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

Need a 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

Need a 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

Need a 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

Need a hint?

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