Swift IOS

swift-ios

Hand-off — the language vs the platform. This skill owns the platform: SwiftUI/UIKit views, app lifecycle, Xcode, iOS APIs, App Store shipping. Language-level Swift — Swift 6 data-race safety and approachable concurrency, actors and task groups, some vs any, ARC and performance, Swift Testing, macros — is this skill's own ground. A concurrency error, a hang, a data race or a retain cycle belongs here. Interface feel, springs and materials: ../motion-craft/SKILL.md.

Native Apple-platform apps: the code is .swift, the UI is SwiftUI, and the compiler enforces actor isolation. If the answer is a @MainActor, @Observable, async let, #expect, NavigationStack, or @Model, you are in the right place.

First move: match the project before you write a line

Mismatched language modes produce diagnostics you cannot reproduce and "fixes" that break the user's build. Pin the ground truth first.

swift --version                               # exact toolchain
xcodebuild -version                           # Xcode version
# In the .xcodeproj / Package.swift, read two settings per target:
#   SWIFT_VERSION (Swift Language Version: 5 or 6)
#   SWIFT_DEFAULT_ACTOR_ISOLATION (Default Actor Isolation: nonisolated or MainActor)

Assumed baseline: Xcode 26 / Swift 6.2 / iOS 26 SDK (current stable seen is Xcode 26.2 / Swift 6.2.3). Do not quote a version you have not confirmed in the user's project. Rule: write for the target's actual language mode. Swift 5-mode code dropped into a Swift 6 target (or vice-versa) is the most common source of "it compiles on my machine" failures.

State & view architecture

For new code use @Observable (the Observation framework), never ObservableObject / @Published. Why: Observation tracks per-property — a view re-renders only when a property it actually reads changes, not on every object mutation.

@Observable gives you zero thread safety. Mutating an observed property off the main actor races the UI. Annotate UI models @MainActor.

// Bad — legacy pattern, whole-object invalidation, no isolation
final class CounterModel: ObservableObject {
    @Published var count = 0
}

// Good — per-property tracking, main-actor isolated
@MainActor @Observable
final class CounterModel {
    var count = 0
}

struct CounterView: View {
    @State private var model = CounterModel()      // view owns it
    var body: some View {
        Stepper("\(model.count)", value: $model.count)  // @State gives bindings to @Observable
    }
}

Concurrency correctness (the core)

The mental model: SwiftUI runs on the main actor. Anything that touches UI state is main-actor work. Background work hops off, then hops back to mutate.

• @MainActor class for UI/view-model state — it is the UI thread.
• actor for shared non-UI mutable state (a cache, a connection pool). Never model a SwiftUI view model as an actor — actors are reentrant and not main-bound, so your UI updates land off-main.
• Task { } inherits the current actor + priority — use it to fire async work from a Button. Task.detached inherits nothing and is almost never what you want; reaching for it to silence a warning is a bug, not a fix.
• async let / TaskGroup for structured parallelism (fan out, then await all).
• Cancellation is mandatory: check Task.isCancelled or try Task.checkCancellation() in loops. SwiftUI's .task {} auto-cancels when the view disappears — prefer it over a bare Task {} for view-lifetime work.

The #1 footgun — mutating observed UI state off the main actor:

// Bad — detached task mutates main-isolated state; intermittent glitches + Swift 6 data-race warning
@MainActor @Observable final class Feed {
    var items: [Item] = []
    func load() {
        Task.detached {
            let data = try? await API.fetch()
            self.items = data ?? []          // off-main mutation: WRONG
        }
    }
}

// Good — Task {} inherits the main actor; await off-main work, mutation lands on-main
@MainActor @Observable final class Feed {
    var items: [Item] = []
    func load() async {
        do {
            items = try await API.fetch()    // await suspends; assignment is back on main
        } catch is CancellationError {
            // view disappeared — nothing to do
        } catch {
            // surface error
        }
    }
}
// In the view: .task { await feed.load() }   // auto-cancels on disappear

Escape hatch for UI-centric apps: set Default Actor Isolation = MainActor (SE-0466) on the target — unannotated code becomes main-actor-isolated by default, killing false-positive data-race warnings; you opt out with nonisolated only where you genuinely need it. The full diagnostic catalog, Sendable rules, and the staged Swift 6 migration live in references/concurrency.md.

