"managed_file" form field type returns null value in my form submission

Question

I am have been struggling with this for the past day or so. I am simply trying to create a custom entity and populate/or update the image field with the fid. Whats weird is that every other form field I am passing I can get the value for but this managed_file field for uploading an image always returns NULL when I call dpm().

use Drupal\Core\Form\FormBase;
use Drupal\Core\Form\FormStateInterface;
use Drupal\file\Entity\File;

Within my form_build function, I use the following code.

// Manage file.
$form['theimage'] = array(
  '#type' => 'managed_file',
  '#title' => 'Managed file',
  '#description' => $this->t('Manage file, #type = managed_file'),
  //'#default_value' => $form_state->getValue('theimage'),
  '#upload_location' => 'public://images/',
  '#required' => FALSE,
);

In the submission handler, I use the following code.

// Get the fid value, upload it and save it to the managed file table. 
$fid = $form_state->getValue(['theimage', 0]);
dpm($fid);
if (!empty($fid)) {
  $file = File::load($fid);
  $file->setPermanent();
  $file->save(); 
}

Any thoughts, solutions, or recommendations would be greatly appreciated.

Answer 1

After alot of digging/debugging i discovered that i could not pull any data from the "manage_file" field while using the PrivateTempStore in my Multi-Step form. After disabling the form cache, i began to get the values and the expected behavior by using:

$form_state->disableCache(); 

My Submit Handler

public function buildForm(array $form, FormStateInterface $form_state) {
 /**
 *  Fixes image upload bug in form. Does not work unless we 
 disable the cache while using PrivateTempStore
 **/

 $form_state->disableCache();

.......

}

See https://www.drupal.org/project/drupal/issues/2647812#comment-11683961

Answer 2

$form_state will show NULL or empty arrays in the file upload if you don't use the proper upload function. The file hasn't been uploaded yet.

In the validation function, for example, you should use the function file_save_upload.

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

  $file = file_save_upload('banner_image', array(), FALSE, 0, FILE_EXISTS_REPLACE);
}