Multistep Ajax form with ManagedFile: current step array dissapears from $form upon file upload callback

Question

I have a custom multi-step Ajax form. On the second step, I have defined a managed_file field where users can upload several attachments:

public function buildForm(array $form, FormStateInterface $form_state)
  {
    $form['#tree'] = TRUE;
    ...
    $step = !empty($form_state->getValue('step')) ? $form_state->getValue('step') : 1;
    $form['step'] = [
      '#type' => 'value',
      '#value' => $step,
    ];
     // Steps previous to current one
    $steps = [1, 2, 3];
    foreach ($steps as $prev_step) {
      if ($prev_step < $step && $formState->hasValue('step' . $prev_step)) {
        $form['step'. $prev_step] = [
          '#type' => 'value',
          '#value' => $formState->getValue('step'. $prev_step),
        ];
      }
    }
    // Current step
    switch ($step) {
      ...
      case 2:
        $form['step' . $step] = [
          '#type' => 'fieldset',
          '#title' => $this->t('Step @number: Compose Emails', [
            '@number' => $step
          ]),
        ];
        ...
        $form['step' . $step]['test_file'] = [
          '#type' => 'managed_file',
          '#upload_location' => 'public://attachments/',
          '#multiple' => TRUE,
          '#description' => 'This is a description',
          '#upload_validators' => [
            'file_validate_extensions' => [],
            'file_validate_size' => array(25600000)
          ],
          '#default_value' => $form_state->hasValue(["step$step", 'test_file']) ? $form_state->getValue(["step$step", 'test_file']) : []
        ];
        ...
    }
    ...
  }
    

However, upon uploading the file, the uploadAjaxCallback of Drupal\file\Element\ManagedFile is not able to find my nested step2 array in $form:

# \Drupal\file\Element\ManagedFile::uploadAjaxCallback, line 190
// Before calling getValue, $form does not have key 'step2', only 'step1'
$form = NestedArray::getValue($form, $form_parents);
// $form is now NULL!

Consequently, nothing is displayed in my form, and the file upload fails.

Am I doing something wrong here?

Answer 1

Try to store the step in $form_state:

$step = $form_state->has('step') ? $form_state->get('step') : 1;
$form_state->set('step', $step);