Why isn't this functionality in submitForm firing?

Question

I'm in the process of writing a module. The module works very well. In the module, I generate a form which consists of one submit button, and in the .module file, I'm calling that form and the submitForm works too. I was asked to move that button into a block, so I did. submitForm still works, but the main functionality doesn't. How come?

<?php

namespace Drupal\my_module\Form;

use Drupal\Core\Form\FormBase;
use Drupal\Core\Form\FormStateInterface;

/**
* Generate button.
*/
class myModuleForm extends FormBase {

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

  /**
  * {@inheritdoc}.
  */
  public function buildForm(array $form, FormStateInterface $form_state) {
    $form['my_module_btn'] = [
      '#type' => 'submit',
      '#value' => 'Do something'        ];
    return $form;
  }

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

  /**
  * {@inheritdoc}.
  */
  public function submitForm(array &$form, FormStateInterface $form_state) {
    $mpdf = new \Mpdf\Mpdf(['tempDir' => 'sites/default/files/tmp', 'default_font' => 'roboto', 'margin_left' => 6, 'margin_top' => 6, 'margin_right' => 6, 'margin_bottom' => 11]);
    ...
    ...
    ...
    $mpdf->Output($title."-".date("Ymd-His").".pdf", 'D');
  }
}

Answer 1

Though 4k4 says its not usually advisable, you can add an exit; at the end of submitForm in your particular case since you are generating a PDF and immediately serving it to the client. This will stop any further code from executing and solve the immediate problem (though it sounds like more scalable ones exist).

Answer 2

I googled the Mpdf library and it supports a string output. So you can use the string output to build a response and return it via $form_state:

use Symfony\Component\HttpFoundation\Response;

  public function submitForm(array &$form, FormStateInterface $form_state) {
    $content = '%PDF-1.3...';
    $headers = [
      'Content-Type' => 'application/pdf',
    ];
    $response = new Response($content, 200, $headers);
    $form_state->setResponse($response);
  }

Answer 3

I think it will be because the form functionality will be attempting to reload/redirect back to the page the form is on and you can't interrupt that mid flow with your own output.

You should probably add a redirect in your form submit to go to a custom controller page that will output your PDF

Answer 4

You can update the code as follows-

public function submitForm(array &$form, FormStateInterface $form_state) {
    $mpdf = new \Mpdf\Mpdf(['tempDir' => 'sites/default/files/tmp', 'default_font' => 'roboto', 'margin_left' => 6, 'margin_top' => 6, 'margin_right' => 6, 'margin_bottom' => 11]);
    ...
    ...
    ...
    $mpdf->Output($title."-".date("Ymd-His").".pdf", 'D');

  // Add the line for custom redirect

  $form_state->setRedirect('NEW PATH');

  exit;

  }