Mock objects and protocols in iOS Swift - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Why use protocols for mocking in Swift?

In Swift, why do developers often use protocols when creating mock objects for testing?

A Protocols define a blueprint that mocks can implement, allowing tests to substitute real objects easily.

B Protocols automatically generate mock objects without extra code.

C Protocols prevent any code from being tested unless mocks are used.

D Protocols are only used to improve app performance, not for mocking.

💡 Hint

Think about how protocols help replace real objects with test doubles.

Protocols in Swift define required methods and properties. Mocks implement these protocols to stand in for real objects during tests, enabling controlled and isolated testing.

❓ ui_behavior

intermediate

Mock object behavior in a login test

Given a protocol Authenticator with a method login(username:password:), which mock implementation correctly simulates a successful login for testing a login screen UI?

iOS Swift

protocol Authenticator {
  func login(username: String, password: String) -> Bool
}

class MockAuthenticator: Authenticator {
  func login(username: String, password: String) -> Bool {
    // Which return value simulates success?
    return ???
  }
}

A return true

B return false

C return nil

D throw an error

💡 Hint

Success means the login method should indicate a positive result.

Returning true simulates a successful login, allowing the UI to proceed as if authentication succeeded.

❓ lifecycle

advanced

Mock object lifecycle in unit tests

In Swift unit tests, when using a mock object that conforms to a protocol, what is the best practice to ensure the mock's state resets between tests?

A Create a new mock instance in the <code>setUp()</code> method before each test runs.

B Reuse the same mock instance for all tests to save memory.

C Create the mock once in <code>init()</code> and never reset it.

D Use global variables to store mock state across tests.

💡 Hint

Think about test isolation and avoiding shared state.

Creating a fresh mock in setUp() ensures each test starts with a clean state, preventing tests from affecting each other.

📝 Syntax

advanced

Correct protocol mock syntax in Swift

Which of the following Swift code snippets correctly defines a mock class that conforms to the protocol DataFetcher with a method fetchData() -> String?

iOS Swift

protocol DataFetcher {
  func fetchData() -> String
}

A

class MockFetcher: DataFetcher {
  func fetchData() -> String {
    return "mock data"
  }
}

B

class MockFetcher: DataFetcher {
  func fetchData() {
    return "mock data"
  }
}

C

class MockFetcher: DataFetcher {
  func fetchData() -> Int {
    return 123
  }
}

D

class MockFetcher {
  func fetchData() -> String {
    return "mock data"
  }
}

💡 Hint

Check method signature matches protocol exactly.

Option A correctly implements the protocol method with matching return type. Others either have wrong return type, missing protocol conformance, or wrong method signature.

🔧 Debug

expert

Why does this mock cause a runtime error?

Consider this mock class in Swift: class MockService: ServiceProtocol { func fetchData() -> String { fatalError("Not implemented") } } Why will using this mock in tests cause a runtime crash?

A Because calling fetchData() triggers fatalError, which stops the program immediately.

B Because the mock does not conform to ServiceProtocol correctly.

C Because fetchData() returns the wrong data type.

D Because fatalError is only allowed in production code, not tests.

💡 Hint

What does fatalError() do when called?

fatalError() immediately stops program execution with a crash. Using it in a mock method causes tests to crash if that method is called.