ios-testing-and-benchmark

Mark test functions as `func testFoo() async throws` and annotate them `@MainActor` when they touch MainActor-isolated view models. Use `#expect` with Swift Testing on iOS 17+, or `XCTAssert*` for established suites, with one assertion focus per test.

Stub URLSession by subclassing URLProtocol and registering it via `URLSessionConfiguration.ephemeral.protocolClasses`. A static handler closure returns canned HTTPURLResponse and Data, giving deterministic, hermetic tests without hitting staging servers.

Swift Testing with `@Test` and `#expect` is preferred for new tests on iOS 17+, offering clearer failure output and async support. Legacy `XCTAssert*` remains fine for established XCTest suites that predate Swift Testing.

A mocked repository that returns the value you passed in proves nothing about SQLite behavior. An in-memory ModelContainer runs the real store, catching wrong column types, bad predicates, and broken migrations while staying fast.

Use XCTMetric subclasses in a `measure` block: XCTClockMetric for startup wall-clock time and XCTOSSignpostMetric.scrollDecelerationMetric for scroll performance. Add OS signposts in code so Instruments traces align with the measured intervals.

Reserve XCUITest for true cross-screen journeys like login to home to detail, since it is slow and flaky on animations. Cover view models and repositories in unit tests, and always query elements by accessibilityIdentifier, never coordinates.