Building Swift Apps

Building Swift/Xcode Apps

Xcode projects fail in ways a package-manager-driven language never does: the build graph lives in a hand-edited project.pbxproj, CI can't sign, and the IDE lints against a module graph it doesn't actually have. The traps below each come from a build that was green locally and broken in CI (or vice versa). The rule throughout: trust xcodebuild, not the IDE, not swift build, not a green make test that runs nothing.

CI can't sign — build unsigned

CI runners don't have the developer team's provisioning profiles, so a normal build dies with "requires a provisioning profile". The half-fix everyone tries first — ad-hoc signing — does not work when the app declares entitlements (CloudKit, App Groups, Keychain sharing): the profile-bound entitlement check still fires.

# DOES NOT fix it — the entitlement check still requires a profile:
CODE_SIGN_IDENTITY=- CODE_SIGNING_REQUIRED=NO

# The working recipe — skip signing AND the entitlement check entirely:
xcodebuild build CODE_SIGNING_ALLOWED=NO
xcodebuild test  CODE_SIGNING_ALLOWED=NO CODE_SIGNING_REQUIRED=NO CODE_SIGN_IDENTITY=

CODE_SIGNING_ALLOWED=NO skips both signing and the entitlement gate. On arm64 the linker still applies an implicit ad-hoc signature, so the host app is launchable and xcodebuild test can host and run the test bundle. Pass these flags through your Makefile (XCODE_FLAGS) so local and CI builds match.

Pin the runner and Xcode version to your local toolchain. Swift 6.2 concurrency semantics are rejected by older toolchains, so a project that builds locally fails on a default runner. Pin both explicitly:

runs-on: macos-26            # not macos-latest
- run: sudo xcode-select -s /Applications/Xcode_26.2.app

The pbxproj is hand-maintained — new files aren't in the build

Adding a .swift file to the folder on disk does not add it to the build. The file must be wired into project.pbxproj in three places: a PBXFileReference, the target's PBXSourcesBuildPhase, and a group. Until then the target fails to compile with Cannot find 'NewType' in scope at the use site — even though the file plainly exists. This often stays latent until CI builds the app target as a test host and surfaces it.

Rule: after adding any Swift file, build the app target (xcodebuild build), not just the file's own preview. Let Xcode add files (it edits the pbxproj) or verify the three entries by hand.

swift test vs xcodebuild test — mind the module boundary

A Package.swift target and an app target are different modules. Test code that does @testable import AppModule only sees symbols compiled into that SwiftPM target. If it references types that live in the app target (Library/… sources not in the package), swift build succeeds but swift test fails to compile with "cannot find X in scope".

• Put logic you want to unit-test in the SwiftPM library target, and test it there — that's what swift test can reach.
• App-target-only types (SwiftData @Models, SwiftUI views, managers wired to the app) are testable only via xcodebuild test against a hosted XCTest target. Don't document swift test as the test command if the tests need the app module.

"no testable scheme" — the test gate that runs nothing

A make test that advertises tests but has no XCTest target (or no shared scheme) either fails with "no testable scheme" or silently no-ops. A hosted test target needs: the XCTest target itself, TEST_HOST pointing at the app, and a shared scheme (checked into xcshareddata, or CI can't see it). Confirm the gate actually runs by asserting the test count in the log, not just exit 0.

SourceKit false positives — trust the build, not the squiggles

In a single-target Xcode project (not SPM), out-of-band SourceKit / IDE diagnostics lack the full module graph and report phantom errors like "Cannot find type 'HTTPServer' in scope" for symbols defined in a sibling file. These are not real — xcodebuild build is the source of truth. Don't chase them or "fix" them by moving code around; verify against a real build first.

Clean-checkout footgun: gitignored base xcconfig

If the project references a Config.xcconfig as a base configuration but that file is gitignored (only Config.xcconfig.example is tracked), a fresh clone fails immediately: "Unable to open base configuration reference file". Keep an .example with placeholder values and make the first build step copy it:

cp -n Config.xcconfig.example Config.xcconfig   # placeholders compile fine

Document this in the README/Makefile — it blocks the very first build, before any code runs.

Secrets: Keychain, not UserDefaults — and OAuth needs state/PKCE

OAuth access/refresh tokens, API keys, and passwords do not belong in UserDefaults — that's a plaintext plist in the app container. Refresh tokens are long-lived credentials; store them in the Keychain. Same discipline as any repo: a credential that reaches a user's disk (or git history) is compromised.

An OAuth flow needs a state parameter (and ideally PKCE) — a localhost callback that accepts any code with no state is a login-CSRF / code-injection surface. Generate a per-attempt state from cryptographic randomness, validate it single-use in the callback before exchanging the code:

var bytes = [UInt8](repeating: 0, count: 32)
_ = SecRandomCopyBytes(kSecRandomDefault, bytes.count, &bytes)
let state = Data(bytes).base64URLEncodedString()   // +→-, /→_, strip =
// ...append &state=\(state); on callback: guard returnedState == state else { reject }

Add PKCE (code_challenge/code_verifier) on top for public clients — a native app cannot keep a client secret. Use base64url (not standard base64) for any value placed in a URL query: +///= aren't URL-safe and + decodes back to a space, corrupting share links and state.

