Fields and the error type

Four ideas carry the whole package: a field holds one typed value, its errors are your type, its state is one snapshot, and a form is a tree of fields. This page covers the first two.

A field controller holds one typed value

AdvancedFieldController<T, E> holds a value of type T, runs validators that return errors of type E, and notifies its listeners when anything about it changes. It is a ChangeNotifier and a ValueListenable<AdvancedFieldState<T, E>>, nothing more exotic.

Four specialisations cover the usual inputs, and the base class is concrete, so a slider or a stepper can hold an int with no subclass at all:

Field controllers has each one's constructor and methods.

E is your error type

The second type argument is the type of the errors a field reports. It is bounded by Object, never nullable, because null already means "no error". Beyond that the package does not care:

// A String is fine on day one.
final name = AdvancedTextFieldController(validator: filled('Name is required'));

// An enum scales to translations…
enum SignupError { required, tooShort, taken }
final email = AdvancedTextFieldController(validator: filled(SignupError.required));

// …and a sealed class can carry data.
sealed class QuantityError {}
class TooMany extends QuantityError { TooMany(this.max); final int max; }

The package never formats a message. Widgets translate E to text where the wording belongs — often through an ErrorTranslator<E>, which is just String Function(E). One field can even report several violations at once by choosing E = List<PasswordRule>; Custom validators shows that.

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