Test doubles (mocks, stubs) in Swift - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a test double in software testing?

A test double is a simple object that stands in for a real object during testing. It helps isolate the part of the code being tested by mimicking the behavior of real components.

intermediate

What is the difference between a mock and a stub?

A stub provides predefined responses to calls made during the test, while a mock also records how it was used, allowing verification of interactions.

beginner

Why use mocks in Swift unit tests?

Mocks help check if certain methods were called and with what data, making sure the tested code interacts correctly with other parts.

intermediate

How does a stub help in testing asynchronous code?

A stub can return fixed data immediately or after a delay, simulating asynchronous responses without real network calls.

beginner

Give an example of when to use a stub instead of a mock.

Use a stub when you only need to provide fixed data for a test, like returning a canned response from a database, without checking how it was called.

Which test double records how it was used during a test?

A Mock

B Stub

C Fake

D Spy

Mocks record interactions so you can verify if methods were called as expected.

What is the main purpose of a stub in testing?

A To verify method calls

B To generate random data

C To replace the entire system

D To provide canned responses

Stubs provide fixed responses to help tests run without real dependencies.

In Swift, which test double would you use to check if a function was called with specific arguments?

A Stub

B Mock

C Dummy

D Real object

Mocks allow verification of method calls and arguments.

Which of these is NOT a typical use of test doubles?

A Isolate the code under test

B Speed up tests by avoiding real dependencies

C Replace user interface elements

D Control test data responses

Test doubles replace code dependencies, not UI elements.

What is a key benefit of using test doubles in unit testing?

A They help test code in isolation

B They make tests slower

C They increase code complexity

D They replace production code permanently

Test doubles isolate the code under test by replacing dependencies.

Explain the roles of mocks and stubs in unit testing and when to use each.

Think about whether you need to verify interactions or just provide data.

Describe how test doubles help improve the quality and speed of Swift unit tests.

Consider what happens if you test with real network or database calls.

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