Making a file downloading system with valid input in a custom drupal 8 form

Question

I am trying to create a custom drupal 8 form, that is going to allow user to enter his/her firstname, lastname and email address, and if the user enters these credentials, will be able to download the file that I will upload to cms. This is how my form looks like right now:

namespace Drupal\whitepaper\Form;

use Drupal\Core\Form\FormBase;
use Drupal\Core\Form\FormStateInterface;
use Drupal\Component\Utility\UrlHelper;

class WhitePaperForm extends FormBase {

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

/**
 * {@inheritdoc}.
 */
   public function buildForm(array $form, FormStateInterface $form_state) {
  $form['iam'] = array(
   '#type' => 'select',
   '#title'=>t('I am:'),
   '#options' => array(
    'dentist' => t('Dentist'),
    'broker' => t('Broker'),
    'engineer' => t('Engineer'),
   ),
   '#required' => TRUE,
  );
  $form['fname'] = array(
   '#type' => 'textfield',
   '#title' => $this->t('First Name:'),
   '#required' => TRUE,
  );
  $form['lname'] = array(
   '#type' => 'textfield',
   '#title' => $this->t('Last Name:'),
   '#required' => TRUE,
  );
  $form['email'] = array(
   '#type' => 'email',
   '#title' => $this->t('Your .com email address.'),
   '#required' => TRUE,
  );
  $form['show'] = array(
   '#type' => 'submit',
   '#value' => $this->t('Download Whitepaper'),
  );

  return $form;
}


/**
 * {@inheritdoc}
 */
public function validateForm(array &$form, FormStateInterface $form_state) {

  if (strpos($form_state['values']['email'], '.com') === FALSE ) {
    $this->setFormError('email', $form_state, $this->t('This is not a .com email address.'));
  } 
 }

 /**
  * {@inheritdoc}
  */
  public function submitForm(array &$form, FormStateInterface $form_state) {

  drupal_set_message($this->t('Your email address is @email', array('@email' => $form_state['values']['email'])));
 }
}

Can anyone help with a block of code that if the inputs are valid, when the user click download paper button, will be able to download? I got really stuck and i guess my point of view is not helpful to solve this problem.Thank you advanced.

Answer 1

you can do this with the $form_state->response($response)

  $response = new BinaryFileResponse($file->getFileUri();
  $form_state->setResponse($response);

Answer 2

You could add a redirect to your form, like:

$form['#redirect'][] = 'path/to/your/file.ext';

In submitForm the form values are valid !

So validateForm has to set errors to prevent the submission of the form.