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iOS Swift Development

A senior iOS engineering skill that encodes deep expertise in building production-quality iOS applications with Swift. It covers the full iOS development spectrum - from SwiftUI declarative interfaces and UIKit imperative patterns to Core Data persistence, App Store submission compliance, and runtime performance optimization. The skill prioritizes modern Swift idioms (async/await, structured concurrency, property wrappers) while maintaining practical UIKit knowledge for legacy and hybrid codebases. Apple's platform is the foundation - lean on system frameworks before reaching for third-party dependencies.

When to use this skill

Trigger this skill when the user:

Asks to build, review, or debug SwiftUI views, modifiers, or navigation
Needs help with UIKit view controllers, Auto Layout, or table/collection views
Wants to design or query a Core Data model, handle migrations, or debug persistence
Asks about App Store Review Guidelines, metadata, or submission requirements
Needs to profile and fix memory leaks, rendering hitches, or energy usage
Is working with Swift concurrency (async/await, actors, TaskGroups) in an iOS context
Wants to implement animations, gestures, or custom drawing on iOS
Asks about integrating SwiftUI and UIKit in the same project

Do NOT trigger this skill for:

General Swift language questions with no iOS/Apple platform context
macOS-only, watchOS-only, or server-side Swift development

Key principles

Declarative first, imperative when necessary - Use SwiftUI for new screens and features. Fall back to UIKit only when SwiftUI lacks the capability (complex collection layouts, certain UIKit-only APIs) or when integrating into a legacy codebase. Mix via
```
UIHostingController
```
and
```
UIViewRepresentable
```
when needed.
The system is your design library - Use SF Symbols, system fonts (
```
.body
```
,
```
.title
```
), standard colors (
```
.primary
```
,
```
.secondary
```
), and built-in controls before custom implementations. System components get Dark Mode, Dynamic Type, and accessibility for free.
State drives the UI, not the other way around - In SwiftUI, the view is a function of state. Pick the right property wrapper (
```
@State
```
,
```
@Binding
```
,
```
@StateObject
```
,
```
@EnvironmentObject
```
,
```
@Observable
```
) based on ownership and scope. In UIKit, keep view controllers thin by moving state logic into separate models.
Measure with Instruments, not intuition - Use Xcode Instruments (Time Profiler, Allocations, Core Animation, Energy Log) before optimizing. Profile on real devices - Simulator performance is not representative. An unmeasured optimization is just added complexity.
Design for App Review from day one - Follow Apple's Human Interface Guidelines and App Store Review Guidelines throughout development, not as a last-minute checklist. Rejections cost weeks. Privacy declarations (App Tracking Transparency, purpose strings), in-app purchase rules, and content policies should be architecture decisions, not afterthoughts.

Core concepts

iOS development centers on four pillars: UI frameworks (SwiftUI and UIKit), data persistence (Core Data, SwiftData, UserDefaults), system integration (notifications, background tasks, permissions), and distribution (App Store submission, TestFlight, signing).

SwiftUI is Apple's declarative UI framework. Views are value types (structs) that declare what the UI looks like for a given state. The framework diffs the view tree and applies minimal updates. State management flows through property wrappers:

@State

for local,

@Binding

for child references,

@StateObject

@ObservedObject

for reference-type models, and

@Environment

for system-provided values. With the Observation framework (

@Observable

), SwiftUI tracks property access at the view level for fine-grained updates.

UIKit is the imperative predecessor - view controllers manage view lifecycles (

viewDidLoad

viewWillAppear

viewDidLayoutSubviews

), and Auto Layout constrains positions. UIKit remains essential for

UICollectionViewCompositionalLayout

, advanced text editing, and existing large codebases.

Core Data is Apple's object graph and persistence framework. It manages an in-memory object graph backed by SQLite (or other stores). The stack consists of

NSPersistentContainer

NSManagedObjectContext

NSManagedObject

. Contexts are not thread-safe - use

perform {}

blocks and separate contexts for background work.

