Field formatter module throws cannot declare class error

Question

I am trying to write a custom module, which defines a field formatter plugin, but when I visit the Manage display page, I get this error:

Fatal error: Cannot declare class Drupal\my_formatter\Plugin\Field\FieldFormatter\MyFormatter, because the name is already in use in /var/www/sites/drupalroot/web/modules/custom/my_formatter/src/Plugin/Field/FieldFormatter/MyFormatter.php on line 0

I've compared my module to several other working field formatter plugins and can't seem to figure out what I'm missing.

My module directory is laid out like this:

web/modules/custom/my_formatter/
├── my_formatter.info.yml
├── my_formatter.module
├── src
│   └── Plugin
│       └── Field
│           └── FieldFormatter
│               └── MyFormatter.php
└── templates
    └── my-formatter.html.twig

My info file:

name: My Formatter
type: module
description: 'Do some fun stuff.'
package: Field types
core: 8.x
dependencies:
  - field
  - file

And my MyFormatter.php file:

<?php
/**
 * @file
 * Contains \Drupal\my_formatter\Plugin\Field\FieldFormatter\MyFormatter.
 */

namespace Drupal\my_formatter\Plugin\Field\FieldFormatter;

use Drupal\Core\Field\FieldItemListInterface;
use Drupal\file\Plugin\Field\FieldFormatter\FileFormatterBase;

/**
 * Plugin implementation of the 'my' formatter.
 *
 * @FieldFormatter(
 *   id = "my",
 *   label = @Translation("My Formatter"),
 *   field_types = {
 *     "file"
 *   }
 * )
 */
class MyFormatter extends FileFormatterBase {

  /**
   * {@inheritdoc}
   */
  public function viewElements(FieldItemListInterface $items, $langcode) {
    $elements = [];
    foreach ($items as $delta => $item) {
      $elements[$delta] = [
        '#theme' => 'my_formatter',
        '#my_var' => 'Hello',
      ];
    }
    return $elements;
  }

}

Any help would be greatly appreciated.

Answer 1

Here are some pointers that might help:

• Run drush cr to clear cache and see if the error persists.
• Enable error backtraces from the Config > Development > Logging & Errors page to see if you get more info in the error message about where the class is being redeclared.
• Search your project for the name MyFormatter and see if you find anything besides the MyFormatter.php file you've mentioned.
• File, directory and class names are case-sensitive – in what you've shared, they seem to be correctly named.
• Make sure you're not manually using require or include anywhere to include this file.
  • Probably, not applicable in your case.

Answer 2

This error is often caused by a "stale" autoloader file. The first and most effective step would be to call

composer dump-autoload

from inside your drupal installation folder (presumably /var/www/sites/drupalroot/) in order to force an updating of the autoloader file in your vendor folder. Try clearing the composer caches as well in combination with this command:

composer clear-cache

Good luck!

P.S.: @see dump-autoload-dumpautoload for more information