Field with enum as value type

GermanState.php

<?php

/**
 * German state
 */
enum GermanState: string
{
    case BW = 'Baden-Württemberg';
    case BY = 'Bayern';
    case BE = 'Berlin';
    case BB = 'Brandenburg';
    case HB = 'Bremen';
    case HH = 'Hamburg';
    case HE = 'Hessen';
    case MV = 'Mecklenburg-Vorpommern';
    case NI = 'Niedersachsen';
    case NW = 'Nordrhein-Westfalen';
    case RP = 'Rheinland-Pfalz';
    case SL = 'Saarland';
    case SN = 'Sachsen';
    case ST = 'Sachsen-Anhalt';
    case SH = 'Schleswig-Holstein';
    case TH = 'Thüringen';

    /**
     * Gets state by it's ISO code
     *
     * @param string $isoCode
     * @return self|null
     */
    public static function tryFromIsoCode(string $isoCode): ?self
    {
        $enumFqn = sprintf('%s::%s', self::class, strtoupper($isoCode));

        return defined($enumFqn) ? constant($enumFqn) : null;
    }

    /**
     * Return all states as array
     *
     * @return string[]
     * @phpstan-return value-of<self>[]
     */
    public static function all(): array
    {
        return array_map(
            fn (self $case) => $case->value,
            self::cases(),
        );
    }

    /**
     * Return all states as options array
     *
     * @return array<int, array{value: string, label: value-of<self>}>
     */
    public static function options(): array
    {
        return array_map(
            fn (self $case) => [
                'value' => $case->name,
                'label' => $case->value,
            ],
            self::cases(),
        );
    }
}

GermanStateField.php

<?php

use Craft;
use craft\base\ElementInterface;
use craft\base\Field;
use craft\base\PreviewableFieldInterface;
use craft\base\SortableFieldInterface;
use craft\fields\conditions\OptionsFieldConditionRule;
use craft\helpers\Cp;

/**
 * Dropdown field with all German states
 */
class GermanStateField extends Field implements SortableFieldInterface, PreviewableFieldInterface
{
    /**
     * @inheritdoc
     */
    public static function displayName(): string
    {
        return Craft::t('site', 'German State');
    }

    /**
     * @inheritdoc
     */
    public static function valueType(): string
    {
        return GermanState::class . '|null';
    }

    /**
     * @inheritdoc
     */
    public function normalizeValue(mixed $value, ?ElementInterface $element = null): mixed
    {
        if ($value instanceof GermanState) {
            return $value;
        }

        if (is_string($value)) {
            return GermanState::tryFromIsoCode($value);
        }

        return null;
    }

    /**
     * @inheritdoc
     */
    public function serializeValue(mixed $value, ?ElementInterface $element = null): mixed
    {
        if ($value instanceof GermanState) {
            return $value->name;
        }

        return null;
    }

    /**
     * @inheritdoc
     */
    public function getTableAttributeHtml(mixed $value, ElementInterface $element): string
    {
        /** @var GermanState|null $value */
        return $value?->label ?? '';
    }

    /**
     * @inheritdoc
     */
    public function getElementConditionRuleType(): ?string
    {
        return OptionsFieldConditionRule::class;
    }

    /**
     * Returns all options
     *
     * @return array<int, array{label: string, value: string}>
     */
    public function getOptions(): array
    {
        return [['label' => '', 'value' => '']] + GermanState::options();
    }

    /**
     * @inheritdoc
     */
    protected function inputHtml(mixed $value, ?ElementInterface $element = null): string
    {
        /** @var GermanState|null $value */
        return Cp::selectHtml([
            'id' => $this->getInputId(),
            'name' => $this->handle,
            'describedBy' => $this->describedBy,
            'value' => $value?->name,
            'options' => $this->getOptions(),
            'required' => $this->required,
        ]);
    }
}