Swiftui Patterns Agent Skill

SwiftUI Patterns

Modern SwiftUI patterns targeting iOS 26+ with Swift 6.3. Covers architecture, state management, view composition, environment wiring, async loading, design polish, and platform/share integration. Navigation, layout, animation, and Liquid Glass patterns live in dedicated sibling skills. Patterns are backward-compatible to iOS 17 unless noted.

Contents

• Architecture: Model-View (MV) Pattern
• Workflow
• State Management
• View Ordering Convention
• View Composition
• Environment
• Async Data Loading
• iOS 26+ New APIs
• Performance Guidelines
• HIG Alignment
• Writing Tools (iOS 18+)
• Common Mistakes
• Review Checklist
• References

Scope boundary: This skill covers architecture, state ownership, composition, environment wiring, async loading, and related SwiftUI app structure patterns. Detailed navigation patterns are covered in the swiftui-navigation skill, including NavigationStack, NavigationSplitView, sheets, tabs, and deep-linking patterns. Detailed layout, container, and component patterns are covered in the swiftui-layout-components skill, including stacks, grids, lists, scroll view patterns, forms, controls, search UI with .searchable, overlays, and related layout components. Detailed animation choreography is covered in swiftui-animation. Liquid Glass adoption, custom glass controls, scroll edge effects, .scrollEdgeEffectStyle, and .backgroundExtensionEffect are covered in swiftui-liquid-glass.

Workflow

• Record the current state ownership, actions, side effects, navigation, and lifecycle behavior.
• Choose the smallest MV/state/composition change that preserves that contract.
• Build after each structural step; fix compiler and isolation errors before continuing.
• Render deterministic previews for loaded, loading, empty, and error states as applicable, including required environment dependencies.
• Exercise important interactions and side effects. If behavior changes, restore the fixture, fix the smallest boundary, and rerun the same build, preview, and interaction checks.

Load Behavior-Preserving View Refactoring for restructuring existing views and Isolated Preview Construction for fixture and dependency patterns.

Architecture: Model-View (MV) Pattern

Default to MV -- views are lightweight state expressions; models and services own business logic. Do not introduce view models unless the existing code already uses them.

Core principles:

• Favor @State, @Environment, @Query, .task, and .onChange for orchestration
• Inject services and shared models via @Environment; keep views small and composable
• Split large views into smaller subviews rather than introducing a view model
• Test models, services, and business logic; keep views simple and declarative

struct FeedView: View {
    @Environment(FeedClient.self) private var client

    enum ViewState {
        case loading, error(String), loaded([Post])
    }

    @State private var viewState: ViewState = .loading

    var body: some View {
        List {
            switch viewState {
            case .loading:
                ProgressView()
            case .error(let message):
                ContentUnavailableView("Error", systemImage: "exclamationmark.triangle",
                                       description: Text(message))
            case .loaded(let posts):
                ForEach(posts) { post in
                    PostRow(post: post)
                }
            }
        }
        .task { await loadFeed() }
        .refreshable { await loadFeed() }
    }

    private func loadFeed() async {
        do {
            let posts = try await client.getFeed()
            viewState = .loaded(posts)
        } catch {
            viewState = .error(error.localizedDescription)
        }
    }
}

For MV pattern rationale, app wiring, and lightweight client examples, see references/architecture-patterns.md.

State Management

@Observable Ownership Rules

Important: Isolate UI-bound @Observable stores and view models on @MainActor when SwiftUI views own them, mutate them, or bind to their properties. Observation tracks changes; it does not make shared mutable state thread-safe. Domain models that do not touch UI state can use their own isolation strategy.

| Wrapper | When to Use |

|---------|-------------|

| @State | View owns the object or value. Creates and manages lifecycle. |

| let | View receives an @Observable object. Read-only observation -- no wrapper needed. |

| @Bindable | View receives an @Observable object and needs two-way bindings ($property). |

| @Environment(Type.self) | Access shared @Observable object from environment. |

| @State (value types) | View-local simple state: toggles, counters, text field values. Always private. |

| @Binding | Two-way connection to parent's @State or @Bindable property. |

Ownership Pattern

// UI-bound @Observable store -- main-actor isolated
@MainActor
@Observable final class ItemStore {
    var title = ""
    var items: [Item] = []
}

