment

   The sink closure prints the new value when count changes. Setting model.count = 5 triggers the print with 5.

3. Final Answer:

   Count changed to 5 -> Option A

4. Quick Check:

   @Published publishes new values = Count changed to 5 [OK]

Hint: Remember: $property publishes changes, sink prints new value [OK]

Common Mistakes:

• Expecting output before assignment
• Confusing $count with count
• Thinking no output without explicit call

4. Identify the error in this code snippet:

class Model: ObservableObject {
  @Published var name: String

  init() {
    name = ""
  }
}

let model = Model()
model.name = "Alice"

medium

A. Missing initial value for @Published property

B. ObservableObject conformance is missing

C. Cannot assign to @Published property outside class

D. No error, code works fine

Solution

1. Step 1: Check @Published property initialization

   The property name is initialized in init(), which is valid for non-optional properties.

2. Step 2: Verify ObservableObject conformance and usage

   Class conforms to ObservableObject and property is assigned outside class, which is allowed.

3. Final Answer:

   No error, code works fine -> Option A

4. Quick Check:

   Proper init and conformance = No error, code works fine [OK]

Hint: Remember: @Published properties can be initialized in init() [OK]

Common Mistakes:

• Assuming @Published needs default value inline
• Thinking ObservableObject conformance is missing
• Believing external assignment is disallowed

5. You want to create a SwiftUI app where a @Published property updates a label text automatically. Which of the following is the best way to connect the @Published property to the UI?

hard

A. Use @EnvironmentObject without passing the object

B. Use @State in the view for the property

C. Manually call setNeedsDisplay() after changing the property

D. Use @ObservedObject in the view to watch the ObservableObject

Solution

1. Step 1: Understand property wrappers for UI updates

   @Published properties notify changes, but the view must observe the object using @ObservedObject to update UI automatically.

2. Step 2: Evaluate other options

   @State is for local view state, not external objects; setNeedsDisplay() is UIKit, not SwiftUI; @EnvironmentObject requires injection.

3. Final Answer:

   Use @ObservedObject in the view to watch the ObservableObject -> Option D

4. Quick Check:

   @Published + @ObservedObject = auto UI update [OK]

Hint: Connect @Published with @ObservedObject in views [OK]

Common Mistakes:

• Using @State instead of @ObservedObject for external data
• Trying manual UI refresh calls in SwiftUI
• Using @EnvironmentObject without proper injection