Flutter Bloc Forms

Form Architecture with BLoC

• Manage form state in a dedicated FormBloc — NOT in widget setState
• Each form field maps to a property in the BLoC state
• Validate on field change (real-time) or on submit (batch) depending on UX requirements
• Emit FormSubmitting, FormSuccess, FormError states for submission flow

Form Events

• FieldChanged(field, value) — update a single field in state
• FormSubmitted — trigger validation and submission
• FormReset — clear all fields and errors

Form State

• Use a single state class with all field values, field-level errors, and form status:

    sealed class FormStatus { initial, submitting, success, failure }

• Field errors: Map<String, String?> keyed by field name — null means valid

Validation Patterns

• Validate in the domain layer — NOT in widgets or BLoCs
• Create pure validator functions that return String? (null = valid, string = error message):

    String? validateEmail(String value) =>
      value.contains('@') ? null : 'Invalid email';

• Compose validators: String? validate(String v) => validateRequired(v) ?? validateEmail(v)
• Use localized error messages via context.l10n — no hardcoded validation strings

Input Widgets

• Use TextFormField with InputDecoration for consistent styling
• Always set textInputAction for proper keyboard behavior (next, done)
• Always set keyboardType matching the field type (emailAddress, phone, number)
• Use inputFormatters to restrict input (e.g., FilteringTextInputFormatter.digitsOnly)
• Assign Key('feature_fieldName') to every form field for test access
• Use AutofillHints for login/signup forms (email, password, name)
• Wrap form fields with Focus or FocusTraversalGroup for proper tab order

Controller Lifecycle

• Declare TextEditingController as late final in initState() — dispose in dispose()
• Sync controllers to BLoC via onChanged callback or controller listener

Form Submission

• Disable submit button while FormStatus.submitting to prevent double-submission
• Show inline field errors below each field — not just a top-level error
• On success: navigate, show success feedback, and reset form if staying on same page
• On failure: show error feedback via SnackBar or inline, keep form data intact

Common Form Patterns

• Search: Use debounce transformer on search events (300-500ms delay)
• Multi-step: Each step is a separate form state within one FormBloc, validated independently
• Dependent fields: Update dependent field options in on<FieldChanged> handler (e.g., country → city)