class FrontMatter in Drupal 9

Component for parsing front matter from a source.

This component allows for an easy and convenient way to parse front matter from a source.

Front matter is used as a way to provide additional static data associated with a source without affecting the contents of the source. Typically this is used in templates to denote special handling or categorization.

Front matter must be the first thing in the source and must take the form of valid YAML set in between triple-hyphen lines:

source.md:

---
important: true
---
My content

example.php:

use Drupal\Component\FrontMatter\FrontMatter;

$frontMatter = FrontMatter::create(file_get_contents('source.md'));
$data = $frontMatter->getData(); // ['important' => TRUE]
$content = $frontMatter->getContent(); // 'My content'
$line => $frontMatter->getLine(); // 4, line where content actually starts.

Hierarchy

• class \Drupal\Component\FrontMatter\FrontMatter

Related topics

Utility classes and functions

Overview of utility classes and functions for developers.

4 files declare their use of FrontMatter

FrontMatterTest.php in core/tests/Drupal/Tests/Component/FrontMatter/FrontMatterTest.php

HelpTopicDiscovery.php in core/modules/help_topics/src/HelpTopicDiscovery.php

HelpTopicTwigLoader.php in core/modules/help_topics/src/HelpTopicTwigLoader.php

TwigEnvironment.php in core/lib/Drupal/Core/Template/TwigEnvironment.php

File

core/lib/Drupal/Component/FrontMatter/FrontMatter.php, line 43

Namespace

Drupal\Component\FrontMatter

class FrontMatter {

  /**
   * The separator used to indicate front matter data.
   *
   * @var string
   */
  const SEPARATOR = '---';

  /**
   * The regular expression used to extract the YAML front matter content.
   *
   * @var string
   */
  const REGEXP = '/\\A(' . self::SEPARATOR . '(.*?)?\\R' . self::SEPARATOR . ')(\\R.*)?\\Z/s';

  /**
   * The parsed source.
   *
   * @var array
   */
  protected $parsed;

  /**
   * A serializer.
   *
   * @var string
   */
  protected $serializer;

  /**
   * The source.
   *
   * @var string
   */
  protected $source;

  /**
   * FrontMatter constructor.
   *
   * @param string $source
   *   A string source.
   * @param string $serializer
   *   The name of a class that implements
   *   \Drupal\Component\Serialization\SerializationInterface.
   */
  public function __construct(string $source, string $serializer = '\\Drupal\\Component\\Serialization\\Yaml') {
    assert(is_subclass_of($serializer, SerializationInterface::class), sprintf('The $serializer parameter must reference a class that implements %s.', SerializationInterface::class));
    $this->serializer = $serializer;
    $this->source = $source;
  }

  /**
   * Creates a new FrontMatter instance.
   *
   * @param string $source
   *   A string source.
   * @param string $serializer
   *   The name of a class that implements
   *   \Drupal\Component\Serialization\SerializationInterface.
   *
   * @return static
   */
  public static function create(string $source, string $serializer = '\\Drupal\\Component\\Serialization\\Yaml') {
    return new static($source, $serializer);
  }

  /**
   * Parses the source.
   *
   * @return array
   *   An associative array containing:
   *   - content: The real content.
   *   - data: The front matter data extracted and decoded.
   *   - line: The line number where the real content starts.
   *
   * @throws \Drupal\Component\FrontMatter\Exception\FrontMatterParseException
   */
  protected function parse() : array {
    if (!$this->parsed) {
      $content = $this->source;
      $data = [];
      $line = 1;

      // Parse front matter data.
      if (preg_match(static::REGEXP, $content, $matches)) {

        // Extract the source content.
        $content = !empty($matches[3]) ? trim($matches[3]) : '';

        // Extract the front matter data and typecast to an array to ensure
        // top level scalars are in an array.
        $raw = !empty($matches[2]) ? trim($matches[2]) : '';
        if ($raw) {
          try {
            $data = (array) $this->serializer::decode($raw);
          } catch (InvalidDataTypeException $exception) {

            // Rethrow a specific front matter parse exception.
            throw new FrontMatterParseException($exception);
          }
        }

        // Determine the real source line by counting all newlines in the first
        // match (which includes the front matter separators) and append a new
        // line to denote that the content should start after it.
        if (!empty($matches[1])) {
          $line += preg_match_all('/\\R/', $matches[1] . "\n");
        }
      }

      // Set the parsed data.
      $this->parsed = [
        'content' => $content,
        'data' => $data,
        'line' => $line,
      ];
    }
    return $this->parsed;
  }

  /**
   * Retrieves the extracted source content.
   *
   * @return string
   *   The extracted source content.
   *
   * @throws \Drupal\Component\FrontMatter\Exception\FrontMatterParseException
   */
  public function getContent() : string {
    return $this
      ->parse()['content'];
  }

  /**
   * Retrieves the extracted front matter data.
   *
   * @return array
   *   The extracted front matter data.
   *
   * @throws \Drupal\Component\FrontMatter\Exception\FrontMatterParseException
   */
  public function getData() : array {
    return $this
      ->parse()['data'];
  }

  /**
   * Retrieves the line where the source content starts, after any data.
   *
   * @return int
   *   The source content line.
   *
   * @throws \Drupal\Component\FrontMatter\Exception\FrontMatterParseException
   */
  public function getLine() : int {
    return $this
      ->parse()['line'];
  }

}

Members

Name	Modifiers	Type	Description	Overrides
FrontMatter::$parsed	protected	property	The parsed source.
FrontMatter::$serializer	protected	property	A serializer.
FrontMatter::$source	protected	property	The source.
FrontMatter::create	public static	function	Creates a new FrontMatter instance.
FrontMatter::getContent	public	function	Retrieves the extracted source content.
FrontMatter::getData	public	function	Retrieves the extracted front matter data.
FrontMatter::getLine	public	function	Retrieves the line where the source content starts, after any data.
FrontMatter::parse	protected	function	Parses the source.
FrontMatter::REGEXP		constant	The regular expression used to extract the YAML front matter content.
FrontMatter::SEPARATOR		constant	The separator used to indicate front matter data.
FrontMatter::__construct	public	function	FrontMatter constructor.