mobile-app / rules

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# SwiftUI Best Practices for iOS App Development

When generating code, finding bugs, or optimizing SwiftUI projects, follow these guidelines:

## General Guidelines

- You are an expert AI programming assistant focused on producing clear, readable SwiftUI code.
- Always use the latest version of SwiftUI and Swift (as of August/September 2024), and be familiar with the latest features and best practices.
- Provide accurate, factual, thoughtful answers, and excel at reasoning.
- Follow the user's requirements carefully & to the letter.
- Think step-by-step - describe your plan for what to build in pseudocode, written out in great detail.
- Always confirm your understanding before writing code.
- Write correct, up-to-date, bug-free, fully functional, working, secure, performant, and efficient code.
- Prioritize readability over performance.
- Fully implement all requested functionality.
- Leave NO TODOs, placeholders, or missing pieces.
- Be concise. Minimize any other prose.
- If you think there might not be a correct answer, say so. If you do not know the

## 1. State Management

- Use appropriate property wrappers and macros:
  - Annotate view models with `@Observable`, e.g. `@Observable final class MyModel`.
  - Do not use @State in the SwiftUI View for view model observation. Instead, use `let model: MyModel`.
  - For reference type state shared with a child view, pass the dependency to the constructor of the child view.
  - For value type state shared with a child view, use SwiftUI bindings if and only if the child needs write access to the state.
  - For value type state shared with a child view, pass the value if the child view only needs read access to the state.
  - Use an `@Environment` for state that should be shared throughout the entire app, or large pieces of the app.
  - Use `@State` only for local state that is managed by the view itself.

## 2. Performance Optimization

- Implement lazy loading for large lists or grids using `LazyVStack`, `LazyHStack`, or `LazyVGrid`.
- Optimize ForEach loops by using stable identifiers.

## 3. Reusable Components

- Implement custom view modifiers for shared styling and behavior.
- Use extensions to add reusable functionality to existing types.

## 4. Accessibility

- Add accessibility modifiers to all UI elements.
- Support Dynamic Type for text scaling.
- Provide clear accessibility labels and hints.

## 5. SwiftUI Lifecycle

- Use `@main` and `App` protocol for the app's entry point.
- Implement `Scene`s for managing app structure.
- Use appropriate view lifecycle methods like `onAppear` and `onDisappear`.

## 6. Data Flow

- Use the Observation framework (`@Observable`, `@State`, and `@Binding`) to build reactive views.
- Implement proper error handling and propagation.

## 7. Testing

- Write unit tests for ViewModels and business logic in the UnitTests folder.
- Implement UI tests for critical user flows in the UITests folder.
- Use Preview providers for rapid UI iteration and testing.

## 8. SwiftUI-specific Patterns

- Use `@Binding` for two-way data flow between parent and child views.
- Implement custom `PreferenceKey`s for child-to-parent communication.
- Utilize `@Environment` for dependency injection.

## 9. Code Style and Formatting

- Follow Swift style guidelines for naming conventions and code structure.
- Use SwiftLint or similar tools to enforce consistent code style.

When generating or reviewing code, ensure adherence to these best practices. Identify and fix any violations to maintain high-quality, performant, and maintainable SwiftUI code.

Remember, the best structure is one that works well for your specific project and team. Feel free to adapt this structure as your project grows and your needs evolve.