Unit testing ViewModels in iOS Swift - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ ui_behavior

intermediate

How does ViewModel update UI state?

Consider a ViewModel that updates a published property to change the UI. What will the UI show after calling loadData() if the data fetch succeeds?

iOS Swift

class SampleViewModel: ObservableObject {
  @Published var message = ""
  func loadData() {
    message = "Loading..."
    DispatchQueue.main.asyncAfter(deadline: .now() + 1) {
      self.message = "Data loaded"
    }
  }
}

let vm = SampleViewModel()
vm.loadData()
// After 2 seconds, what is vm.message?

A "Loading..."

B ""

C "Data loaded"

D nil

💡 Hint

Think about asynchronous updates and when the message changes.

The ViewModel first sets message to "Loading..." immediately, then after 1 second updates it to "Data loaded". After 2 seconds, the message is "Data loaded".

❓ lifecycle

intermediate

When is ViewModel deinitialized?

Given this ViewModel and usage, when will the deinit print statement run?

iOS Swift

class MyViewModel {
  deinit {
    print("ViewModel deinitialized")
  }
}

var vm: MyViewModel? = MyViewModel()
vm = nil
// When does "ViewModel deinitialized" print?

A Immediately after setting vm = nil

B Never, because vm is a strong reference

C After a delay of 5 seconds

D Only when the app closes

💡 Hint

Think about reference counting and when objects are freed.

Setting vm to nil removes the last strong reference, so the ViewModel is deinitialized immediately.

🔧 Debug

advanced

Why does this ViewModel test fail?

This unit test expects the ViewModel's isLoading to be false after fetchData(). Why does it fail?

iOS Swift

class TestViewModel {
  var isLoading = false
  func fetchData() {
    isLoading = true
    DispatchQueue.global().async {
      sleep(1)
      self.isLoading = false
    }
  }
}

let vm = TestViewModel()
vm.fetchData()
assert(vm.isLoading == false)  // Fails here

A Because isLoading is set to false asynchronously after the assert runs

B Because isLoading is never set to true

C Because sleep(1) blocks the main thread

D Because assert syntax is incorrect

💡 Hint

Consider when the assert runs compared to the async update.

The assert runs immediately after fetchData(), before the async block sets isLoading to false, so isLoading is still true at assert time.

🧠 Conceptual

advanced

What is the main benefit of unit testing ViewModels?

Why do developers write unit tests specifically for ViewModels in an iOS app?

A To test UI animations and transitions

B To verify business logic without UI dependencies

C To check network connectivity directly

D To improve app launch speed

💡 Hint

Think about separation of concerns in app architecture.

ViewModels contain business logic and state, so testing them isolates logic from UI and external dependencies.

📝 Syntax

expert

What error does this ViewModel test code produce?

What error occurs when running this Swift unit test code?

iOS Swift

class VM {
  var count = 0
  func increment() {
    count += 1
  }
}

func testIncrement() {
  let vm: VM? = VM()
  vm.increment()
  assert(vm?.count == 1)
}
testIncrement()

A Compile-time error: Missing return in function

B Runtime error: nil unwrapping

C Test passes with no error

D Compile-time error: Cannot call method on optional without unwrapping

💡 Hint

Look at how vm is declared and how increment() is called.

vm is an optional, so calling vm.increment() without unwrapping causes a compile error.