IOS

iOS Native

Purpose

Build iOS native applications using Swift, SwiftUI/UIKit, MVVM+Coordinator architecture, Combine/async-await, Core Data, and XCTest.

Agent Protocol

Trigger

User request includes: ios, swift, swiftui, uikit, ios architecture, ios testing, ios native.

Input Context

• Xcode version
• Deployment target
• Swift version
• Architecture pattern (MVVM, TCA, VIPER)

Output Artifact

A markdown document containing:

• Project structure
• SwiftUI/UIKit setup
• MVVM+Coordinator pattern
• Combine/async-await concurrency
• Core Data / SwiftData setup
• XCTest test plan

Response Format

Produce the artifact directly. No preamble, no postamble, no explanations. No filler, no hedging, no transitions. Strip articles a/an/the where unambiguous. Compress output — why use many token when few do trick.

Max Response Length

4096 tokens

Workflow

Step 1: Set Up Project Structure

Organize code with feature-based folders: App, Features, Core (Network, Persistence, DI), and Common components.

Step 2: Implement MVVM with Coordinator

Create ViewModels with ObservableObject, Coordinators for navigation logic, and dependency injection container.

Step 3: Build UI with SwiftUI or UIKit

Use SwiftUI with @StateObject/@Published for reactive UI, or UIKit with programmatic view controllers and delegates.

Step 4: Set Up Persistence

Configure Core Data or SwiftData with NSPersistentContainer, context management, and save flows.

Step 5: Write Tests

Cover ViewModels with XCTest unit tests using mock services and UI flows with XCUITest.

Architecture Decision Trees

UI Framework Selection

Deployment target?
├── iOS 16+ → SwiftUI (native NavigationStack, SwiftData, Observable macros)
├── iOS 14-15 → SwiftUI with UIKit fallbacks for complex interactions
└── iOS 13- → UIKit (SwiftUI limited by deployment target requirement)
    
Team expertise?
├── New Swift team → SwiftUI (faster to build, less code)
├── Experienced UIKit team → UIKit for complex features, SwiftUI for simpler screens
└── Enterprise app → UIKit (proven, better accessibility, broader testing tools)

Architecture Pattern Selection

App complexity + team size?
├── Simple (<10 screens, CRUD) → MVVM
│   ViewModel @Observable, lightweight, quick to implement
├── Complex state management → TCA (The Composable Architecture)
│   Reducer pattern, testable, unidirectional, own your dependencies
├── Legacy codebase → VIPER
│   Strong separation, best for huge teams, more boilerplate
└── Multi-platform Swift → Clean Swift (VIP)
    Testable, clear boundaries, used with Swift on server too

Data Persistence Strategy

Data complexity?
├── Simple objects, light queries → SwiftData (iOS 17+)
│   Macro-based, Swift-native, auto-save, iCloud sync
├── Relational, existing Core Data → Core Data
│   Mature, complex queries, migration, iCloud, multi-threaded
├── Ephemeral, in-memory → UserDefaults + Codable
└── Full-text search, large datasets → GRDB (SQLite)
    Raw SQL control, migrations, combine publishers

Sync needed?
├── Yes → CloudKit + Core Data / SwiftData
├── No → Local persistence only
└── Real-time → Firebase Firestore

Concurrency Model

Task type?
├── One-shot network call → async/await (structured concurrency)
├── Stream of values → AsyncSequence (AsyncStream, AsyncAlgorithms)
├── Reactive chain → Combine (pre iOS 13-16, being superseded by Swift concurrency)
└── Long-running observation → AsyncStream + @Observable (iOS 17+)

Project Structure

App/
├── Sources/
│   ├── App/
│   │   └── App.swift
│   ├── Features/
│   │   ├── Orders/
│   │   │   ├── OrderListView.swift
│   │   │   ├── OrderDetailView.swift
│   │   │   ├── OrderViewModel.swift
│   │   │   └── OrderCoordinator.swift
│   │   └── ...
│   ├── Core/
│   │   ├── Network/
│   │   ├── Persistence/
│   │   └── DI/
│   └── Common/
│       ├── Extensions/
│       └── Components/
└── Tests/
    ├── UnitTests/
    └── UITests/