App Store distribution requires provisioning profiles, code signing, metadata (screenshots, descriptions, privacy labels), and compliance with App Store Review Guidelines. TestFlight enables beta testing with up to 10,000 external testers.

Common tasks

1. Build a SwiftUI list with navigation

Create a list that navigates to a detail view. Use

NavigationStack

(iOS 16+) for type-safe, value-based navigation.

swift

struct ItemListView: View {
    @State private var items: [Item] = Item.samples
    @State private var path = NavigationPath()

    var body: some View {
        NavigationStack(path: $path) {
            List(items) { item in
                NavigationLink(value: item) {
                    ItemRow(item: item)
                }
            }
            .navigationTitle("Items")
            .navigationDestination(for: Item.self) { item in
                ItemDetailView(item: item)
            }
        }
    }
}

Avoid the deprecated
NavigationView
and
NavigationLink(destination:)
patterns in new code.
NavigationStack
supports programmatic navigation and deep linking.

2. Set up a Core Data stack with background saving

Initialize

NSPersistentContainer

and perform writes on a background context to keep the main thread responsive.

swift

class PersistenceController {
    static let shared = PersistenceController()
    let container: NSPersistentContainer

    init() {
        container = NSPersistentContainer(name: "Model")
        container.loadPersistentStores { _, error in
            if let error { fatalError("Core Data load failed: \(error)") }
        }
        container.viewContext.automaticallyMergesChangesFromParent = true
    }

    func save(block: @escaping (NSManagedObjectContext) -> Void) {
        let context = container.newBackgroundContext()
        context.perform {
            block(context)
            if context.hasChanges {
                try? context.save()
            }
        }
    }
}

Never perform writes on
viewContext
for large operations - it blocks the main thread. Always use
newBackgroundContext()
or
performBackgroundTask
.

3. Bridge SwiftUI and UIKit

Wrap a UIKit view for use in SwiftUI with

UIViewRepresentable

, or host SwiftUI inside UIKit with

UIHostingController

swift

// UIKit view in SwiftUI
struct MapViewWrapper: UIViewRepresentable {
    @Binding var region: MKCoordinateRegion

    func makeUIView(context: Context) -> MKMapView {
        let mapView = MKMapView()
        mapView.delegate = context.coordinator
        return mapView
    }

    func updateUIView(_ mapView: MKMapView, context: Context) {
        mapView.setRegion(region, animated: true)
    }

    func makeCoordinator() -> Coordinator { Coordinator(self) }

    class Coordinator: NSObject, MKMapViewDelegate {
        var parent: MapViewWrapper
        init(_ parent: MapViewWrapper) { self.parent = parent }
    }
}

swift

// SwiftUI view in UIKit
let hostingController = UIHostingController(rootView: MySwiftUIView())
navigationController?.pushViewController(hostingController, animated: true)

4. Profile and fix memory leaks

Use Instruments Allocations and Leaks to find retain cycles. The most common iOS memory leak is a strong reference cycle in closures.

Checklist:

Run the Leaks instrument on a real device while exercising the suspected screen
Check for closures capturing
```
self
```
strongly - use
```
[weak self]
```
in escaping closures
Verify delegates are declared
```
weak
```
(e.g.,
```
weak var delegate: MyDelegate?
```
)
Look for
```
NotificationCenter
```
observers not removed on
```
deinit
```
Check
```
Timer
```
instances -
```
Timer.scheduledTimer
```
retains its target
In SwiftUI, verify
```
@StateObject
```
is used for creation,
```
@ObservedObject
```
for injection

Use the Debug Memory Graph in Xcode (Runtime -> Debug Memory Graph) for a visual view of retain cycles without launching Instruments.

5. Handle App Store submission requirements

Prepare an app for App Store Review compliance.

