Test doubles (mocks, stubs) in Swift - Mini Project: Build & Apply

Test doubles (mocks, stubs) in Swift

📖 Scenario: You are building a simple app that fetches user data from a server. To test your code without calling the real server, you will use test doubles called mocks and stubs.

🎯 Goal: Learn how to create and use test doubles (mocks and stubs) in Swift to test code that depends on external services.

📋 What You'll Learn

Create a protocol for user data fetching

Create a stub class that returns fixed user data

Create a mock class that records method calls

Write a test function that uses the stub and mock to verify behavior

💡 Why This Matters

🌍 Real World

Test doubles help developers test code that depends on external systems without needing the real systems available.

💼 Career

Understanding mocks and stubs is essential for writing reliable automated tests in software development jobs.

1

Create a protocol for user data fetching

Create a protocol called UserDataFetcher with a function fetchUser(id: Int) -> String that returns a String.

Swift

# Your code here

Hint

A protocol defines a blueprint of methods. Use protocol UserDataFetcher and declare the function inside.

2

Create a stub class that returns fixed user data

Create a class called UserDataStub that conforms to UserDataFetcher. Implement fetchUser(id: Int) to always return the string "Stub User".

Swift

protocol UserDataFetcher {
    func fetchUser(id: Int) -> String
}

# Your code here

Hint

Make a class that says it follows the protocol. Then write the function to return the fixed string.

3

Create a mock class that records method calls

Create a class called UserDataMock that conforms to UserDataFetcher. Add a variable calledWithId of type Int? to record the id passed. Implement fetchUser(id: Int) to save the id to calledWithId and return "Mock User".

Swift

protocol UserDataFetcher {
    func fetchUser(id: Int) -> String
}

class UserDataStub: UserDataFetcher {
    func fetchUser(id: Int) -> String {
        return "Stub User"
    }
}

# Your code here

Hint

Make a class that saves the id it receives and returns a fixed string.

4

Write a test function that uses the stub and mock

Write a function called testUserDataFetchers(). Inside, create an instance of UserDataStub and call fetchUser(id: 1). Then create an instance of UserDataMock, call fetchUser(id: 42), and print calledWithId from the mock.

Swift

protocol UserDataFetcher {
    func fetchUser(id: Int) -> String
}

class UserDataStub: UserDataFetcher {
    func fetchUser(id: Int) -> String {
        return "Stub User"
    }
}

class UserDataMock: UserDataFetcher {
    var calledWithId: Int? = nil
    func fetchUser(id: Int) -> String {
        calledWithId = id
        return "Mock User"
    }
}

# Your code here

Hint

Call the stub and mock, then print the mock's recorded id.

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