MVVM pattern in iOS Swift - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a ViewModel class in Swift.

iOS Swift

class [1] {
  // ViewModel properties and methods
}

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

AViewModel

BModel

CViewController

DView

Attempts:

3 left

💡 Hint

Common Mistakes

Naming the ViewModel as ViewController or Model.

The ViewModel class holds the data and logic for the view. Naming it ViewModel follows the MVVM pattern.

2fill in blank

medium

Complete the code to bind a ViewModel property to update the view using Combine.

iOS Swift

@Published var name: String = ""

var cancellables = Set<AnyCancellable>()

viewModel.$name
  .[1] { newName in
    self.label.text = newName
  }
  .store(in: &cancellables)

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

Asink

Bassign

Cmap

Dfilter

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'assign' which requires a writable key path, or 'map' which transforms values.

The sink operator subscribes to changes and executes the closure to update the UI.

3fill in blank

hard

Fix the error in the ViewModel initializer to properly initialize the model property.

iOS Swift

class ViewModel {
  private var model: Model

  init() {
    self.model = [1]
  }
}

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

Anil

BModel

Cself.model

DModel()

Attempts:

3 left

💡 Hint

Common Mistakes

Assigning the type name or nil instead of an instance.

You must create a new instance of Model by calling Model() to initialize the property.

4fill in blank

hard

Fill both blanks to create a computed property in ViewModel that returns the model's title in uppercase.

iOS Swift

var uppercaseTitle: String {
  return model.title[1][2]()
}

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

A.

Buppercased

CuppercaseString

D()

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'uppercaseString' which is Objective-C style, or missing the dot operator.

Use the dot operator to call the uppercased() method on the string.

5fill in blank

hard

Fill all three blanks to create a dictionary comprehension that filters model data with values greater than 10.

iOS Swift

let filteredData = model.data.filter { $0.value [1] 10 }.reduce(into: [:]) { $0[[2]] = [3] }

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

A>

B$1.key

C$1.value

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<' instead of '>', or mixing up key and value in reduce.

The filter uses > to keep values greater than 10. The reduce builds a dictionary using the key and value from each element.

Practice

1. What is the main role of the ViewModel in the MVVM pattern?

easy

A. To display the user interface elements

B. To hold data and business logic separate from the UI

C. To store raw data from the database only

D. To handle user input directly without any data processing

Solution

1. Step 1: Understand MVVM roles

   The ViewModel acts as a middleman between the Model and the View, holding data and logic.

2. Step 2: Identify ViewModel responsibility

   It keeps data and business logic separate from UI, so the View only handles display.

3. Final Answer:

   To hold data and business logic separate from the UI -> Option B

4. Quick Check:

   ViewModel = data + logic [OK]

Hint: ViewModel holds data and logic, not UI [OK]

Common Mistakes:

• Confusing ViewModel with View
• Thinking ViewModel handles UI rendering
• Assuming ViewModel stores raw database data only

2. Which of the following is the correct way to declare a ViewModel property in Swift for binding to a SwiftUI View?

easy

A. @State var viewModel = MyViewModel()

B. @EnvironmentObject var viewModel = MyViewModel()

C. var viewModel = MyViewModel()

D. @ObservedObject var viewModel = MyViewModel()

Solution

1. Step 1: Identify property wrapper for ViewModel

   In SwiftUI, @ObservedObject is used to observe ViewModel changes.

2. Step 2: Check other options

   @State is for local state, @EnvironmentObject requires external injection, plain var lacks observation.

3. Final Answer:

   @ObservedObject var viewModel = MyViewModel() -> Option D

4. Quick Check:

   ViewModel binding uses @ObservedObject [OK]

Hint: Use @ObservedObject for ViewModel in SwiftUI [OK]

Common Mistakes:

• Using @State for ViewModel instead of @ObservedObject
• Forgetting to mark ViewModel as ObservableObject
• Trying to instantiate ViewModel inside @EnvironmentObject

3. Given the following SwiftUI code snippet, what will be displayed when the button is tapped once?

class CounterViewModel: ObservableObject {
  @Published var count = 0
  func increment() { count += 1 }
}

struct ContentView: View {
  @ObservedObject var viewModel = CounterViewModel()
  var body: some View {
    VStack {
      Text("Count: \(viewModel.count)")
      Button("Tap me") {
        viewModel.increment()
      }
    }
  }
}

medium

A. Count: 0

B. Count: Tap me

C. Count: 1

D. Count: nil

Solution

1. Step 1: Understand initial state

   The count starts at 0 as declared in ViewModel.

2. Step 2: Analyze button tap effect

   When tapped, increment() adds 1 to count, updating the published property.

3. Final Answer:

   Count: 1 -> Option C

4. Quick Check:

   Button tap increments count to 1 [OK]

Hint: Button tap calls increment, count updates [OK]

Common Mistakes:

• Assuming count stays 0 after tap
• Confusing button label with count value
• Expecting nil instead of initial 0

4. Identify the error in this ViewModel code snippet and how to fix it:

class UserViewModel {
  @Published var username: String = ""
  func updateName(newName: String) {
    username = newName
  }
}

medium

A. Missing ObservableObject conformance; add ': ObservableObject' to class

B. Cannot use @Published on String; change to Int

C. updateName should return String; add return type

D. username should be private; add 'private' keyword

Solution

1. Step 1: Check @Published usage

   @Published requires the class to conform to ObservableObject to work properly.

2. Step 2: Fix class declaration

   Add ': ObservableObject' after class name to enable property observation.

3. Final Answer:

   Missing ObservableObject conformance; add ': ObservableObject' to class -> Option A

4. Quick Check:

   @Published needs ObservableObject [OK]

Hint: Add ObservableObject to use @Published [OK]

Common Mistakes:

• Forgetting ObservableObject conformance
• Thinking @Published works without ObservableObject
• Changing data type unnecessarily

5. You want to create a SwiftUI app using MVVM where the ViewModel fetches a list of users asynchronously and updates the View. Which approach correctly handles this scenario?

hard

A. Make ViewModel conform to ObservableObject, use @Published for users array, fetch data asynchronously and update @Published property

B. Fetch data directly in the View and update a local @State variable

C. Store users array in the Model only and let View read it directly

D. Use a global variable for users and update it from anywhere

Solution

1. Step 1: Understand MVVM async data flow

   ViewModel should handle data fetching and update @Published properties to notify View.

2. Step 2: Evaluate options

   Make ViewModel conform to ObservableObject, use @Published for users array, fetch data asynchronously and update @Published property correctly uses ObservableObject and @Published with async fetch; others break MVVM or cause bad design.

3. Final Answer:

   Make ViewModel conform to ObservableObject, use @Published for users array, fetch data asynchronously and update @Published property -> Option A

4. Quick Check:

   Async fetch in ViewModel updates @Published [OK]

Hint: Async fetch in ViewModel updates @Published property [OK]

Common Mistakes:

• Fetching data in View instead of ViewModel
• Using global variables for state
• Letting View read Model data directly