MVVM + Coordinator

// OrderCoordinator.swift
final class OrderCoordinator: Coordinator {
    var navigationController: UINavigationController
    private let di: DIContainer

    init(navigationController: UINavigationController, di: DIContainer) {
        self.navigationController = navigationController
        self.di = di
    }

    func start() {
        let vm = OrderViewModel(service: di.orderService)
        vm.onSelectOrder = { [weak self] order in
            self?.showDetail(order)
        }
        let vc = OrderListViewController(viewModel: vm)
        navigationController.pushViewController(vc, animated: false)
    }

    private func showDetail(_ order: Order) {
        let vm = OrderDetailViewModel(order: order)
        let vc = OrderDetailViewController(viewModel: vm)
        navigationController.pushViewController(vc, animated: true)
    }
}

SwiftUI + async-await

// OrderViewModel.swift
@MainActor
@Observable
final class OrderViewModel {
    var orders: [Order] = []
    var isLoading = false
    var errorMessage: String?
    private let service: OrderService

    init(service: OrderService) {
        self.service = service
    }

    func loadOrders() async {
        isLoading = true
        errorMessage = nil
        do {
            orders = try await service.fetchOrders()
        } catch {
            errorMessage = error.localizedDescription
        }
        isLoading = false
    }
}

// OrderListView.swift
struct OrderListView: View {
    @State private var viewModel: OrderViewModel

    var body: some View {
        Group {
            if viewModel.isLoading {
                ProgressView()
            } else if let error = viewModel.errorMessage {
                ContentUnavailableView("Error", systemImage: "exclamationmark.triangle",
                    description: Text(error))
            } else {
                List(viewModel.orders) { order in
                    Text(order.customerName)
                }
            }
        }
        .task { await viewModel.loadOrders() }
    }
}

UIKit Programmatic

final class OrderListViewController: UIViewController {
    private let viewModel: OrderViewModel
    private let tableView = UITableView()
    private var dataSource: UITableViewDiffableDataSource<String, Order>!

    init(viewModel: OrderViewModel) {
        self.viewModel = viewModel
        super.init(nibName: nil, bundle: nil)
    }

    override func viewDidLoad() {
        super.viewDidLoad()
        setupTableView()
        bindViewModel()
    }

    private func setupTableView() {
        view.addSubview(tableView)
        tableView.translatesAutoresizingMaskIntoConstraints = false
        NSLayoutConstraint.activate([
            tableView.topAnchor.constraint(equalTo: view.topAnchor),
            tableView.leadingAnchor.constraint(equalTo: view.leadingAnchor),
            tableView.trailingAnchor.constraint(equalTo: view.trailingAnchor),
            tableView.bottomAnchor.constraint(equalTo: view.bottomAnchor)
        ])
        tableView.register(UITableViewCell.self, forCellReuseIdentifier: "cell")
        dataSource = UITableViewDiffableDataSource(tableView: tableView) { tableView, indexPath, order in
            let cell = tableView.dequeueReusableCell(withIdentifier: "cell", for: indexPath)
            cell.textLabel?.text = order.customerName
            return cell
        }
    }

    private func bindViewModel() {
        Task { @MainActor in
            for await orders in viewModel.$orders.values {
                var snapshot = NSDiffableDataSourceSnapshot<String, Order>()
                snapshot.appendSections(["main"])
                snapshot.appendItems(orders)
                await dataSource.apply(snapshot)
            }
        }
    }
}

Core Data

// PersistenceController.swift
struct PersistenceController {
    static let shared = PersistenceController()
    let container: NSPersistentContainer

    init(inMemory: Bool = false) {
        container = NSPersistentContainer(name: "AppModel")
        if inMemory {
            container.persistentStoreDescriptions.first?.url = URL(fileURLWithPath: "/dev/null")
        }
        container.loadPersistentStores { _, error in
            if let error = error { fatalError("\(error)") }
        }
        container.viewContext.automaticallyMergesChangesFromParent = true
    }

    var context: NSManagedObjectContext { container.viewContext }

    func save() {
        if context.hasChanges {
            try? context.save()
        }
    }
}

