Why do i need dirty and pure using formz package in Dart?

Question

I don't really get the concept of the formz package in dart.

According to https://pub.dev/packages/formz

import 'package:formz/formz.dart';

// Define input validation errors
enum NameInputError { empty }

// Extend FormzInput and provide the input type and error type.
class NameInput extends FormzInput<String, NameInputError> {
  // Call super.pure to represent an unmodified form input.
  const NameInput.pure() : super.pure('');

  // Call super.dirty to represent a modified form input.
  const NameInput.dirty({String value = ''}) : super.dirty(value);

  // Override validator to handle validating a given input value.
  @override
  NameInputError? validator(String value) {
    return value.isEmpty ? NameInputError.empty : null;
  }
}

There is a dirty and a pure representation of a form input, but why do i need this?

Isn't it just about validating the form input via the overwritten validator method? Therefore it would be enough to just have one constructor and run the validation?

Answer 1

I think i can answer it by myself. Dirty or clean form elements can be used as an indicator to decide for example if a save button is enabled or disabled in a form.

Imagine the following situation.

First Name -> John -> pure
Last Name -> Doe -> pure
Age -> 32 -> pure

And then check with

Formz.isPure([firstName, lastname, age])

will return true and the save button should be disabled

Now, the age is changed to 33

First Name -> John -> pure
Last Name -> Doe -> pure
Age -> 33 -> dirty

And

Formz.isPure([firstName, lastname, age])

will be false and the button should be enabled