What is the difference between unit tests and UI tests?

Answer: They test different layers of the app and run at different speeds.

	Unit Tests	UI Tests
What	Isolated logic	Full app user flows
Speed	Very fast (ms)	Slow (seconds)
Framework	XCTest	XCUITest
Dependencies	Mocked	Real app running
Flakiness	Low	Higher

Code Example:

// Unit test — tests a function in isolation
class PriceFormatterTests: XCTestCase {
    func test_formatPrice_returnsCurrencyString() {
        let formatter = PriceFormatter()
        let result = formatter.format(9.99)
        XCTAssertEqual(result, "$9.99")
    }
}

// UI test — launches the app and interacts with it
class CheckoutUITests: XCTestCase {
    let app = XCUIApplication()

    override func setUp() { app.launch() }

    func test_addToCart_showsCartBadge() {
        app.buttons["Add to Cart"].tap()
        XCTAssertTrue(app.staticTexts["1"].exists)
    }
}