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Writing custom fields in Drupal 8 – Part 2

Continuing from Evan’s blog post on building pages with Paragraphs and writing custom blocks of content as fields, I will walk you through how to create a custom field-formatter in Drupal 8 by example.

field-formatter is the last piece of code to go with the field-type and the field-widget that Evan wrote about in the previous blog post. While the field-type tells Drupal about what data comprises a field, the field-formatter is responsible for telling Drupal how to display the data stored in the field.

To recap, we defined a hashtag_search field type in the previous blog post whose instances will be composed of two items: the hashtag to search for, and the number of items to display. We want to convert this data into a list of the most recent n tweets with the specified hashtag.

A field-formatter is a Drupal plugin, just like its respective field-type and field-widget. They live in <module_path>/src/Plugin/Field/FieldFormatter/ and are namespaced appropriately: Drupal\<module_name>\Plugin\Field\FieldFormatter.

<?php

namespace Drupal\my_module\Plugin\Field\FieldFormatter;


use Drupal\Core\Field\FieldItemListInterface;
use Drupal\Core\Field\FormatterBase;
use Drupal\Core\Form\FormStateInterface;

/**
 * @FieldFormatter(
 *     id = "hashtag_formatter",
 *     label = @Translation("Hashtag Search"),
 *     field_types = {
 *      "hashtag_search"
 *     }
 * )
 */
class HashtagFormatter extends FormatterBase
{

    public function viewElements(FieldItemListInterface $items, $langcode)
    {
        return array();
    }
}

We tell Drupal important details about our new field-formatter using a @FieldFormatter class annotation. We declare its unique id; a human-readable, translatable label; and a list of field_types that it supports.

The most important method in a field-formatter is the viewElements method. It’s responsibility is returning a render array based on field data being passed as <Drupal\Core\Field\FieldItemListInterface> $items.

Let’s look at the code:

<?php

use Drupal\my_module\Twitter\TwitterClient;
use Drupal\my_module\Twitter\TweetFormatter;

...

    /**
     * @var TwitterClient
     */
    protected $twitter;

    /**
     * @var TweetFormatter
     */
    protected $formatter;

    ...

    public function viewElements(FieldItemListInterface $items, $langcode)
    {
        $element = [];

        // Iterate through all  values from one ore more fields...
        foreach ($items as $delta => $item) {

            try {

                // Use service to fetch `$item->count` no. of tweets matching the hashtag...
                $results = $this->twitter->search($item->hashtag_search, $item->count);

                // Map through each tweet and generate an HTML-rich version
                // complete with hashtags, mentions, URLs, etc. as links, using a formatter service.
                // Assign the HTML-rich version to a "formatted_text" property.
                $statuses = array_map(function ($s) {
                    $s['formatted_text'] = $this->formatter->toHtml($s['text'], $s['entities']);
                    return $s;
                }, $results['statuses']);

                // Add a header...
                if (!empty($statuses)) {
                    $element[$delta]['header'] = [
                        '#markup' => '<h4>#' . $item->hashtag_search . '</h4>'
                    ];
                }

                // Tell Drupal that each status is to be rendered
                // by the `my_module_status` theme.
                foreach ($statuses as $status) {
                    $element[$delta]['status'][] = [
                        '#theme' => 'my_module_status',
                        '#status' => $status
                    ];
                }

            } catch (\Exception $e) {
                // If an error/exception occur i.e. Twitter auth errors, log it, and carry-on
                // gracefully...
                $this->logger->error('[:exception]: %message', [
                    ':exception' => get_class($e),
                    '%message' => $e->getMessage(),
                ]);
                continue;
            }
        }

        // Include the `twitter_intents` library defined by this module which holds
        // `Drupal.behavior`s that dictate functionality of the Twitter block UI.
        $element['#attached']['library'][] = 'my_module/twitter_intents';

        return $element;
    }

    ...

See https://github.com/bezhermoso/tweet-to-html-php for how TweetFormatter works. Also, you can find the source-code for the basic Twitter HTTP client here: https://gist.github.com/bezhermoso/5a04e03cedbc77f6662c03d774f784c5

Custom theme renderer

As shown above, each individual tweets are using the my_module_status render theme. We’ll define it in the my_module.module file:

<?php

/**
 * Implements hook_theme().
 */
function my_module_theme($existing, $type, $theme, $path) {
  $theme = [];
  $theme['my_module_status'] = array(
    'variables' => array(
      'status' => NULL
    ),
    'template' => 'twitter-status',
    'render element' => 'element',
    'path' => $path . '/templates'
  );

  return $theme;
}

With this, we are telling Drupal to use the template file modules/my_module/templates/twitter-status.twig.html for any render array using my_module_status as its theme.

 

 

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