# Flutter AI rules — language, style and architecture

Dart/Flutter style, package management, code quality, and how a project is laid out.

_Verbatim from [flutter/flutter docs/rules/rules.md](https://github.com/flutter/flutter/blob/main/docs/rules/rules.md), split across four files only to keep each under the repo's 250-line
cap. Re-fetch periodically to stay current; do not hand-edit._

### Interaction Guidelines
* **User Persona:** Assume the user is familiar with programming concepts but
  may be new to Dart.
* **Explanations:** When generating code, provide explanations for Dart-specific
  features like null safety, futures, and streams.
* **Clarification:** If a request is ambiguous, ask for clarification on the
  intended functionality and the target platform (e.g., command-line, web,
  server).
* **Dependencies:** When suggesting new dependencies from `pub.dev`, explain
  their benefits.
* **Formatting:** Use the `dart_format` tool to ensure consistent code
  formatting.
* **Fixes:** Use the `dart_fix` tool to automatically fix many common errors,
  and to help code conform to configured analysis options.
* **Linting:** Use the Dart linter with a recommended set of rules to catch
  common issues. Use the `analyze_files` tool to run the linter.

### Project Structure
* **Standard Structure:** Assumes a standard Flutter project structure with
  `lib/main.dart` as the primary application entry point.

### Flutter style guide
* **SOLID Principles:** Apply SOLID principles throughout the codebase.
* **Concise and Declarative:** Write concise, modern, technical Dart code.
  Prefer functional and declarative patterns.
* **Composition over Inheritance:** Favor composition for building complex
  widgets and logic.
* **Immutability:** Prefer immutable data structures. Widgets (especially
  `StatelessWidget`) should be immutable.
* **State Management:** Separate ephemeral state and app state. Use a state
  management solution for app state to handle the separation of concerns.
* **Widgets are for UI:** Everything in Flutter's UI is a widget. Compose
  complex UIs from smaller, reusable widgets.
* **Navigation:** Use a modern routing package like `auto_route` or `go_router`.
  For more guidelines around navigation, see the section on [routing](#routing).

### Package Management
* **Pub Tool:** To manage packages, use the `pub` tool, if available.
* **External Packages:** If a new feature requires an external package, use the
  `pub_dev_search` tool, if it is available. Otherwise, identify the most
  suitable and stable package from pub.dev.
* **Adding Dependencies:** To add a regular dependency, use the `pub` tool, if
  it is available. Otherwise, run `flutter pub add <package_name>`.
* **Adding Dev Dependencies:** To add a development dependency, use the `pub`
  tool, if it is available, with `dev:<package name>`. Otherwise, run `flutter
  pub add dev:<package_name>`.
* **Dependency Overrides:** To add a dependency override, use the `pub` tool, if
  it is available, with `override:<package name>:1.0.0`. Otherwise, run `flutter
  pub add override:<package_name>:1.0.0`.
* **Removing Dependencies:** To remove a dependency, use the `pub` tool, if it
  is available. Otherwise, run `dart pub remove <package_name>`.

### Code Quality
* **Code structure:** Adhere to maintainable code structure and separation of
  concerns (e.g., UI logic separate from business logic).
* **Naming conventions:** Avoid abbreviations and use meaningful, consistent,
  descriptive names for variables, functions, and classes.
* **Conciseness:** Write code that is as short as it can be while remaining
  clear.
* **Simplicity:** Write straightforward code. Code that is clever or
  obscure is difficult to maintain.
* **Error Handling:** Anticipate and handle potential errors. Don't let your
  code fail silently.
* **Styling:**
    * Line length: Lines should be 80 characters or fewer.
    * Use `PascalCase` for classes, `camelCase` for
      members/variables/functions/enums, and `snake_case` for files.
* **Functions:**
    * Keep functions short and with a single purpose.
      Strive for less than 20 lines.
* **Testing:** Write code with testing in mind. Use the `file`, `process`, and
  `platform` packages, if appropriate, so you can inject in-memory and fake
  versions of the objects.
* **Logging:** Use the `logging` package instead of `print`.

### Dart Best Practices
* **Effective Dart:** Follow the official Effective Dart guidelines
  (https://dart.dev/effective-dart)
* **Class Organization:** Define related classes within the same library file.
  For large libraries, export smaller, private libraries from a single top-level
  library.
* **Library Organization:** Group related libraries in the same folder.
* **API Documentation:** Add documentation comments to all public APIs,
  including classes, constructors, methods, and top-level functions.
* **Comments:** Write clear comments for complex or non-obvious code. Avoid
  over-commenting.
* **Trailing Comments:** Don't add trailing comments.
* **Async/Await:** Ensure proper use of `async`/`await` for asynchronous
  operations with robust error handling.
    * Use `Future`s, `async`, and `await` for asynchronous operations.
    * Use `Stream`s for sequences of asynchronous events.
* **Null Safety:** Write code that is soundly null-safe. Leverage Dart's null
  safety features. Avoid `!` unless the value is guaranteed to be non-null.
* **Pattern Matching:** Use pattern matching features where they simplify the
  code.
* **Records:** Use records to return multiple types in situations where defining
  an entire class is cumbersome.
* **Switch Statements:** Prefer using exhaustive `switch` statements or
  expressions, which don't require `break` statements.
* **Exception Handling:** Use `try-catch` blocks for handling exceptions, and
  use exceptions appropriate for the type of exception. Use custom exceptions
  for situations specific to your code.
* **Arrow Functions:** Use arrow syntax for simple one-line functions.

### Flutter Best Practices
* **Immutability:** Widgets (especially `StatelessWidget`) are immutable; when
  the UI needs to change, Flutter rebuilds the widget tree.
* **Composition:** Prefer composing smaller widgets over extending existing
  ones. Use this to avoid deep widget nesting.
* **Private Widgets:** Use small, private `Widget` classes instead of private
  helper methods that return a `Widget`.
* **Build Methods:** Break down large `build()` methods into smaller, reusable
  private Widget classes.
* **List Performance:** Use `ListView.builder` or `SliverList` for long lists to
  create lazy-loaded lists for performance.
* **Isolates:** Use `compute()` to run expensive calculations in a separate
  isolate to avoid blocking the UI thread, such as JSON parsing.
* **Const Constructors:** Use `const` constructors for widgets and in `build()`
  methods whenever possible to reduce rebuilds.
* **Build Method Performance:** Avoid performing expensive operations, like
  network calls or complex computations, directly within `build()` methods.

### API Design Principles
When building reusable APIs, such as a library, follow these principles.

* **Consider the User:** Design APIs from the perspective of the person who will
  be using them. The API should be intuitive and easy to use correctly.
* **Documentation is Essential:** Good documentation is a part of good API
  design. It should be clear, concise, and provide examples.

### Application Architecture
* **Separation of Concerns:** Aim for separation of concerns similar to MVC/MVVM, with defined Model,
  View, and ViewModel/Controller roles.
* **Logical Layers:** Organize the project into logical layers:
    * Presentation (widgets, screens)
    * Domain (business logic classes)
    * Data (model classes, API clients)
    * Core (shared classes, utilities, and extension types)
* **Feature-based Organization:** For larger projects, organize code by feature,
  where each feature has its own presentation, domain, and data subfolders. This
  improves navigability and scalability.

### Lint Rules

Include the package in the `analysis_options.yaml` file. Use the following
`analysis_options.yaml` file as a starting point:

```yaml
include: package:flutter_lints/flutter.yaml

linter:
  rules:
    # Add additional lint rules here:
    # avoid_print: false
    # prefer_single_quotes: true
```