Checklist:

Add all required
```
Info.plist
```
purpose strings for permissions (camera, location, photos, microphone, etc.)
Implement App Tracking Transparency (
```
ATTrackingManager.requestTrackingAuthorization
```
) before any tracking
Complete the App Privacy section in App Store Connect - declare all data collected
Use StoreKit 2 for in-app purchases; never process payments outside Apple's system for digital goods
Ensure login-based apps provide Sign in with Apple alongside other third-party login options
Provide a "Restore Purchases" button if the app offers non-consumable IAPs or subscriptions
Include a privacy policy URL accessible from both the app and App Store listing
Test on the minimum supported iOS version declared in your deployment target

Load
references/app-store-guidelines.md
for the full Review Guidelines checklist and common rejection reasons.

6. Optimize SwiftUI rendering performance

Reduce unnecessary view re-evaluations and layout passes.

Rules:

Mark view models with
```
@Observable
```
(iOS 17+) for fine-grained tracking instead of
```
ObservableObject
```
Extract expensive subviews into separate structs so SwiftUI can skip re-evaluation
Use
```
EquatableView
```
or conform views to
```
Equatable
```
to control diffing
Prefer
```
LazyVStack
```
/
```
LazyHStack
```
inside
```
ScrollView
```
for large lists
Avoid
```
.id()
```
modifier changes that destroy and recreate views
Use
```
task {}
```
instead of
```
onAppear
```
for async work - it cancels automatically

swift

// Bad: entire body re-evaluates when unrelated state changes
struct BadView: View {
    @ObservedObject var model: LargeModel
    var body: some View {
        VStack {
            Text(model.title)
            ExpensiveChart(data: model.chartData) // re-evaluated even if chartData unchanged
        }
    }
}

// Good: extracted subview only re-evaluates when its input changes
struct GoodView: View {
    @State var model = LargeModel() // @Observable macro
    var body: some View {
        VStack {
            Text(model.title)
            ChartView(data: model.chartData)
        }
    }
}

7. Implement structured concurrency for networking

Use Swift's async/await with proper task management for iOS networking.

swift

class ItemService {
    private let session: URLSession
    private let decoder = JSONDecoder()

    init(session: URLSession = .shared) {
        self.session = session
        decoder.keyDecodingStrategy = .convertFromSnakeCase
    }

    func fetchItems() async throws -> [Item] {
        let url = URL(string: "https://api.example.com/items")!
        let (data, response) = try await session.data(from: url)
        guard let httpResponse = response as? HTTPURLResponse,
              (200...299).contains(httpResponse.statusCode) else {
            throw APIError.invalidResponse
        }
        return try decoder.decode([Item].self, from: data)
    }
}

// In SwiftUI
struct ItemListView: View {
    @State private var items: [Item] = []

    var body: some View {
        List(items) { item in
            Text(item.name)
        }
        .task {
            do {
                items = try await ItemService().fetchItems()
            } catch {
                // handle error
            }
        }
    }
}

Use
.task {}
in SwiftUI - it runs when the view appears, cancels when it disappears, and restarts if the view identity changes. Never use
Task {}
inside
onAppear
without manual cancellation.

Anti-patterns / common mistakes

Mistake	Why it's wrong	What to do instead
Force unwrapping optionals	Crashes at runtime with no recovery path	Use `guard let` , `if let` , or nil-coalescing `??`
Writing to Core Data on the main context	Blocks the main thread during saves, causes UI hitches	Use `newBackgroundContext()` with `perform {}`
Massive view controllers	UIKit VCs with 1000+ lines become unmaintainable	Extract logic into view models, coordinators, or child VCs
Strong self in escaping closures	Creates retain cycles and memory leaks	Use `[weak self]` in escaping closures, `[unowned self]` only when lifetime is guaranteed
Ignoring the main actor	Updating UI from background threads causes undefined behavior	Use `@MainActor` annotation or `MainActor.run {}` for UI updates
Hardcoded strings and colors	Breaks localization and Dark Mode	Use `LocalizedStringKey` , asset catalog colors, and semantic system colors
Skipping `LazyVStack` for long lists	Eager `VStack` in `ScrollView` instantiates all views at once	Use `LazyVStack` or `List` for scrollable content with many items
Storing images in Core Data	Bloats the SQLite store, slows fetches	Store image data on disk, keep file paths in Core Data; use `allowsExternalBinaryDataStorage` for large blobs
Testing on Simulator only	Simulator does not reflect real device performance, memory, or thermal behavior	Always profile and test on physical devices before submission
Skipping privacy purpose strings	Automatic App Store rejection	Add `NSCameraUsageDescription` , `NSLocationWhenInUseUsageDescription` , etc. for every permission

