Testing iOS Apps (XCTest)

Let's think about it this way for a second

A unit test tests an individual function/class (ViewModel logic, data transformation) without needing UI/device — it uses the XCTest framework (built into Xcode), runs fast, and can run frequently in a CI pipeline. A UI test (XCUITest), on the other hand, automates actual UI interaction (button taps, text input) on the Simulator — you can use `XCUIApplication` to verify things like 'does the text change when I tap this button'.

Let's connect this to a real-world scenario

To unit test the logic of `TodoViewModel.addTodo()`, you could write `func testAddTodo_increasesListSize() { viewModel.addTodo(title: "Test"); XCTAssertEqual(viewModel.todos.count, 1) }` — for a UI test, you could write `app.buttons["Add"].tap(); XCTAssertTrue(app.staticTexts["Test"].exists)` to verify that the todo item actually shows up on screen after tapping the button.

• Unit Test = tests function/class logic without UI/device (XCTest, fast)
• UI Test (XCUITest) = automates actual UI interaction on the Simulator
• XCTAssertEqual/XCTAssertTrue = the core XCTest API for writing test assertions

Let's look at it together

// Unit test
import XCTest
@testable import MyApp

final class TodoViewModelTests: XCTestCase {
    func testAddTodo_increasesListSize() {
        let viewModel = TodoViewModel()
        viewModel.addTodo(title: "Buy milk")
        XCTAssertEqual(viewModel.todos.count, 1)
    }
}

// UI test
final class TodoAppUITests: XCTestCase {
    func testTappingAddButton_showsNewTodo() {
        let app = XCUIApplication()
        app.launch()
        app.buttons["Add"].tap()
        XCTAssertTrue(app.staticTexts["Buy milk"].exists)
    }
}

$ xcodebuild test
Test Suite 'All tests' passed — 2 tests, 0 failures