PluginFormFactory.php

<?php

namespace Drupal\Core\Plugin;

use Drupal\Component\Plugin\Exception\InvalidPluginDefinitionException;
use Drupal\Component\Plugin\PluginAwareInterface;
use Drupal\Core\DependencyInjection\ClassResolverInterface;

/**
 * Provides form discovery capabilities for plugins.
 */
class PluginFormFactory implements PluginFormFactoryInterface {
    
    /**
     * The class resolver.
     *
     * @var \Drupal\Core\DependencyInjection\ClassResolverInterface
     */
    protected $classResolver;
    
    /**
     * PluginFormFactory constructor.
     *
     * @param \Drupal\Core\DependencyInjection\ClassResolverInterface $class_resolver
     *   The class resolver.
     */
    public function __construct(ClassResolverInterface $class_resolver) {
        $this->classResolver = $class_resolver;
    }
    
    /**
     * {@inheritdoc}
     */
    public function createInstance(PluginWithFormsInterface $plugin, $operation, $fallback_operation = NULL) {
        if (!$plugin->hasFormClass($operation)) {
            // Use the default form class if no form is specified for this operation.
            if ($fallback_operation && $plugin->hasFormClass($fallback_operation)) {
                $operation = $fallback_operation;
            }
            else {
                throw new InvalidPluginDefinitionException($plugin->getPluginId(), sprintf('The "%s" plugin did not specify a "%s" form class', $plugin->getPluginId(), $operation));
            }
        }
        $form_class = $plugin->getFormClass($operation);
        // If the form specified is the plugin itself, use it directly.
        if (ltrim(get_class($plugin), '\\') === ltrim($form_class, '\\')) {
            $form_object = $plugin;
        }
        else {
            $form_object = $this->classResolver
                ->getInstanceFromDefinition($form_class);
        }
        // Ensure the resulting object is a plugin form.
        if (!$form_object instanceof PluginFormInterface) {
            throw new InvalidPluginDefinitionException($plugin->getPluginId(), sprintf('The "%s" plugin did not specify a valid "%s" form class, must implement \\Drupal\\Core\\Plugin\\PluginFormInterface', $plugin->getPluginId(), $operation));
        }
        if ($form_object instanceof PluginAwareInterface) {
            $form_object->setPlugin($plugin);
        }
        return $form_object;
    }

}