Drupal 8: Admin Form Example

AdminFormExample.php

<?php

namespace Drupal\my_module\Form;

use Drupal\Core\Form\ConfigFormBase;
use Drupal\Core\Form\FormStateInterface;

/**
 * Builds the example admin form
 */
class AdminFormExample extends ConfigFormBase {

  /**
   * {@inheritdoc}
   */
  protected function getEditableConfigNames() {
    return [
      'my_module.admin_form_example',
    ];
  }

  /**
   * {@inheritdoc}
   */
  public function getFormId() {
    return 'admin_form_example';
  }

  /**
   * {@inheritdoc}
   */
  public function buildForm(array $form, FormStateInterface $form_state) {
    $options = [
      1 => 'Option 1',
      2 => 'Option 2',
    ];
    // Add options to select form element
    $config = $this->config('my_module.admin_form_example');
    $item_selected = $config->get('items');
    $form['items'] = [
      '#type' => 'select',
      '#title' => $this->t('Title'),
      '#options' => $options,
      '#default_value' => empty($item_selected) ? false : $item_selected,
    ];
    
    // Set node to feature in a view
    $config = $this->config('my_module.featured_node');
    $featured_node = $config->get('featured_node');
    $form['featured_node'] = array(
      '#type' => 'entity_autocomplete',
      '#title' => $this->t('Set Featured Node'),
      '#target_type' => 'node',
      '#selection_settings' => ['target_bundles' => ['my_content_type']],
      '#default_value' => empty($featured_node) ? false : Node::load($featured_node),
    );
    
    return parent::buildForm($form, $form_state);
  }

  /**
   * {@inheritdoc}
   */
  public function submitForm(array &$form, FormStateInterface $form_state) {
    // Final submit
    parent::submitForm($form, $form_state);
    // get submitted values
    $values = $form_state->getValues();
    // add to config
    $this->config('my_module.admin_form_example')
      ->set('items', $values['items'])
      ->set('featured_node', $values['featured_node'])
      ->save();
  }
}

my_module.links.menu.yml

my_module.admin_form_example:
  title: 'Admin Form Example'
  route_name: my_module.admin_form_example
  description: 'Example administration form'
  parent: system.admin_config
  weight: -10

my_module.routing.yml

my_module.admin_form_example:
  path: '/admin/config/admin-form-example'
  defaults:
    _form: '\Drupal\my_module\Form\AdminFormExample'
    _title: 'Admin Form Example'
  requirements:
    _permission: 'administer content'
  options:
    _admin_route: TRUE