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.

Practice

1. What is the main purpose of using a mock in Swift testing?

easy

A. To record how a function was called during a test

B. To provide fixed data without recording calls

C. To replace the entire app with a test version

D. To speed up the app in production

Solution

1. Step 1: Understand what a mock does

   A mock is a test double that records how it was used, such as which methods were called and with what arguments.

2. Step 2: Compare with other test doubles

   Unlike stubs, mocks focus on interaction verification, not just providing fixed answers.

3. Final Answer:

   To record how a function was called during a test -> Option A

4. Quick Check:

   Mock = Records calls [OK]

Hint: Mocks remember calls; stubs just return data [OK]

Common Mistakes:

• Confusing mocks with stubs
• Thinking mocks speed up production
• Believing mocks replace the whole app

2. Which of the following is the correct way to define a stub method in Swift that always returns 42?

easy

A. func getNumber() -> Int { return 42 }

B. func getNumber() -> Int { print(42) }

C. func getNumber() { return 42 }

D. func getNumber() -> String { return "42" }

Solution

1. Step 1: Check method signature and return type

   The method must return an Int, so the signature should be func getNumber() -> Int.

2. Step 2: Verify the return statement

   The method should return the integer 42, so return 42 is correct.

3. Final Answer:

   func getNumber() -> Int { return 42 } -> Option A

4. Quick Check:

   Stub returns fixed value [OK]

Hint: Stub methods return fixed values with correct return type [OK]

Common Mistakes:

• Missing return type in function signature
• Returning wrong data type
• Using print instead of return

3. Consider this Swift mock class snippet:

class ServiceMock {
    var fetchCalled = false
    func fetchData() {
        fetchCalled = true
    }
}

let mock = ServiceMock()
mock.fetchData()
print(mock.fetchCalled)

What will be printed?

medium

A. false

B. true

C. nil

D. Compilation error

Solution

1. Step 1: Understand the mock class behavior

   The class has a boolean property fetchCalled initially false. The method fetchData() sets it to true.

2. Step 2: Trace the code execution

   After creating mock, calling mock.fetchData() sets fetchCalled to true. Printing it outputs true.

3. Final Answer:

   true -> Option B

4. Quick Check:

   Mock records call = true [OK]

Hint: Mock flags change after method call [OK]

Common Mistakes:

• Assuming fetchCalled stays false
• Expecting nil instead of Bool
• Thinking code won't compile

4. This Swift stub class is intended to return a fixed string but has a bug:

class DataStub {
    func getData() -> String {
        print("Fixed data")
    }
}

What is the error and how to fix it?

medium

A. print cannot be used in stubs; remove print

B. Wrong method name; rename to fetchData()

C. Return type should be Void; change signature

D. Missing return statement; add return "Fixed data"

Solution

1. Step 1: Identify the method signature and body

   The method declares it returns a String but only calls print without returning anything.

2. Step 2: Fix the missing return

   Replace print("Fixed data") with return "Fixed data" to satisfy the return type.

3. Final Answer:

   Missing return statement; add return "Fixed data" -> Option D

4. Quick Check:

   Stub must return value, not print [OK]

Hint: Return values, don't print in stubs [OK]

Common Mistakes:

• Confusing print with return
• Changing method name unnecessarily
• Changing return type incorrectly

5. You want to test a Swift class Downloader that calls a network service. To avoid real network calls, you create a mock service that records if download() was called and a stub that returns fixed data. Which approach correctly combines mocks and stubs for this test?

hard

A. Use only a mock that returns fixed data without recording calls

B. Use only a stub that records calls and returns data

C. Use a mock to record download() calls and a stub to return fixed data when fetchData() is called

D. Use real network service to ensure accurate testing

Solution

1. Step 1: Understand roles of mocks and stubs

   Mocks record interactions like method calls; stubs provide fixed return data.

2. Step 2: Apply correct combination

   Use a mock to check if download() was called, and a stub to supply fixed data for fetchData() to avoid real network calls.

3. Final Answer:

   Use a mock to record download() calls and a stub to return fixed data when fetchData() is called -> Option C

4. Quick Check:

   Mock records calls + Stub returns data [OK]

Hint: Mocks track calls; stubs provide data [OK]

Common Mistakes:

• Using only one test double type
• Relying on real network in tests
• Confusing mock and stub roles