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name: flutter-error-handling description: Implement functional repository error recovery with Either/Failure patterns in Flutter. Use when handling exceptions or dartz Either types in data flows; defer UI-only retry buttons. metadata: triggers: files: - 'lib/domain/' - 'lib/infrastructure/' keywords: - Either - fold - Left - Right - Failure - dartz

Error Handling

Priority: P1 (HIGH)

Implementation Workflow

1. Define failures — Create domain-specific failures using @freezed unions (e.g., UnauthorizedFailure, OutOfStockFailure).
2. Return Either — Repositories return Either<Failure, T>. No exceptions in UI/BLoC.
3. Catch in Infrastructure only — Infrastructure catches exceptions (e.g., DioException) and returns Left(Failure). Never rethrow to UI.
4. Fold in BLoC — Use .fold(failure, success) in BLoC to emit corresponding states. Remove try/catch from BLoC.
5. Localize messages — Use failure.failureMessage (returns TRObject or localized string) for UI-safe text.
6. Log with stable templates — Use low-cardinality message templates; pass variable data via metadata/context.
7. No Silent Catch: Never swallow errors without logging or documented retry.
8. Crashlytics Routing: All UI/BLoC catch blocks MUST route errors via AppLogger.error(AppException.fromException(e).message, error: e, stackTrace: st) for observability and type-safe UI messages.

Repository & BLoC Examples

See implementation examples for repository error mapping and BLoC consumption patterns.

Reference & Examples

For Failure definitions and API error mapping: See references/REFERENCE.md.

Anti-Patterns

• No Try-Catch in BLoC: BLoC receives Either and folds; try/catch belongs in Infrastructure
• No Plain String Failures: Define typed @freezed Failure subclasses instead of Left('Something went wrong')
• No Empty Catch Blocks: Always log and propagate; never swallow errors silently
• No Repositories Throwing Status: Return Left(Failure) instead of throwing Exception
• No Missing Log Registration: Use AppLogger.error in BLoC/UI catch to ensure Crashlytics tracking and type-safe UI messages

Related Topics

layer-based-clean-architecture | bloc-state-management

Canonical response anchors

When this skill applies, preserve the following domain terminology or equivalent concrete examples in the answer when relevant:

• TRObject