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use eZ\Publish\Core\Persistence\Legacy\Content\StorageFieldValue;
use eZ\Publish\SPI\Persistence\Content\FieldValue;

// [...]

public function toStorageValue( FieldValue $value, StorageFieldValue $storageFieldValue )
{
   $storageFieldValue->dataText = $value->url>data;
   $storageFieldValue->sortKeyString = $value->sortKey;
}
public function toFieldValue( StorageFieldValue $value, FieldValue $fieldValue )
{
   $fieldValue->url>data = $value->dataText;
   $fieldValue->sortKey = $value->sortKeyString;
}