Persistence

import SwiftData

@Model final class Workout {
    var name: String
    var minutes: Int
    init(name: String, minutes: Int) { self.name = name; self.minutes = minutes }
}

// App entry: attach the container once
WindowGroup { ContentView() }.modelContainer(for: Workout.self)

// In a view: query + mutate
struct ListView: View {
    @Query(sort: \Workout.name) private var workouts: [Workout]
    @Environment(\.modelContext) private var context
    var body: some View {
        List(workouts) { Text($0.name) }
            .toolbar { Button("Add") { context.insert(Workout(name: "Run", minutes: 30)) } }
    }
}

Bad: hand-rolling JSON-to-disk for relational data with cross-references. Good: model the relationship with @Model and let SwiftData own identity and autosave.

Navigation

Use NavigationStack with a value-typed path + navigationDestination(for:) — type-safe, programmatic, and deep-linkable. NavigationView is deprecated.

// Bad
NavigationView { List(items) { NavigationLink(item.name, destination: DetailView(item: item)) } }

// Good — push by value, deep-link by mutating the path
@State private var path: [Item] = []
NavigationStack(path: $path) {
    List(items) { item in NavigationLink(item.name, value: item) }
        .navigationDestination(for: Item.self) { DetailView(item: $0) }
}
// Deep link: path = [parent, child]

Use NavigationSplitView for iPad/Mac multi-column layouts.

Testing

Swift Testing is the default (Xcode 16+, matured through Xcode 26): @Test, #expect (soft — keeps running), #require (hard — throws/unwraps), @Test(arguments:) for parameterized cases, @Suite types, parallel + in-process by default.

import Testing
@testable import MyApp

@Suite struct ScoreTests {
    @Test func startsAtZero() { #expect(Score().value == 0) }

    @Test(arguments: [1, 2, 3]) func adds(_ n: Int) {
        var s = Score(); s.add(n)
        #expect(s.value == n)
    }

    @Test func requiredUser() throws {
        let user = try #require(UserStore().current)   // hard stop if nil
        #expect(user.isActive)
    }
}

UI automation (XCUIApplication) and performance (XCTMetric) stay in XCTest — both frameworks coexist in one target. Run: xcodebuild test -scheme MyApp -destination 'platform=iOS Simulator,name=iPhone 17'. Depth (traits, confirmation for async events, in-memory ModelContainer for SwiftData, the XCTest migration table) in references/testing.md.

Build / run loop

xcodebuild -scheme MyApp -destination 'platform=iOS Simulator,name=iPhone 17' build
xcodebuild -scheme MyApp -destination 'platform=iOS Simulator,name=iPhone 17' test
xcrun simctl list devices                 # manage simulators
swift build && swift test                 # SwiftPM packages (libraries / modular targets)

App = .xcodeproj/.xcworkspace built with xcodebuild. Library or shared module = Package.swift built with swift build; use SwiftPM to pin dependency versions.

iOS 26 adoption (brief)

New surface: Liquid Glass via .glassEffect() (no UIVisualEffectView), the @Animatable macro for custom animatable shapes, native SwiftUI WebView, Chart3D, richer TabView roles. Adopt them behind if #available(iOS 26, *); do not casually raise the whole deployment target — most apps still support N-1/N-2.

Anti-patterns

Cross-references

• Native Android in Kotlin/Compose → ../kotlin-android/SKILL.md
• One codebase for iOS + Android via KMP/CMP → ../compose-multiplatform/SKILL.md
• Cross-platform Dart → ../flutter/SKILL.md
• React Native runtime / native modules → ../react-native/SKILL.md; the EAS/OTA shipping pipeline → ../expo/SKILL.md
• Store submission / release process → ../ship/SKILL.md
• Cloud CI build/sign and the non-Swift test stack are siblings (github-actions, testing-web) — see recommends.