Shipping Bug‑Free iOS Apps With a Lean SDET Strategy

An SDET is not “QA with a cooler title”.
An SDET owns the code that ships and the code that proves it works—continuously, deterministically, and cheaply.
This post dissects the exact tool‑chain I use in production.

1 · The Mobile‑First Test Pyramid

Key design decision: no business logic in the view layer.
Every tap routes through a single Swift reducer (handle(token:)).
Result: six‑second green bar for ~300 unit assertions.

2 · Unit Tests – Math > Pixels

mutating func handle(_ token: Token) {
    switch token {
    case .digit(let d):      accumulate(d)
    case .op(let op):        pushOperator(op)
    case .equals:            solve()
    case .percent:           percentify()
    case .sign:              toggleSign()
    case .backspace:         backspace()
    }
}

func test_order_of_operations() {
    var calc = Calculator()
    ["2", "+", "3", "×", "4", "="].map(Token.init).forEach(calc.handle)
    XCTAssertEqual(calc.display, "14")   // precedence preserved
}

• Arithmetic uses Decimal, not Double; no IEEE‑754 traps.
• justCalculated is a state flag, toggled strictly inside the reducer—never from SwiftUI gestures.

3 · UI Automation – Python Loves Swift

3.1 Session‑Scoped Driver

@pytest.fixture(scope="session")
def driver():
    caps = {
        "platformName": "iOS",
        "automationName": "XCUITest",
        "deviceName": "iPhone 15 Pro Max",
        "platformVersion": "17.5",
        "app": BUILD_ARTIFACT,
        "newCommandTimeout": 300
    }
    drv = webdriver.Remote("http://127.0.0.1:4723", caps)
    yield drv
    drv.quit()

Reuse the same simulator session for the whole suite—saves ~25 s on a 50‑case run.

3.2 Data‑Driven Scenarios

@pytest.mark.parametrize("seq, expected", [
    (["5","0","%","+","2","5","%","="], "0.75"),
    (["8","8","+","⌫","⌫","⌫","⌫"],     "0"),        # brutal backspaces
])
def test_sequences(driver, seq, expected):
    clear_all(driver)
    for key in seq: tap(driver, key)
    assert display(driver) == expected

One additional edge case = one extra tuple. 92 E2E cases, still readable.

4 · Accessibility IDs – The Contract You Forgot to Write

Text(expression.isEmpty ? display
     : "\(expression) \(display)")
    .accessibilityIdentifier("Display")

• Testing – Appium anchors on "Display", zero XPath.
• Accessibility – VoiceOver inherits the same identifier.
• Refactors – swap the whole UI, tests stay green.

5 · CI – 35 s Pull Request, 150 s Nightly

Flaky spec? CI quarantines after three consecutive reds—mean‑time‑to‑green < 24 h.

6 · Mutation Testing – Trust, but Verify

Running Stryker‑SwiftMutation on reducers surfaces false‑positive unit tests—the ones that never fail because they never assert:

• killed mutants ≥ 85 % ⇒ tests are honestly exercising logic
• each surviving mutant becomes a new TDD card in Jira

7 · Portable Playbooks

Same mental model, three stacks—on‑boarding a new SDET takes one sprint, not a quarter.

8 · Outcome Snapshot (Solo Implementation)

9 · What’s Next

1. Analytics contract tests – every UI event validated against Amplitude schema.
2. On‑device fuzzing – random rotations / locale switches via XCTest.
3. Real‑device farm – iOS 17 + 18 smoke on physical hardware.