Swap the Service

How do you photograph three backends without rebuilding the app?

Wire the screen to a protocol. Pass live, stub, or fail at launch. Keep a hardwired Live card as the control that ignores the argument.

Protocol first

protocol BuildStatusClient: Sendable {
  var sourceLabel: String { get }
  func fetchLatest() async -> BuildPulseOutcome
}

Three conformers. LiveBuildClient sleeps ~280 ms then returns Harbor. StubBuildClient returns the same READY instantly. FailingBuildClient returns NETWORK DOWN.

The injected card takes any BuildStatusClient from outside. That is initializer injection — the dependency arrives when the view is built, not from a singleton inside the body.

Live — both look the same

-mode live pins the client selector to LIVE.

Same numbers. Injection is invisible until you swap the client.

Stub — source changes, payload matches

-mode stub.

Hardwired still says SOURCE LIVE and ignores -mode. Injected flips to SOURCE STUB with the same Harbor pulse — a deterministic stand-in for tests and screenshots.

Fail — the frame that proves DI

-mode fail.

One screen. One launch argument. The control stays healthy while the injected path shows the outage. That is substitutability photographed — not argued.

Mini-exercise

Add a fourth client, SlowStubBuildClient, that sleeps two seconds then returns build 999. Pin -mode slow and confirm HARDWIRED still shows 418 while INJECTED shows 999.

Challenges

1. Move client resolution into a tiny BuildStatusFactory with one make(mode:) method — Design Patterns’ factory, kept local.
2. Unit-test FailingBuildClient without launching the simulator.
3. Property-inject a second optional logger after init; leave the required client on the initializer.
4. Replace the hardwired card with a Service Locator singleton and photograph why the control got harder to pin.
5. Wire XCUITest to assert NETWORK DOWN appears under -mode fail and never under -mode live.

Key Points

• Observed LIVE: both panels SOURCE LIVE, READY Harbor 418.
• Observed STUB: HARDWIRED LIVE / INJECTED STUB, both READY 418.
• Observed FAIL: HARDWIRED READY 418; INJECTED ERROR NETWORK DOWN.
• Protocol + initializer injection enables substitutability; hardcoding Live inside the view does not.
• Capture -mode is the same switch production tests can use.
• The control is the card that refuses to accept a client from outside.

Next up: architecture patterns — the same feature as MV, MVVM, and a reducer, with body-run counts on each so the comparison is measured.