Test doubles (mocks, stubs) in Swift - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a stubbed method call

What is the output of this Swift code using a stub to replace a method?

Swift

protocol DataService {
    func fetchData() -> String
}

class StubDataService: DataService {
    func fetchData() -> String {
        return "Stubbed Data"
    }
}

func printData(service: DataService) {
    print(service.fetchData())
}

let stub = StubDataService()
printData(service: stub)

A Stubbed Data

B fetchData()

C DataService

D Runtime error

💡 Hint

The stub replaces the real method with a fixed return value.

The stub class implements the protocol and returns a fixed string. When printData calls fetchData, it prints "Stubbed Data".

❓ Predict Output

intermediate

Mock method call count

What will be printed after running this Swift code that uses a mock to count method calls?

Swift

class MockLogger {
    var logCount = 0
    func log(_ message: String) {
        logCount += 1
        print("Log: \(message)")
    }
}

let mock = MockLogger()
mock.log("Start")
mock.log("Process")
print(mock.logCount)

A 2

B 0

C Compile error

D 1

💡 Hint

The mock increments logCount every time log() is called.

The log() method is called twice, so logCount becomes 2 and prints 2.

🔧 Debug

advanced

Why does this mock not record calls?

This Swift mock class is supposed to record calls to the method but fails. What is the cause?

Swift

class MockService {
    var callCount = 0
    func performAction() {
        var callCount = 0
        callCount += 1
    }
}

let mock = MockService()
mock.performAction()
mock.performAction()
print(mock.callCount)

A callCount is a constant and cannot be changed.

B The performAction method is never called.

C The callCount inside performAction is a new local variable, not the class property.

D print statement is outside the class and cannot access callCount.

💡 Hint

Check variable scope inside the method.

Inside performAction, a new local variable callCount is declared, shadowing the class property. So the class property never increments.

🧠 Conceptual

advanced

Purpose of a stub in testing

Which statement best describes the purpose of a stub in unit testing?

A A stub replaces the entire application logic with a fake implementation for integration testing.

B A stub provides canned responses to calls made during the test, isolating the tested code from external dependencies.

C A stub records every call made to it and verifies the order of calls.

D A stub automatically generates test data based on input parameters.

💡 Hint

Think about how stubs help isolate code under test.

Stubs provide fixed responses to isolate the tested code from real dependencies, making tests predictable and fast.

❓ Predict Output

expert

Output of a mock verifying call sequence

What is the output of this Swift code that uses a mock to verify the order of method calls?

Swift

class MockSequence {
    var calls: [String] = []
    func first() { calls.append("first") }
    func second() { calls.append("second") }
    func verify() -> Bool {
        return calls == ["first", "second"]
    }
}

let mock = MockSequence()
mock.second()
mock.first()
print(mock.verify())

A Runtime error

B true

C Compile error

D false

💡 Hint

Check the order in which methods are called and recorded.

The calls array records ["second", "first"], which does not match the expected ["first", "second"], so verify() returns false.