Gotchas

@StateObject
vs
@ObservedObject
on the wrong owner causes views to reset - Using
```
@ObservedObject
```
to create a view model (instead of injecting one) means SwiftUI may recreate the object every time the parent view re-renders, destroying all state. Use
```
@StateObject
```
when the view owns the object's lifecycle; use
```
@ObservedObject
```
only when the object is injected from outside.
Core Data
NSManagedObjectContext
is not thread-safe and crashes are non-obvious - Accessing a managed object or its context from any thread other than the one it was created on causes data corruption or crashes that appear intermittent. Always use
```
context.perform {}
```
for background context work, and never pass
```
NSManagedObject
```
instances across threads - pass object IDs instead.
App Store rejection for missing purpose strings is instant and takes days to resolve - If your app accesses camera, photos, location, microphone, contacts, or any other private data without a corresponding
```
NS*UsageDescription
```
key in
```
Info.plist
```
, Apple rejects the binary automatically within hours of submission. Audit
```
Info.plist
```
against your permission calls before every submission, not just the first one.
NavigationView
is deprecated but mixing it with
NavigationStack
breaks navigation state - In Xcode projects with mixed iOS version support, using
```
NavigationView
```
on older iOS alongside
```
NavigationStack
```
on iOS 16+ causes navigation state corruption. Pick one per navigation hierarchy - use
```
NavigationStack
```
with availability checks for older OS rather than mixing both.
Storing large blobs in Core Data's SQLite store bloats the database and slows all fetches - SQLite stores all column data in the same file. Even one row with a 5MB image makes every fetch of that entity slow because SQLite reads past the image data. Store binary assets on disk via FileManager, keep only the file path in Core Data, and use
```
allowsExternalBinaryDataStorage
```
for smaller blobs that Apple should manage externally.

References

For detailed guidance on specific iOS topics, load the relevant reference file:

```
references/swiftui-patterns.md
```
- Navigation patterns, state management deep dive, custom modifiers, animations, and accessibility in SwiftUI
```
references/uikit-patterns.md
```
- View controller lifecycle, Auto Layout best practices, collection view compositional layouts, and coordinator pattern
```
references/core-data-guide.md
```
- Model design, relationships, fetch request optimization, migrations, and CloudKit sync
```
references/app-store-guidelines.md
```
- Review Guidelines checklist, common rejection reasons, privacy requirements, and in-app purchase rules
```
references/performance-tuning.md
```
- Instruments workflows, memory profiling, rendering optimization, energy efficiency, and launch time reduction

Only load a reference file when the current task requires that depth - they are detailed and will consume context.

Companion check

On first activation of this skill in a conversation: check which companion skills are installed by running
ls ~/.claude/skills/ ~/.agent/skills/ ~/.agents/skills/ .claude/skills/ .agent/skills/ .agents/skills/ 2>/dev/null
. Compare the results against the
recommended_skills
field in this file's frontmatter. For any that are missing, mention them once and offer to install:
npx skills add AbsolutelySkilled/AbsolutelySkilled --skill <name>
Skip entirely if
recommended_skills
is empty or all companions are already installed.

ios-swift

NPX Install

Tags

SKILL.md Content

iOS Swift Development

When to use this skill

Key principles

Core concepts

Common tasks

1. Build a SwiftUI list with navigation

2. Set up a Core Data stack with background saving

3. Bridge SwiftUI and UIKit

4. Profile and fix memory leaks

5. Handle App Store submission requirements

6. Optimize SwiftUI rendering performance

7. Implement structured concurrency for networking

Anti-patterns / common mistakes

Gotchas

References

Companion check