// Background context for heavy operations
func performBackgroundTask(_ block: @escaping (NSManagedObjectContext) -> Void) {
    let context = container.newBackgroundContext()
    context.perform {
        block(context)
        try? context.save()
    }
}

Testing (XCTest)

import XCTest
@testable import App

final class OrderViewModelTests: XCTestCase {
    var sut: OrderViewModel!
    var mockService: MockOrderService!

    override func setUp() {
        mockService = MockOrderService()
        sut = OrderViewModel(service: mockService)
    }

    override func tearDown() {
        sut = nil
        mockService = nil
    }

    func testLoadOrders() async {
        mockService.stubOrders = [Order(id: "1", customerName: "Test")]
        await sut.loadOrders()
        XCTAssertEqual(sut.orders.count, 1)
        XCTAssertFalse(sut.isLoading)
    }

    func testLoadOrdersFailure() async {
        mockService.shouldFail = true
        await sut.loadOrders()
        XCTAssertTrue(sut.orders.isEmpty)
        XCTAssertNotNil(sut.errorMessage)
    }
}

Production Considerations

• Memory: Use Instruments Allocations and Leaks instrument each sprint
• Startup: Static framework reduces launch time vs dynamic frameworks
• Size: Asset catalog with Preserve Vector Data for PDF assets
• Crash-free rate target: >99.5% before App Store release
• Thread sanitizer: Enable in test scheme to catch data race bugs
• OSLog for structured logging — never print() in production
• MetricKit for production performance monitoring (hang rate, launch time, power)
• BackgroundTasks framework for deferrable background work (BGAppRefreshTask, BGProcessingTask)

SwiftUI Performance Patterns

• equatable() modifier for view equality checks
• LazyVStack inside ScrollView for large lists (not VStack)
• @ViewBuilder for conditional content instead of AnyView
• PreferenceKey for child-to-parent communication
• .drawingGroup() for Metal-backed offscreen rendering (complex shapes)
• id(_:) modifier for stable identity in animations and transitions
• @ObservedObject vs @StateObject — parent creates, child observes

UIKit Performance Patterns

• UITableViewDiffableDataSource for declarative, animated updates
• UICollectionViewCompositionalLayout for complex, responsive layouts
• Cell reuse identifiers must match registered cells — runtime crash otherwise
• Self-sizing cells with estimatedRowHeight and UITableView.automaticDimension
• prefetchDataSource for loading data before cells appear
• cellReuseIdentifier + prepareForReuse to reset cell state

Concurrency Safety

• @MainActor on all ViewModels and UI controllers
• Task { @MainActor in ... } for dispatching back to main thread
• AsyncStream for bridging callback-based APIs to async sequences
• withThrowingTaskGroup for parallel async operations (multiple API calls)
• Task lifetime tied to view lifecycle with .task modifier
• Cancellation: check Task.isCancelled in long-running loops
• await Task.yield() for cooperative cancellation points

Anti-Patterns

• Massive View Controller: UIKit MVC with business logic in view controllers — extract to ViewModel/Coordinator
• SwiftUI view with business logic: Keep logic in @Observable classes, not in View body
• Force-unwrapping optionals: Use guard let or optional chaining — never ! outside tests
• Shared mutable state: Prefer value types (structs) for model data, actors for shared mutable state
• NotificationCenter abuse: Use Combine publishers or async streams instead of NotificationCenter for app-internal events
• Retain cycles: Always [weak self] in escaping closures; use WeakReference<T> wrapper for Combine cancellables
• Over-reliance on @Published: Use @Observable macro (iOS 17+) — fewer view updates
• Singleton proliferation: Use DI container (Swinject, Factory) for service dependencies
• Long-running blocking tasks: Never block main thread — use Task.detached or DispatchQueue.global()

Rules

• Coordinators own all navigation logic — views never push or present directly
• ViewModels use @Published/@Observable properties and are marked @MainActor
• Core Data operations on background context — never block main thread
• Use async/await over Combine for simple one-shot network calls
• SwiftUI views should be lightweight — push business logic to ViewModels
• Avoid retain cycles: use [weak self] in escaping closures and Combine cancellables
• All network calls handle errors with do/catch and show user-facing error states
• Use UITableViewDiffableDataSource for UIKit table/collection views
• Prefer static frameworks over dynamic to reduce launch time
• MetricKit for production performance: hang rate, launch time, power consumption
• OSLog over print() — structured logging with privacy classification
• BackgroundTasks for deferrable work — not app delegate background fetch

