Clean build folder (Cmd+Shift+K) fixes most "impossible" build errors — derived data gets corrupted regularly
Simulator reset (Device > Erase All Content and Settings) clears cached app state that survives reinstalls
Archive builds use Release config — bugs that only appear in production often stem from optimization differences
xcodebuild -showsdks lists available SDKs — useful when builds fail with "SDK not found"
Parallel builds can cause race conditions in script phases — add input/output file lists to enforce ordering
Code Signing
"No signing certificate" usually means certificate is expired or revoked — check in Keychain Access, not just Xcode
Provisioning profiles embed device UDIDs — new test device requires profile regeneration and reinstall
Automatic signing fails in CI — always use manual signing with exported credentials for builds
Distribution certificates are limited to 3 per account — don't create new ones, export and share the existing one
After renewing a certificate, update ALL provisioning profiles that used the old one
SwiftUI Patterns
@State for view-local data, @StateObject for owned ObservableObjects, @ObservedObject for passed-in objects — mixing them wrong causes crashes or lost state
List with id: \.self on non-Hashable types causes silent failures — always use explicit id parameter with stable identifiers
task modifier cancels automatically on view disappear — no manual cancellation needed, but check Task.isCancelled in loops
Previews crash silently with real network calls — use mock data or dependency injection for previews
@Environment values are nil in previews unless explicitly provided — wrap previews in container with environment set
App Store Rules
Apps must work offline or show clear offline state — silent failures cause rejection
Login must be skippable if app has non-account features — reviewers reject mandatory login for content browsing
"Sign in with Apple" is required if you offer any third-party social login — no exceptions
Privacy labels must match actual data collection — Apple verifies and rejects mismatches
In-app purchases must use StoreKit for digital goods — external payment links get rejected
Info.plist Requirements
ITSAppUsesNonExemptEncryption = NO avoids export compliance questions for most apps — add it to skip the daily annoyance
Camera/microphone/location usage descriptions are mandatory — missing them crashes the app on access attempt
LSApplicationQueriesSchemes must list URL schemes before canOpenURL works — iOS 9+ security requirement
UIRequiresFullScreen = YES on iPad opts out of multitasking — use only if your app truly can't support split view
Performance
Instruments > Time Profiler reveals actual bottlenecks — don't guess, measure
Images in Assets.xcassets get optimized automatically — loose files in bundle don't
@MainActor annotation ensures UI updates happen on main thread — missing it causes random crashes under load
Memory leaks often hide in closures — use [weak self] in escaping closures that reference self
List with thousands of items is fast, ForEach in ScrollView is not — List uses cell reuse, ScrollView loads everything
Debugging
po in LLDB prints object description, p prints raw value — use po for most debugging
Purple warnings in Console indicate main thread violations — fix these, they cause jank
-com.apple.CoreData.SQLDebug 1 in launch arguments shows all Core Data queries — essential for debugging fetch performance
Crash logs without symbols are useless — keep dSYM files for every release build
TestFlight crashes appear in Xcode Organizer — check there, not just in App Store Connect