CloudKit + SwiftData: optional is deliberate, don't "tidy" it

CloudKit-synced SwiftData @Model types must have every stored property either optional or defaulted, and relationships optional — it's a hard CloudKit constraint, not sloppiness. Projects express this by keeping stored props optional and exposing non-optional resolvedX computed fallbacks:

@Model final class Idea {
    var title: String?                    // optional for CloudKit — required
    var resolvedTitle: String { title ?? "Untitled" }   // callers use this
}

Do not refactor these to non-optional to "clean them up" — it breaks CloudKit sync. When reviewing, recognize the pattern and leave it.

Codable-embedded value types need save migration. If a persisted model embeds whole structs via Codable (a saved game snapshotting a PlantType, say), later changing those static definitions leaves stale copies frozen in existing saves. Version the save format and migrate, or reference stable IDs instead of embedding the whole value.

A property default is not a decoding default. Swift's synthesized Codable decoder still calls decode(_:forKey:) for every non-optional stored property. If an older save lacks a newly added key, decoding throws keyNotFound; a load path such as try? decoder.decode(...) then silently treats the entire save as missing. Avoid adding non-optional persisted fields without a migration. Make the new field optional when absence is meaningful, or implement init(from:) with decodeIfPresent and an explicit fallback:

struct Save: Codable {
    var score: Int
    var seed: UInt64 = 0 // this initializer alone does not help decoding

    init(from decoder: Decoder) throws {
        let values = try decoder.container(keyedBy: CodingKeys.self)
        score = try values.decode(Int.self, forKey: .score)
        seed = try values.decodeIfPresent(UInt64.self, forKey: .seed) ?? 0
    }
}

Do not hide decode failures behind try?: distinguish “no save exists” from “the save exists but is incompatible or corrupt,” then migrate or report the failure rather than resetting user data.

Determinism: dates, formatters, and RNG

Fixed-format DateFormatter needs a POSIX locale and a time zone. Parsing yyyyMMdd without locale = Locale(identifier: "en_US_POSIX") misbehaves under non-Gregorian device calendars/locales (Apple's documented footgun). Set locale and timeZone on every fixed-format formatter.

Don't mix a seeded RNG with system randomness. .shuffled(), Array.randomElement(), and Date() inside logic make output nondeterministic, so tests get written loosely ("contains any of these words") and can't assert precise behavior. If reproducibility matters (game seeds, simulations), route all draws through the seeded generator; mixing in system randomness breaks it. Home-rolled LCGs that derive values via next() % N have poor low-bit distribution — don't rely on them for anything statistical.

Seed the run, not just the step. A seed derived only from a day, turn, or level makes every new run replay the same sequence. Give each run a persisted stable identity, then combine it with the step:

func stableSeed(runID: UUID, step: UInt64) -> UInt64 {
    let bytes = withUnsafeBytes(of: runID.uuid) { Array($0) }
    var seed: UInt64 = 0xcbf29ce484222325
    for byte in bytes {
        seed = (seed ^ UInt64(byte)) &* 0x100000001b3
    }
    return seed ^ (step &* 0x9e3779b97f4a7c15)
}

Never use runID.hashValue: Swift deliberately randomizes Hashable seeding per process, so the same persisted UUID can produce a different sequence after an app relaunch. Fold the UUID's bytes with an explicitly defined algorithm, as above, and keep that algorithm stable as part of the save format.

Use the resulting generator for every decision in that step—including nested effects—and pass it inout through helpers. Prefer a standard generator or a small generator with documented statistical properties; avoid mapping next() % upperBound, which both overuses weak low bits and introduces modulo bias when the generator's range is not divisible by upperBound.

Pin both sides of the contract in tests:

• The same run ID and step reproduce the exact full outcome.
• Different run IDs at the same step do not replay the same script.
• Encoding and decoding the run preserves subsequent outcomes.

Don't fabricate data on failure

A fetch path that, on any network error, silently substitutes randomized sample data makes the UI show fabricated numbers indistinguishable from real ones (only an errorMessage betrays it). Surface the failure to the user; never let a fallback masquerade as live data.

Checklist

• CI builds/tests unsigned with CODE_SIGNING_ALLOWED=NO (not ad-hoc)
• Runner OS + Xcode version pinned to the local toolchain (not -latest)
• Every new .swift file wired into the pbxproj; app target compiles
• Test command matches reality: swift test only if tests stay in the SPM module
• Test gate actually runs (hosted target + shared scheme; assert test count)
• Clean checkout builds (base xcconfig copied from .example)
• Tokens/keys in Keychain, not UserDefaults; OAuth uses state/PKCE; base64url in URLs
• CloudKit SwiftData props stay optional with resolved* fallbacks
• Save format versioned if it Codable-embeds value types
• New persisted fields decode older saves explicitly; decode failures are not hidden by try?
• Fixed-format DateFormatters set en_US_POSIX locale + timeZone
• Seed includes stable run identity plus step; never uses hashValue
• Every nested draw uses the seeded RNG; bounded draws avoid raw next() % N
• No silent sample-data fallback on fetch failure