// View that OWNS the model
struct ParentView: View {
    @State private var viewModel = ItemStore()

    var body: some View {
        ChildView(store: viewModel)
            .environment(viewModel)
    }
}

// View that READS (no wrapper needed for @Observable)
struct ChildView: View {
    let store: ItemStore

    var body: some View { Text(store.title) }
}

// View that BINDS (needs two-way access)
struct EditView: View {
    @Bindable var store: ItemStore

    var body: some View {
        TextField("Title", text: $store.title)
    }
}

// View that reads from ENVIRONMENT
struct DeepView: View {
    @Environment(ItemStore.self) private var store

    var body: some View {
        @Bindable var s = store
        TextField("Title", text: $s.title)
    }
}

Granular tracking: SwiftUI only re-renders views that read properties that changed. If a view reads items but not isLoading, changing isLoading does not trigger a re-render. This is a major performance advantage over ObservableObject.

Legacy ObservableObject

Only use if supporting iOS 16 or earlier. @StateObject → @State, @ObservedObject → let, @EnvironmentObject → @Environment(Type.self).

View Ordering Convention

Order members top to bottom: 1) @Environment 2) let properties 3) @State / stored properties 4) computed var 5) init 6) body 7) view builders / helpers 8) async functions

View Composition

Extract Subviews

Break views into focused subviews. Each should have a single responsibility.

When restructuring an existing view, load Behavior-Preserving View Refactoring

for action/side-effect boundaries and build/preview proof.

var body: some View {
    VStack {
        HeaderSection(title: title, isPinned: isPinned)
        DetailsSection(details: details)
        ActionsSection(onSave: onSave, onCancel: onCancel)
    }
}

Computed View Properties

Keep computed some View properties for small, stateless fragments. Extract a section into a dedicated View type when it has any of these signals:

• meaningful branching or substantial layout
• its own state or async lifecycle
• narrower Observation dependencies than the parent
• a useful independent preview
• enough complexity to obscure the parent's data flow

When narrowing dependencies, pass only the values, bindings, and actions the child needs. If they form a large but cohesive interface, pass a feature-scoped @Observable model. Observation limits invalidation to properties the child reads, but an app-wide store still creates a broad interface; reserve it for children that genuinely need that cohesive state.

Reuse is a useful outcome, not a prerequisite for decomposition.

Extensions and // MARK: - organize a large file; they do not create view boundaries or replace extraction.

ViewBuilder Functions

For conditional logic that does not warrant a separate struct:

@ViewBuilder
private func statusBadge(for status: Status) -> some View {
    switch status {
    case .active: Text("Active").foregroundStyle(.green)
    case .inactive: Text("Inactive").foregroundStyle(.secondary)
    }
}

Custom View Modifiers

Extract repeated styling into ViewModifier:

struct CardStyle: ViewModifier {
    func body(content: Content) -> some View {
        content
            .padding()
            .background(.background)
            .clipShape(.rect(cornerRadius: 12))
            .shadow(radius: 2)
    }
}
extension View { func cardStyle() -> some View { modifier(CardStyle()) } }

Stable View Tree

Avoid top-level conditional view swapping. Prefer a single stable base view with conditions inside sections or modifiers.

When extracted views need independent state coverage, deterministic fixtures, or environment setup, load Isolated Preview Construction.

Environment

Custom Environment Values

Use @Entry for custom environment values and actions. It generates the entry boilerplate for EnvironmentValues.

extension EnvironmentValues {
    @Entry var theme: Theme = .default
    @Entry var refreshFeed: @Sendable () async -> Void = {}
}

// Usage
.environment(\.theme, customTheme)
.environment(\.refreshFeed) { await feedStore.refresh() }

@Environment(\.theme) private var theme
@Environment(\.refreshFeed) private var refreshFeed

For iOS 17-compatible code or older compatibility shims, use manual EnvironmentKey types instead.

