Error Handling Patterns

Mandatory Rules

✅ ALWAYS handle errors explicitly - never silently swallow exceptions ✅ Use try-catch blocks for async operations that can fail ✅ Provide user-friendly error messages in UI ✅ Log errors with context using developer.log() ❌ NEVER use empty catch blocks without logging

Basic Pattern

dart

try {
  await operation();
} on SpecificException catch (e) {
  developer.log('Operation failed', error: e, name: 'ServiceName');
  // Handle specific error
} catch (e, st) {
  developer.log('Unexpected error', error: e, stackTrace: st, name: 'ServiceName');
  // Handle generic error
}

Async Operations

Always handle errors in async methods:

dart

Future<Result<String>> fetchData() async {
  try {
    final data = await repository.fetch();
    return Result.success(data);
  } on NetworkException catch (e) {
    developer.log('Network error', error: e, name: 'DataService');
    return Result.failure('Connection failed. Please check your internet.');
  } on ValidationException catch (e) {
    developer.log('Validation error', error: e, name: 'DataService');
    return Result.failure('Invalid data format.');
  } catch (e, st) {
    developer.log('Unexpected error', error: e, stackTrace: st, name: 'DataService');
    return Result.failure('Something went wrong. Please try again.');
  }
}

Service Error Handling

dart

class MyService {
  final AppLogger _logger = const AppLogger(defaultTag: 'MyService');
  
  Future<void> performOperation() async {
    try {
      await riskyOperation();
      _logger.info('Operation completed successfully');
    } on SpecificException catch (e) {
      _logger.error('Specific error occurred', error: e);
      throw ServiceException('User-friendly message');
    } catch (e, st) {
      _logger.error('Unexpected error', error: e, stackTrace: st);
      throw ServiceException('Something went wrong');
    }
  }
}

BLoC Error Handling

dart

Future<void> _onLoadData(
  LoadData event,
  Emitter<MyState> emit,
) async {
  emit(MyLoading());
  try {
    final data = await useCase.getData();
    emit(MyLoaded(data));
  } on NetworkException catch (e) {
    emit(MyError('Connection failed. Please check your internet.'));
  } catch (e, st) {
    developer.log('Unexpected error', error: e, stackTrace: st, name: 'MyBloc');
    emit(MyError('Something went wrong. Please try again.'));
  }
}

Result/Either Patterns

For operations that can fail, consider Result pattern:

dart

// Result pattern
sealed class Result<T> {
  const Result();
}

class Success<T> extends Result<T> {
  final T value;
  const Success(this.value);
}

class Failure<T> extends Result<T> {
  final String error;
  const Failure(this.error);
}

// Usage
Future<Result<User>> getUser(String id) async {
  try {
    final user = await repository.getUser(id);
    return Success(user);
  } catch (e) {
    return Failure('Failed to load user: ${e.toString()}');
  }
}

UI Error Display

Always show user-friendly messages:

dart

// In widget
if (state is MyError) {
  ScaffoldMessenger.of(context).showSnackBar(
    SnackBar(
      content: Text(state.message), // User-friendly message
      backgroundColor: AppColors.error,
    ),
  );
}

Anti-Patterns

❌ Silent Failure:

dart

try {
  await operation();
} catch (e) {
  // Silent failure - BAD!
}

❌ Generic Catch Only:

dart

try {
  await operation();
} catch (e) {
  // Missing specific exception handling - BAD!
}

❌ No Logging:

dart

try {
  await operation();
} catch (e) {
  showError('Error'); // Missing error logging - BAD!
}

Best Practices

•Catch specific exceptions first, then generic
•Always log errors with context and stack trace
•Provide user-friendly messages (hide technical details)
•Use Result/Either patterns for operations that can fail
•Don't swallow exceptions without handling or logging