How do I put a <div> around a form?

Question

I have created a form that I will embed in my page with a custom module. How can I wrap this form with a div that has a specific class?

I need to add it to the build which is made with the following code.

$build = array(
          'form' => $form,
          '#type' => 'markup',
          '#markup' => $html,
          '#attached' => array(

             'library' => array('pup_viewfiles/sortfiles_libraries'),
            ),
        );

Answer 1

There are many different ways to add markup to a form in Drupal 8. In your build array, you could add a wrapper element, and then nest the form within:

$build['wrapper'] = [
  '#type' => 'container',
  '#attributes' => ['id' => 'my-form-wrapper', 'class' => ['my-form-wrapper']],
];

$build['wrapper']['form'] = \Drupal::formBuilder()->getForm('Drupal\mymodule\Form\MyForm');

Alternately you could add a '#prefix' and a '#suffix' to your form.

$form['#prefix'] = '<div class="my-form-wrapper">';
$form['#suffix'] = '</div>';

Or you could use a template for your form and add it to your theme, using the suggestion form--form-id.html.twig and add custom markup there. (Be sure to replace underscores with hyphens in the template name.)

<div id="my-form-wrapper">
  <form{{ attributes }}>
    {{ children }}
  </form>
</dv>

Answer 2

I believe that you have described the perfect use case for the container render element.

https://api.drupal.org/api/drupal/core%21lib%21Drupal%21Core%21Render%21Element%21Container.php/class/Container/8.2.x

I believe your build would be something like

$build = [
  'some_name' => [
    '#type' => 'container',
    '#attributes' => [
        'class' => 'some class',
    ],
    'form' => $form,
  ],
  'some_other_name' => [
    '#type' => 'markup',
    '#markup' => $html,
  ],
  '#attached' => array(
    'library' => array('pup_viewfiles/sortfiles_libraries'),
  ],
];

Wasn't exactly sure where you wanted the html markup (this has it outside the container).