Test doubles (mocks, stubs) in Swift - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a stub class that returns a fixed value.

Swift

class UserServiceStub: UserServiceProtocol {
    func fetchUser() -> User {
        return [1]
    }
}

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AUser(name: "TestUser")

BfetchUser()

CUserService()

DUser()

Attempts:

3 left

💡 Hint

Common Mistakes

Returning a method call instead of a User object.

Returning an empty User without parameters.

The stub returns a fixed User instance with name "TestUser" to simulate data.

2fill in blank

medium

Complete the code to verify the mock method was called once.

Swift

class UserServiceMock: UserServiceProtocol {
    var fetchUserCalled = false
    func fetchUser() -> User {
        fetchUserCalled = true
        return User(name: "MockUser")
    }
}

let mock = UserServiceMock()
_ = mock.fetchUser()
assert([1] == true)

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AfetchUserCalled

Bmock.fetchUser()

Cmock.fetchUserCalled

DUserServiceMock.fetchUserCalled

Attempts:

3 left

💡 Hint

Common Mistakes

Calling the method again inside assert.

Using class name instead of instance.

We check the mock's property fetchUserCalled to confirm the method was called.

3fill in blank

hard

Fix the error in the mock class to count how many times fetchUser is called.

Swift

class UserServiceMock: UserServiceProtocol {
    var fetchUserCallCount = 0
    func fetchUser() -> User {
        [1]
        return User(name: "MockUser")
    }
}

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AfetchUserCallCount--

BfetchUserCallCount = 1

CfetchUserCallCount =+ 1

DfetchUserCallCount += 1

Attempts:

3 left

💡 Hint

Common Mistakes

Using = 1 resets count every call.

Using -- decreases count incorrectly.

Using =+ 1 is a syntax error.

Increment the call count by 1 each time fetchUser is called using += 1.

4fill in blank

hard

Fill both blanks to create a stub that returns a user with a dynamic name and verify the name.

Swift

class UserServiceStub: UserServiceProtocol {
    var userName: String
    init(name: String) {
        self.userName = [1]
    }
    func fetchUser() -> User {
        return User(name: [2])
    }
}

let stub = UserServiceStub(name: "Alice")
let user = stub.fetchUser()
assert(user.name == "Alice")

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Aname

B"Alice"

CuserName

Dself.userName

Attempts:

3 left

💡 Hint

Common Mistakes

Using a fixed string instead of the parameter.

Returning the parameter directly instead of the property.

The initializer assigns the parameter 'name' to the property 'userName'. The fetchUser returns a User with 'self.userName'.

5fill in blank

hard

Fill all three blanks to create a mock that records calls and returns a stubbed user.

Swift

class UserServiceMock: UserServiceProtocol {
    var fetchUserCallCount = 0
    var stubUser: User
    init(stubUser: User) {
        [1] = stubUser
    }
    func fetchUser() -> User {
        [2]
        return [3]
    }
}

let stub = User(name: "Bob")
let mock = UserServiceMock(stubUser: stub)
_ = mock.fetchUser()
assert(mock.fetchUserCallCount == 1)
assert(mock.stubUser.name == "Bob")

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Aself.stubUser

BfetchUserCallCount += 1

CstubUser

Dself.fetchUserCallCount

Attempts:

3 left

💡 Hint

Common Mistakes

Assigning stubUser without self.

Returning stubUser without self.

Not incrementing call count.

The initializer assigns stubUser to self.stubUser. The fetchUser increments call count and returns self.stubUser.