Test doubles (mocks, stubs) in Swift

Introduction

Test doubles help us check parts of our code by pretending to be other parts. They make testing easier and safer.

When you want to test a function without calling a real database.

When you want to check if a function calls another function correctly.

When you want to simulate different responses from a service without using the real one.

When you want to isolate a part of your code to find bugs easily.

Syntax

Swift

protocol Service {
    func fetchData() -> String
}

class StubService: Service {
    func fetchData() -> String {
        return "Stub data"
    }
}

class MockService: Service {
    var fetchDataCalled = false
    func fetchData() -> String {
        fetchDataCalled = true
        return "Mock data"
    }
}

A stub provides fixed answers to calls.

A mock records if and how it was used.

Examples

This stub always returns the same data without doing anything else.

Swift

class StubService: Service {
    func fetchData() -> String {
        return "Fixed stub data"
    }
}

This mock remembers if fetchData() was called, so tests can check that.

Swift

class MockService: Service {
    var fetchDataCalled = false
    func fetchData() -> String {
        fetchDataCalled = true
        return "Mock data"
    }
}

Sample Program

This program shows how a stub returns fixed data and a mock tracks if its method was called.

Swift

protocol Service {
    func fetchData() -> String
}

class StubService: Service {
    func fetchData() -> String {
        return "Stub data"
    }
}

class MockService: Service {
    var fetchDataCalled = false
    func fetchData() -> String {
        fetchDataCalled = true
        return "Mock data"
    }
}

func testWithStub() {
    let service = StubService()
    let data = service.fetchData()
    print("Stub returned: \(data)")
}

func testWithMock() {
    let service = MockService()
    _ = service.fetchData()
    print("Mock fetchDataCalled: \(service.fetchDataCalled)")
}

testWithStub()
testWithMock()

Important Notes

Use stubs when you only need to provide simple data for tests.

Use mocks when you want to check if certain methods were called or how many times.

Test doubles help keep tests fast and independent from real services.

Summary

Test doubles replace real parts to make testing easier.

Stubs give fixed answers; mocks remember how they were used.

They help test code without relying on real external things.

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