Common Built-in Environment Values

@Environment(\.dismiss) var dismiss
@Environment(\.colorScheme) var colorScheme
@Environment(\.dynamicTypeSize) var dynamicTypeSize
@Environment(\.horizontalSizeClass) var sizeClass
@Environment(\.isSearching) var isSearching
@Environment(\.openURL) var openURL
@Environment(\.modelContext) var modelContext

Async Data Loading

Always use .task -- it cancels automatically on view disappear:

struct ItemListView: View {
    @State var store = ItemStore()

    var body: some View {
        List(store.items) { item in
            ItemRow(item: item)
        }
        .task { await store.load() }
        .refreshable { await store.refresh() }
    }
}

Use .task(id:) to re-run when a dependency changes:

.task(id: searchText) {
    guard !searchText.isEmpty else { return }
    await search(query: searchText)
}

Never create manual Task in onAppear unless you need to store a reference for cancellation. Exception: Task {} is acceptable in synchronous action closures (e.g., Button actions) for immediate state updates before async work.

Use swift-concurrency for cancellation handlers, debounce and clocks, AsyncSequence, or actor isolation.

iOS 26+ New APIs

Route .scrollEdgeEffectStyle, .backgroundExtensionEffect, and glass controls to swiftui-liquid-glass; route @Animatable to swiftui-animation. TextEditor(text: Binding<AttributedString>) is the iOS 26 rich-text editing path. Keep availability checks beside code that adopts these APIs.

Clipboard command modifiers are not iOS 26 defaults: .copyable, .cuttable, and command-based .pasteDestination(for:action:validator:) are macOS 13+ and iOS/iPadOS/Mac Catalyst 27 beta in current Apple docs. For iOS 26 targets, use UIPasteboard for custom clipboard commands, or use drag/drop and ShareLink for Transferable flows. See references/platform-and-sharing.md.

Performance Guidelines

• Lazy stacks/grids: Use LazyVStack, LazyHStack, LazyVGrid, LazyHGrid for large collections. Regular stacks render all children immediately.
• Stable IDs: All items in List/ForEach must conform to Identifiable with stable IDs. Never use array indices.
• Avoid body recomputation: Move filtering and sorting to computed properties or the model, not inline in body.
• Equatable views: For complex views that re-render unnecessarily, conform to Equatable.

HIG Alignment

Follow Apple Human Interface Guidelines for layout, typography, color, and accessibility. Key rules:

• Use semantic colors (Color.primary, .secondary, Color(uiColor: .systemBackground)) for automatic light/dark mode
• Use system font styles (.title, .headline, .body, .caption) for Dynamic Type support
• Use ContentUnavailableView for empty and error states
• Omit spacing: on stacks unless a specific value is required — nil (the default) uses platform-appropriate adaptive spacing
• Support adaptive layouts via horizontalSizeClass
• Provide VoiceOver labels (.accessibilityLabel) and support Dynamic Type accessibility sizes by switching layout orientation

See references/design-polish.md for HIG, theming, haptics, focus, transitions, and loading patterns.

Writing Tools (iOS 18+)

Control the Apple Intelligence Writing Tools experience on text views with .writingToolsBehavior(_:).

| Level | Effect | When to use |

|-------|--------|-------------|

| .complete | Full inline rewriting (proofread, rewrite, transform) | Notes, email, documents |

| .limited | Reduced overlay-panel experience | Code editors, validated forms |

| .disabled | Writing Tools hidden entirely | Passwords, search bars |

| .automatic | System chooses based on context (default) | Most views |

TextEditor(text: $body)
    .writingToolsBehavior(.complete)
TextField("Search…", text: $query)
    .writingToolsBehavior(.disabled)

Detecting active sessions: Read isWritingToolsActive on UITextView (UIKit) to defer validation or suspend undo grouping until a rewrite finishes.

