# SwiftUI UI Patterns

## Quick start

Choose a track based on your goal:

### Existing project

- Identify the feature or screen and the primary interaction model (list, detail, editor, settings, tabbed).
- Find a nearby example in the repo with `rg "TabView\("` or similar, then read the closest SwiftUI view.
- Apply local conventions: prefer SwiftUI-native state, keep state local when possible, and use environment injection for shared dependencies.
- Choose the relevant component reference from `references/components-index.md` and follow its guidance.
- If the interaction reveals secondary content by dragging or scrolling the primary content away, read `references/scroll-reveal.md` before implementing gestures manually.
- Build the view with small, focused subviews and SwiftUI-native data flow.

### New project scaffolding

- Start with `references/app-wiring.md` to wire TabView + NavigationStack + sheets.
- Add a minimal `AppTab` and `RouterPath` based on the provided skeletons.
- Choose the next component reference based on the UI you need first (TabView, NavigationStack, Sheets).
- Expand the route and sheet enums as new screens are added.

## General rules to follow

- Use modern SwiftUI state (`@State`, `@Binding`, `@Observable`, `@Environment`) and avoid unnecessary view models.
- If the deployment target includes iOS 16 or earlier and cannot use the Observation API introduced in iOS 17, fall back to `ObservableObject` with `@StateObject` for root ownership, `@ObservedObject` for injected observation, and `@EnvironmentObject` only for truly shared app-level state.
- Prefer composition; keep views small and focused.
- Use async/await with `.task` and explicit loading/error states. For restart, cancellation, and debouncing guidance, read `references/async-state.md`.
- Keep shared app services in `@Environment`, but prefer explicit initializer injection for feature-local dependencies and models. For root wiring patterns, read `references/app-wiring.md`.
- Prefer the newest SwiftUI API that fits the deployment target and call out the minimum OS whenever a pattern depends on it.
- Maintain existing legacy patterns only when editing legacy files.
- Follow the project's formatter and style guide.
- **Sheets**: Prefer `.sheet(item:)` over `.sheet(isPresented:)` when state represents a selected model. Avoid `if let` inside a sheet body. Sheets should own their actions and call `dismiss()` internally instead of forwarding `onCancel`/`onConfirm` closures.
- **Scroll-driven reveals**: Prefer deriving a normalized progress value from scroll offset and driving the visual state from that single source of truth. Avoid parallel gesture state machines unless scroll alone cannot express the interaction.

## State ownership summary

Use the narrowest state tool that matches the ownership model:

| Scenario | Preferred pattern |
| --- | --- |
| Local UI state owned by one view | `@State` |
| Child mutates parent-owned value state | `@Binding` |
| Root-owned reference model on iOS 17+ | `@State` with an `@Observable` type |
| Child reads or mutates an injected `@Observable` model on iOS 17+ | Pass it explicitly as a stored property |
| Shared app service or configuration | `@Environment(Type.self)` |
| Legacy reference model on iOS 16 and earlier | `@StateObject` at the root, `@ObservedObject` when injected |

Choose the ownership location first, then pick the wrapper. Do not introduce a reference model when plain value state is enough.

## Cross-cutting references

- `references/navigationstack.md`: navigation ownership, per-tab history, and enum routing.
- `references/sheets.md`: centralized modal presentation and enum-driven sheets.
- `references/deeplinks.md`: URL handling and routing external links into app destinations.
- `references/app-wiring.md`: root dependency graph, environment usage, and app shell wiring.
- `references/async-state.md`: `.task`, `.task(id:)`, cancellation, debouncing, and async UI state.
- `references/previews.md`: `#Preview`, fixtures, mock environments, and isolated preview setup.
- `references/performance.md`: stable identity, observation scope, lazy containers, and render-cost guardrails.

## Anti-patterns

- Giant views that mix layout, business logic, networking, routing, and formatting in one file.
- Multiple boolean flags for mutually exclusive sheets, alerts, or navigation destinations.
- Live service calls directly inside `body`-driven code paths instead of view lifecycle hooks or injected models/services.
- Reaching for `AnyView` to work around type mismatches that should be solved with better composition.
- Defaulting every shared dependency to `@EnvironmentObject` or a global router without a clear ownership reason.

## Workflow for a new SwiftUI view

1. Define the view's state, ownership location, and minimum OS assumptions before writing UI code.
2. Identify which dependencies belong in `@Environment` and which should stay as explicit initializer inputs.
3. Sketch the view hie

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