Add a Form in to a Basic Controller

NOTE: I don't know this is the best practice, anyway it's working !!!.

Here an Example To add A Exposed filter to a EntityListBuilder.

1. Create the Form using FormBase. (How to create a Form, here ID = my_test_form_id).

2. Create the basic page (or, here EntityListBuilder) (How to create a Simple Page).

3. Insert the form into the page (Tuto).

4. Handle form submit using \Drupal::request()

Example:

//Form

class MyEntityExposedFilters extends FormBase {

  public function getFormId() {

    return 'my_test_form_id';

  }

  public function buildForm(array $form, FormStateInterface $form_state) {

    $form = array();

    $form['filters']['text'] = array(

      '#title' => $this->t('Text'),

      '#type' => 'textfield',

      '#default_value' => \Drupal::request()->get('text'),

    );

    $form['filters']['submit_apply'] = [

      '#type' => 'submit',

      '#value' => t('Filter'),

    ];

    return $form;

  }

  public function submitForm(array &$form, FormStateInterface $form_state) {

    //Handle Submit on MyExposedListBuilder.

  }

}

//Page

class MyExposedListBuilder extends EntityListBuilder {

  public function render() {

    //Exposed form

    $form = \Drupal::formBuilder()->getForm('\Drupal\mymodule\Form\MyEntityExposedFilters');

    $build['form'] = $form;

    //Result table

    $build += parent::render();

    return $build;

  }

  /**

   * Loads entity IDs using a pager sorted by the entity id.

   */

  protected function getEntityIds() {

    $form_id = \Drupal::request()->get('form_id');

    if ($form_id && $form_id === 'my_test_form_id') {

      $query = \Drupal::entityQuery($this->entityTypeId);

      $text = \Drupal::request()->get('text');

      $query->condition('text', $text, 'CONTAINS');

      if ($this->limit) {

        $query->pager($this->limit);

      }

      $res = $query->execute();

    }

    else {

      $res = parent::getEntityIds();

    }

    return $res;

  }

}