References

• references/ios-advanced.md — Ios Advanced Topics
• references/ios-architecture.md — iOS Architecture — MVVM, Coordinator, Combine, Swift Concurrency
• references/ios-fundamentals.md — Ios Fundamentals
• references/swiftui-patterns.md — SwiftUI Patterns
• references/swiftui-performance.md — SwiftUI Performance Optimization
• references/testing.md — iOS Testing (XCTest)
• references/uikit-interop.md — SwiftUI / UIKit Interop

Handoff

Hand off to mobile/universal/deployment/SKILL.md for deployment.

Implementation Patterns

Observer Pattern for Event Handling

` interface EventObserver { onEvent(event: T): Promise; }

class EventBus { private observers: Set<EventObserver> = new Set(); subscribe(observer: EventObserver): void { this.observers.add(observer); } unsubscribe(observer: EventObserver): void { this.observers.delete(observer); } async emit(event: T): Promise { const results = Array.from(this.observers).map(o => o.onEvent(event)); await Promise.allSettled(results); } } `

Configuration-Driven Approach

config: defaults: timeout: 30s retryCount: 3 overrides: production: timeout: 60s retryCount: 5 development: timeout: 300s retryCount: 1

Production Considerations

Deployment Checklist

• Configuration validated against schema before startup
• Health check endpoints registered and monitored
• Graceful shutdown with draining period (30s timeout)
• Resource limits configured (CPU, memory, file descriptors)
• Log level set appropriate for environment
• Metrics endpoint secured and exposed
• Rate limiting configured per-tier
• TLS certificates valid and auto-renewing
• Database migrations run as separate deployment step
• Feature flags ready for gradual rollout

Monitoring and Alerting

Anti-Patterns

Performance Optimization

Caching Strategy

Cache hierarchy: L1 (in-memory local) → L2 (distributed Redis/Memcached) → L3 (CDN/Edge). Cache invalidation: TTL-based (simple, stale), event-based (complex, fresh), write-through (consistent, higher write latency), write-behind (fast writes, eventual consistency).

Resource Pooling

• Database connections: Pool of reusable connections (HikariCP, pgBouncer)
• HTTP connections: Keep-alive + connection pooling for external calls
• Thread pool: Bounded thread pools for async task execution

Profiling Methodology

1. Establish baseline with production traffic profile
2. Profile CPU with sampling profiler (pprof, perf, async-profiler)
3. Profile memory with heap dumps and allocation tracking
4. Profile I/O with strace/perf trace for syscall analysis
5. Profile latency with distributed tracing (OpenTelemetry)
6. Identify bottleneck, formulate hypothesis, implement fix
7. Re-profile to verify improvement, repeat

Security Considerations

Threat Modeling (STRIDE)

• Spoofing: Identity validation, authentication
• Tampering: Integrity checks, digital signatures
• Repudiation: Audit logs, non-repudiation
• Information disclosure: Encryption, access control
• Denial of service: Rate limiting, resource quotas
• Elevation of privilege: Principle of least privilege

Supply Chain Security

• Dependency scanning: Snyk, Dependabot, Trivy
• SBOM generation: CycloneDX or SPDX format
• Signed commits: GPG or SSH commit signing
• Artifact verification: Checksum validation, signature verification

Secrets Management

• Secrets never in code — always in secrets manager (Vault, AWS Secrets Manager)
• Rotation policy: Rotate database credentials every 90 days
• Access audit: Log every secrets access, alert on anomalies
• Encryption at rest and in transit for all secrets
• Principle of least privilege: each service gets only its own secrets

Rules

• Default-deny security posture — allow only explicitly required access.
• All inputs validated, all outputs encoded, all errors handled.
• Defend in depth — multiple layers of security controls.
• Fail securely — errors default to safe behavior.
• Log security-relevant events for audit and investigation.
• Keep dependencies updated — automate vulnerability scanning.
• Design for observability from day one, not as an afterthought.
• Document all architectural decisions with rationale.
• Review code for security, performance, and correctness before merging.