Add Error Display with isFailed()

This skill adds error state handling to widgets using context.isFailed() and context.getException().

What This Skill Does

Shows error UI in widgets by:

•Using context.isFailed(key) to check if an operation failed
•Using context.getException(key) to get the error message
•Displaying appropriate error UI with retry option
•Automatically clearing errors when operation succeeds

Instructions

Step 1: Ensure Cubit Uses mix()

The Cubit method must use mix() with a key:

dart

class UserCubit extends Cubit<User?> {
  void loadUser() => mix(
    key: this,
    () async {
      final user = await api.getUser();
      if (user == null) throw UserException('User not found');
      emit(user);
    },
  );
}

Step 2: Add isFailed() to Widget

Check context.isFailed(key) in the widget's build method:

dart

class UserScreen extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    // Check if failed
    if (context.isFailed(UserCubit)) {
      return Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: [
            Text('Error: ${context.getException(UserCubit)}'),
            ElevatedButton(
              onPressed: () => context.read<UserCubit>().loadUser(),
              child: const Text('Retry'),
            ),
          ],
        ),
      );
    }

    // Check if loading
    if (context.isWaiting(UserCubit)) {
      return const Center(child: CircularProgressIndicator());
    }

    // Show data
    final user = context.watch<UserCubit>().state;
    return Text('Hello, ${user?.name}');
  }
}

Context Extensions

Extension	Returns	Description
`context.isFailed(key)`	`bool`	True if operation threw an exception
`context.getException(key)`	`UserException?`	The exception that was thrown
`context.clearException(key)`	`void`	Manually clears the error state

Error UI Patterns

Full Screen Error

dart

Widget build(BuildContext context) {
  if (context.isFailed(UserCubit)) {
    return Scaffold(
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: [
            const Icon(Icons.error_outline, size: 64, color: Colors.red),
            const SizedBox(height: 16),
            Text(
              '${context.getException(UserCubit)}',
              textAlign: TextAlign.center,
            ),
            const SizedBox(height: 16),
            ElevatedButton(
              onPressed: () => context.read<UserCubit>().loadUser(),
              child: const Text('Try Again'),
            ),
          ],
        ),
      ),
    );
  }
  // ...
}

Inline Error Banner

dart

Widget build(BuildContext context) {
  return Column(
    children: [
      if (context.isFailed(UserCubit))
        MaterialBanner(
          content: Text('${context.getException(UserCubit)}'),
          actions: [
            TextButton(
              onPressed: () => context.read<UserCubit>().loadUser(),
              child: const Text('Retry'),
            ),
            TextButton(
              onPressed: () => context.clearException(UserCubit),
              child: const Text('Dismiss'),
            ),
          ],
        ),
      // Rest of content
      UserContent(),
    ],
  );
}

Error with Stale Data

Show error but keep displaying old data:

dart

Widget build(BuildContext context) {
  final state = context.watch<ProductCubit>().state;

  return Column(
    children: [
      // Show error banner if failed, but keep showing products
      if (context.isFailed(ProductCubit))
        Container(
          color: Colors.red.shade100,
          padding: const EdgeInsets.all(8),
          child: Row(
            children: [
              Expanded(
                child: Text('Failed to refresh: ${context.getException(ProductCubit)}'),
              ),
              TextButton(
                onPressed: () => context.read<ProductCubit>().loadProducts(),
                child: const Text('Retry'),
              ),
            ],
          ),
        ),

      // Show existing products even if refresh failed
      Expanded(
        child: ProductsList(products: state.products),
      ),
    ],
  );
}

Per-Item Error

dart

Widget build(BuildContext context) {
  return ListView.builder(
    itemCount: items.length,
    itemBuilder: (context, index) {
      final item = items[index];
      final key = (DeleteItem, item.id);

      // Check error for this specific item
      if (context.isFailed(key)) {
        return ListTile(
          title: Text(item.name),
          subtitle: Text(
            'Delete failed: ${context.getException(key)}',
            style: const TextStyle(color: Colors.red),
          ),
          trailing: IconButton(
            icon: const Icon(Icons.refresh),
            onPressed: () => cubit.deleteItem(item.id),
          ),
        );
      }

      return ListTile(
        title: Text(item.name),
        trailing: IconButton(
          icon: const Icon(Icons.delete),
          onPressed: () => cubit.deleteItem(item.id),
        ),
      );
    },
  );
}

Form Submission Error

dart

Widget build(BuildContext context) {
  return Form(
    child: Column(
      children: [
        // Form fields
        TextFormField(...),
        TextFormField(...),

        // Error message
        if (context.isFailed(SubmitForm))
          Padding(
            padding: const EdgeInsets.all(8),
            child: Text(
              '${context.getException(SubmitForm)}',
              style: const TextStyle(color: Colors.red),
            ),
          ),

        // Submit button
        ElevatedButton(
          onPressed: context.isWaiting(SubmitForm)
              ? null
              : () => context.read<FormCubit>().submit(formData),
          child: context.isWaiting(SubmitForm)
              ? const CircularProgressIndicator()
              : const Text('Submit'),
        ),
      ],
    ),
  );
}

Error Auto-Clearing

Errors are automatically cleared when:

•The operation runs again (and starts successfully)
•You call context.clearException(key)

dart

// Error cleared when loadUser() is called again
context.read<UserCubit>().loadUser();

// Manually clear error
context.clearException(UserCubit);

Complete Example

dart

class DataScreen extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: const Text('Data')),
      body: _buildBody(context),
    );
  }

  Widget _buildBody(BuildContext context) {
    // Error state - full screen
    if (context.isFailed(DataCubit)) {
      return _buildErrorView(context);
    }

    // Loading state
    if (context.isWaiting(DataCubit)) {
      return const Center(child: CircularProgressIndicator());
    }

    // Success state
    final data = context.watch<DataCubit>().state;
    return _buildDataView(context, data);
  }

  Widget _buildErrorView(BuildContext context) {
    return Center(
      child: Padding(
        padding: const EdgeInsets.all(24),
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: [
            const Icon(
              Icons.cloud_off,
              size: 64,
              color: Colors.grey,
            ),
            const SizedBox(height: 16),
            Text(
              '${context.getException(DataCubit)}',
              textAlign: TextAlign.center,
              style: Theme.of(context).textTheme.titleMedium,
            ),
            const SizedBox(height: 24),
            ElevatedButton.icon(
              onPressed: () => context.read<DataCubit>().loadData(),
              icon: const Icon(Icons.refresh),
              label: const Text('Try Again'),
            ),
          ],
        ),
      ),
    );
  }

  Widget _buildDataView(BuildContext context, DataState data) {
    return RefreshIndicator(
      onRefresh: () async {
        context.read<DataCubit>().loadData();
      },
      child: ListView.builder(
        itemCount: data.items.length,
        itemBuilder: (context, index) => ListTile(
          title: Text(data.items[index].name),
        ),
      ),
    );
  }
}

User Preferences

Ask the user:

•What key should be checked? (Cubit type, enum, string, or record)
•What error UI style? (full screen, banner, inline text)
•Should retry button be included?
•Keep showing stale data on error?