xctest-patterns

# XCTest Patterns — Expert Decisions

Expert decision frameworks for testing choices. Claude knows XCTest syntax — this skill provides judgment calls for test strategy and mock design.

---

## Decision Trees

### Test Type Selection

```
What are you testing?
├─ Pure business logic (no I/O, no UI)
│  └─ Unit test
│     Fast, isolated, many of these
│
├─ Component interactions (services, repositories)
│  └─ Integration test
│     Test real interactions, fewer than unit
│
├─ User-visible behavior
│  └─ Does it require visual verification?
│     ├─ YES → Snapshot test or manual QA
│     └─ NO → UI test (XCUITest)
│        Slowest, fewest of these
│
└─ Performance characteristics
   └─ Performance test with measure {}
```

### Mock vs Stub vs Spy

```
What do you need from the test double?
├─ Just return canned data
│  └─ Stub
│     Simplest, no verification
│
├─ Verify interactions (was method called?)
│  └─ Spy
│     Records calls, verifiable
│
└─ Both return data AND verify calls
   └─ Mock (stub + spy)
      Most flexible, most complex
```

### When to Mock

```
Is this dependency...
├─ External (network, database, filesystem)?
│  └─ Always mock
│     Tests must be fast and deterministic
│
├─ Internal but slow or stateful?
│  └─ Mock if it makes test significantly faster
│
└─ Internal and fast?
   └─ Consider using real implementation
      Integration coverage > isolation purity
```

### Async Test Strategy

```
Is the async operation...
├─ Returning a value (async/await)?
│  └─ Use async test function
│     func testFetch() async throws { }
│
├─ Using completion handlers?
│  └─ Use XCTestExpectation
│     expectation.fulfill() in callback
│
└─ Publishing via Combine?
   └─ Use XCTestExpectation + sink
      Or use async-aware Combine helpers
```

---

## NEVER Do

### Test Design

**NEVER** test implementation details:
```swift
// ❌ Testing internal state
func testLogin() async {
    await sut.login(email: "test@example.com", password: "pass")

    XCTAssertEqual