Flutter Presentation Layer

Flutter Presentation Layer (Clean Architecture)

Purpose: Provide a concise, opinionated set of conventions and quick prompts for implementing the presentation layer (UI + state) in a Flutter Clean Architecture project.

Principles

• Each UI page lives in its own folder so all page-specific artifacts are colocated.
• Each Cubit has a single responsibility: one cubit per action/intent (fetch, post, delete, etc.).
• Widgets live as close as possible to where they're used. Only genuinely shared components go into a global widgets folder.
• Keep build() methods clean: extract widget-building functions into private class methods or small private widgets.
• All user-facing strings must come from the localization system (ARB, S, AppLocalizations, etc.).
• For CRUD-like flows, split by intent explicitly: one cubit to list items, one to add/create item, one to delete item, and one to remove item from a local collection/state (if this action differs from backend delete).

Folder layout (recommended)

Use a feature-first layout. Example for a user feature with a list page:

• lib/features/user/presentation/user_list/
  • user_list_page.dart # page entry (Widget)
  • cubit/
    • user_list/
      • user_list_cubit.dart # cubit with single responsibility (e.g., fetch list)
      • user_list_state.dart # states for the cubit
  • widgets/ # page-scoped widgets used only by this page
    • user_row.dart
    • user_loading.dart

Shared UI components (only for true reuse)

• lib/shared/widgets/ # cross-feature shared widgets (buttons, inputs, avatars)
• lib/core/localization/ # localization ARB files and generated accessors

Cubit conventions

• Name: <feature>_<page>_<action>_cubit.dart OR for simpler cases <page>_cubit.dart with clearly separated methods. The important rule is one cubit per action/intent.
• Each cubit should:
  • Represent a single asynchronous intent (fetch, create, delete, update).
  • Expose a minimal API: one method to trigger the action and one public state stream.
  • Map domain errors to UI-friendly states. Do not embed domain logic—call usecases.

Examples:

• user_list_cubit — only responsible for fetching the list of users.
• user_create_cubit — only responsible for sending a create-user request and reporting success/failure.
• item_list_cubit — only responsible for listing items.
• item_add_cubit — only responsible for adding a new item.
• item_delete_cubit — only responsible for deleting an item in the backend/source of truth.
• item_remove_cubit — only responsible for removing an item from current UI state/local cache when modeled as a distinct action.

Practical SRP split for Cubits

When the page supports multiple intents, avoid a “god cubit”. Prefer one cubit per intent:

• ListItemsCubit: loads and refreshes item collections.
• AddItemCubit: handles create/add submission flow.
• DeleteItemCubit: executes delete use case against remote/local source of truth.
• RemoveItemCubit: removes an element from an in-memory list/UI state when this is a separate intent from delete.

This separation keeps each cubit focused, easier to test, and aligned with the Single Responsibility Principle (SRP).

Widget placement and composition

• Keep small, page-private widgets under the page's widgets/ folder.
• Only move a widget to lib/shared/widgets/ when at least two pages use it and its contract is stable.
• Prefer small focused widgets instead of large build methods with many nested conditionals.

Clean build() pattern

Bad:

class UserListPage extends StatelessWidget { @override Widget build(BuildContext context) { return Scaffold(body: ButtonPrimary(onPressed: () { // handle click }, ...)) } }

Good:

class UserListPage extends StatefulWidget { ... }

class _UserListPageState extends State { void _onClick() { // handle click }

@override Widget build(BuildContext context) => Scaffold(body: ButtonPrimary(onPressed: _onClick, ...)); }

• Extract callbacks private methods instead of inline functions inside build().
• Extract complex UI sections into small widgets to keep build() focused on composition.
• Avoid functions that return widgets with complex logic; prefer small widgets that can be tested in isolation.
• For conditional UI, consider using separate widgets for each state (loading, empty, data) instead of nested conditionals in build().

Strings & Localization

• All visible strings must come from the project's localization accessor (S.of(context), AppLocalizations.of(context), or the project's convention).
• Add keys to ARB files during scaffolding; include a comment describing the context for translators.
• Tests should verify that pages reference localization keys (not hardcoded strings).

Testing hints

• Write Cubit unit tests that assert state transitions for success/failure cases.
• Add widget tests that:
  • Pump the page with mocked cubits and verify correct localized text is displayed.
  • Verify that page-scoped widgets render expected states (loading, empty, data).

Quick prompts (examples you can paste into chat to generate scaffolding)

• "Scaffold a user_list page with a user_list_cubit that fetches users, a widgets/user_row.dart, and add localization keys user_list.title and user_list.empty."
• "Generate user_create_cubit that posts a new user and returns success/failure states; include unit tests for success and server-error mapping."

Checklist (pre-PR)

• Page folder exists and contains page.dart, cubits/, and widgets/ if needed.
• Cubit implements a single responsibility and is covered by unit tests.
• No hardcoded strings remain in the page; all visible text uses localization keys.
• Shared widgets only live in lib/shared/widgets/ and are used by at least two pages.
• Build methods are small; UI complexity is extracted to private methods or widgets.