> Docs: WritingToolsBehavior · writingToolsBehavior(_:))

Common Mistakes

• Using @ObservedObject to create objects -- use @StateObject (legacy) or @State (modern)
• Heavy computation in view body -- move to model or computed property
• Not using .task for async work -- manual Task in onAppear leaks if not cancelled
• Array indices as ForEach IDs -- causes incorrect diffing and UI bugs
• Forgetting @Bindable -- $property syntax on @Observable requires @Bindable
• Over-using @State -- only for view-local state; shared state belongs in @Observable
• Keeping complex or independently previewable sections computed -- extract View types; extensions and // MARK: only organize
• Using NavigationView -- deprecated; use NavigationStack
• Reaching for foregroundColor(_:) when foregroundStyle(_:) better matches semantic styling

10. Inline closures in body -- extract complex closures to methods

11. .sheet(isPresented:) when state represents a model -- use .sheet(item:) instead

12. Using AnyView for routine branching -- type erasure hides structure and can hurt performance or identity-sensitive transitions. Use @ViewBuilder, Group, or generics unless an API genuinely needs heterogeneous view storage. See references/deprecated-migration.md

13. Putting @AppStorage inside an @Observable class. @AppStorage is a view DynamicProperty; keep it in a View, or expose a normal observed property backed by UserDefaults in the model.

14. Hard-coding spacing: on every stack -- omit it to get adaptive platform spacing; only specify when the value is intentional

15. Treating .copyable, .cuttable, or command-based .pasteDestination(for:action:validator:) as iOS 16/iOS 26 APIs -- they are macOS 13+ and iOS/iPadOS/Mac Catalyst 27 beta in current Apple docs. Use UIPasteboard, drag/drop, or ShareLink for iOS 26 targets.

16. Treating modern defaults as formal deprecations -- #Preview is the modern preview default, but PreviewProvider is legacy rather than compiler-deprecated. EditButton, .onDelete, and .onMove remain valid for edit-mode list workflows; use .swipeActions for contextual row actions.

17. Making a required dependency optional to stop a preview crash -- install deterministic preview dependencies instead, without live networking, authentication, production databases, or global singletons

Review Checklist

• [ ] @Observable used for shared state models (not ObservableObject on iOS 17+)
• [ ] @State owns objects; let/@Bindable receives them
• [ ] Migration and availability claims checked for current platform support, especially clipboard and sharing APIs
• [ ] NavigationStack used (not NavigationView)
• [ ] .task modifier for async data loading
• [ ] LazyVStack/LazyHStack for large collections
• [ ] Stable Identifiable IDs (not array indices)
• [ ] Extraction uses branching/layout, lifecycle, dependency, preview, or parent-flow signals; small stateless fragments stay computed
• [ ] Extensions and // MARK: only organize files
• [ ] Structure-only refactors preserve behavior; use thin action/lifecycle methods, keep reusable logic in services/models, then build and render useful previews
• [ ] Previews cover meaningful loaded/loading/empty/error states with deterministic fixtures and every required environment dependency
• [ ] No heavy computation in view body
• [ ] Environment used for deeply shared state
• [ ] foregroundStyle(_:) used when semantic styling is preferable to a fixed color
• [ ] Custom ViewModifier for repeated styling
• [ ] .sheet(item:) preferred over .sheet(isPresented:)
• [ ] Sheets own their actions and call dismiss() internally
• [ ] MV pattern followed -- no unnecessary view models
• [ ] UI-bound @Observable stores and view models are @MainActor-isolated
• [ ] Model types passed across concurrency boundaries are Sendable
• [ ] Stack spacing: omitted unless a specific value is required (prefer adaptive default)

References

• Architecture, app wiring, and lightweight clients: references/architecture-patterns.md
• Design polish (HIG, theming, haptics, transitions, loading, focus): references/design-polish.md
• Deprecated API migration: references/deprecated-migration.md
• Platform and sharing patterns (Transferable, clipboard availability, media, menus, macOS settings): references/platform-and-sharing.md
• Isolated preview construction (state coverage, fixtures, and environment dependencies): references/preview-isolation.md
• Existing-view restructuring (behavior contract, action/side-effect boundaries, and verification): references/view-refactoring.md