Why submit button is losing disabled attribute after deletion of uploaded file?

Question

The task is simple: submit button must be enabled only if file is attached to file input.

The form code:

function custom_module_form($form, &$form_state) {

    $form['#attributes']['enctype'] = 'multipart/form-data';
    $form['xml_file'] = array(
        '#field_prefix'      => '<span class="description">'.t('Only XML files are allowed').'</span>',
        '#type'              => 'managed_file',
        '#title'             => '',
        '#default_value'     => '',
        '#upload_location'   => CUSTOM_MODULE_UPLOAD_DIR,
        '#upload_validators' => array('file_validate_extensions' => array('xml')),
        '#required'          => TRUE,
    );

    $form['submit'] = array(
      '#type' => 'submit',
      '#value' => t('Submit'),
    );

    return $form;
}

The form alter code:

function custom_module_FORM_ID_form_alter(&$form, &$form_state, $form_id) {
    if (! isset($form_state['clicked_button'])
        || $form_state['clicked_button']['#name'] == 'xml_file_remove_button') {
        $form['submit']['#disabled'] = TRUE;
    }
}

It works, but when I removed uploaded file by Remove button, refreshed form has enabled submit button, even if I am sure that above code was executed. What am I doing wrong?

Use case:

1. Go to page with a form, submit button is disabled - ok
2. Uploaded sample xml file:

3. Click the Remove button, form is refreshed but submit button is enabled instead of disabled

Answer 1

You can't do it, because on remove event, only HTML for file input field is reloaded. There is a need of using custom AJAX callback for this (such as ajaxSuccess). I don't know nothing about Drupal Ajax callbacks for this type of event too.