Issue #3091478 by Tim Bozeman, lauriii, malcomio, EclipseGc, larowlan,...

   */

  public static function generateSampleValue(FieldDefinitionInterface $field_definition) {

    $random = new Random();

    $values['value'] = $random->word(mt_rand(1, $field_definition->getSetting('max_length')));

    $max_length = $field_definition->getSetting('max_length');

    // When the maximum length is less than 15, generate a random word instead

    // of sentences.

    if ($max_length <= 15) {

      $values['value'] = ucfirst($random->word($max_length));

      return $values;

    }

    // The minimum length is either 10% of the maximum length, or 15 characters

    // long, whichever is greater.

    $min_length = max(ceil($max_length * 0.10), 15);

    // The random value is generated multiple times to create a slight

    // preference towards values that are closer to the minimum length of the

    // string. For values larger than 255 (which is the default maximum value),

    // the bias towards minimum length is increased. This is because the default

    // maximum length of 255 is often used for fields that include shorter

    // values (i.e. title).

    $length = mt_rand($min_length, mt_rand($min_length, $max_length >= 255 ? mt_rand($min_length, $max_length) : $max_length));

    $string = $random->sentences(1);

    while (mb_strlen($string) < $length) {

      $string .= " {$random->sentences(1)}";

    }

    // If the length of the generated string is longer than the maximum length,

    // re-create the string word by word to reduce the chances of the string

    // ending middle of a word.

    if (mb_strlen($string) > $max_length) {

      $words = explode(' ', $string);

      $string = array_reduce($words, function ($current, $item) use ($length) {

        return mb_strlen($current) > $length ? $current : "$current$item ";

      }, '');

    }

    // Normalize the string to be shorter than the maximum length.

    $normalized_value = rtrim(mb_substr($string, 0, $max_length - 1), ' .');

    // Ensure that the string ends with a full stop if there are multiple

    // sentences.

    $values['value'] = $normalized_value . (str_contains($normalized_value, '.') ? '.' : '');

    return $values;

  }

   */

  protected $defaultTheme = 'stark';

  /**

   * {@inheritdoc}

   */

  protected function getModifiedEntityForPostTesting(): array {

    $values = parent::getModifiedEntityForPostTesting();

    // The ID field for a workspace uses a string field, with a validation

    // constraint that applies a regex pattern that prevents whitespace.

    // \Drupal\Core\Field\Plugin\Field\FieldType\StringItem::generateSampleValue

    // used in the parent implementation of ::getModifiedEntityForPostTesting

    // can generate whitespace, so we use the same regex pattern here to ensure

    // the generated value is valid for the sake of the test.

    //

    // @see \Drupal\Core\Field\Plugin\Field\FieldType\StringItem::generateSampleValue

    // @see \Drupal\Tests\rest\Functional\EntityResource\EntityResourceTestBase::getModifiedEntityForPostTesting

    // @see \Drupal\workspaces\Entity\Workspace::baseFieldDefinitions

    $values['id'] = preg_replace('/[^a-z0-9_]/', '', $values['id']);

    return $values;

  }

}

<?php

namespace Drupal\Tests\Core\Field;

use Drupal\Core\Field\FieldDefinitionInterface;

use Drupal\Core\Field\Plugin\Field\FieldType\StringItem;

use Drupal\Tests\UnitTestCase;

/**

 * Defines a test for the StringItem field-type.

 *

 * @group Field

 * @coversDefaultClass \Drupal\Core\Field\Plugin\Field\FieldType\StringItem

 */

class StringItemTest extends UnitTestCase {

  /**

   * Tests generating sample values.

   *

   * @param int $max_length

   *   Maximum field length.

   *

   * @covers ::generateSampleValue

   * @dataProvider providerMaxLength

   */

  public function testGenerateSampleValue(int $max_length): void {

    $definition = $this->prophesize(FieldDefinitionInterface::class);

    $definition->getSetting('max_length')->willReturn($max_length);

    $sample_value = StringItem::generateSampleValue($definition->reveal());

    $this->assertLessThanOrEqual($max_length, mb_strlen($sample_value['value']));

  }

  /**

   * Data provider for maximum-lengths.

   *

   * @return array

   *   Test cases.

   */

  public function providerMaxLength(): array {

    return [

      '32' => [32],

      '255' => [255],

      '500' => [500],

      '15' => [15],

      '4' => [4],

      '64' => [64],

    ];

  }

}