How can I get the currently submitted form state in a block on the same page?

Question

How can I retrieve the currently submitted and validated form state?

Please assume the following scenario.

A page has a form on it created using the form API. The data in the submitted form is transient. In other words it is not saved or stored in any way.

The form is created by a module I do not control and therefore I should not alter the codebase.

On the same page as the form is a block (mine) which displays data based on the form submission.

I want to get access to the validated data from the anonymous user submitted form from within the block plugin code.

The form does all the necessary validation of the user submitted data. I do not want to replicate this validation so I need to get access to the validated form_state so I can extracted the submitted values.

I cannot see a way to retrieve the form_state object from the current request.

How can this be done?

Answer 1

As with most things you are learning. The answer is often simpler than first thought.

To resolve my issue I was able to use hook_form_alter and implement my own submission handler.

From here I was able to disable the redirect to prevent the page reloading.

Once redirect was disabled then my block plugin was able to get access to the form and use the validated results within it.

My block build for demo purposes contained the following

public function build() {

  // Get the current form.
  $form = \Drupal::formBuilder()->getForm('Drupal\modulename\Form\moduleForm');

  return [
    '#markup' => "<pre>" . print_r($form, TRUE) . "</pre>",
  ];
}

The important point is \Drupal::formBuilder()->getForm('name') returns the cached form results and therefore does not require any further validation.

This can be verified by checking $form['#validated'] == 1.