class ManagedFile

class ManagedFile extends FormElement {
    
    /**
     * {@inheritdoc}
     */
    public function getInfo() {
        $class = get_class($this);
        return [
            '#input' => TRUE,
            '#process' => [
                [
                    $class,
                    'processManagedFile',
                ],
            ],
            '#element_validate' => [
                [
                    $class,
                    'validateManagedFile',
                ],
            ],
            '#pre_render' => [
                [
                    $class,
                    'preRenderManagedFile',
                ],
            ],
            '#theme' => 'file_managed_file',
            '#theme_wrappers' => [
                'form_element',
            ],
            '#progress_indicator' => 'throbber',
            '#progress_message' => NULL,
            '#upload_validators' => [],
            '#upload_location' => NULL,
            '#size' => 22,
            '#multiple' => FALSE,
            '#extended' => FALSE,
            '#attached' => [
                'library' => [
                    'file/drupal.file',
                ],
            ],
            '#accept' => NULL,
        ];
    }
    
    /**
     * {@inheritdoc}
     */
    public static function valueCallback(&$element, $input, FormStateInterface $form_state) {
        // Find the current value of this field.
        $fids = !empty($input['fids']) ? explode(' ', $input['fids']) : [];
        foreach ($fids as $key => $fid) {
            $fids[$key] = (int) $fid;
        }
        $force_default = FALSE;
        // Process any input and save new uploads.
        if ($input !== FALSE) {
            $input['fids'] = $fids;
            $return = $input;
            // Uploads take priority over all other values.
            if ($files = file_managed_file_save_upload($element, $form_state)) {
                if ($element['#multiple']) {
                    $fids = array_merge($fids, array_keys($files));
                }
                else {
                    $fids = array_keys($files);
                }
            }
            else {
                // Check for #filefield_value_callback values.
                // Because FAPI does not allow multiple #value_callback values like it
                // does for #element_validate and #process, this fills the missing
                // functionality to allow File fields to be extended through FAPI.
                if (isset($element['#file_value_callbacks'])) {
                    foreach ($element['#file_value_callbacks'] as $callback) {
                        $callback($element, $input, $form_state);
                    }
                }
                // Load files if the FIDs have changed to confirm they exist.
                if (!empty($input['fids'])) {
                    $fids = [];
                    foreach ($input['fids'] as $fid) {
                        if ($file = File::load($fid)) {
                            $fids[] = $file->id();
                            if (!$file->access('download')) {
                                $force_default = TRUE;
                                break;
                            }
                            // Temporary files that belong to other users should never be
                            // allowed.
                            if ($file->isTemporary()) {
                                if ($file->getOwnerId() != \Drupal::currentUser()->id()) {
                                    $force_default = TRUE;
                                    break;
                                }
                                elseif (\Drupal::currentUser()->isAnonymous()) {
                                    $token = NestedArray::getValue($form_state->getUserInput(), array_merge($element['#parents'], [
                                        'file_' . $file->id(),
                                        'fid_token',
                                    ]));
                                    $file_hmac = Crypt::hmacBase64('file-' . $file->id(), \Drupal::service('private_key')->get() . Settings::getHashSalt());
                                    if ($token === NULL || !hash_equals($file_hmac, $token)) {
                                        $force_default = TRUE;
                                        break;
                                    }
                                }
                            }
                        }
                    }
                    if ($force_default) {
                        $fids = [];
                    }
                }
            }
        }
        // If there is no input or if the default value was requested above, use the
        // default value.
        if ($input === FALSE || $force_default) {
            if ($element['#extended']) {
                $default_fids = isset($element['#default_value']['fids']) ? $element['#default_value']['fids'] : [];
                $return = isset($element['#default_value']) ? $element['#default_value'] : [
                    'fids' => [],
                ];
            }
            else {
                $default_fids = isset($element['#default_value']) ? $element['#default_value'] : [];
                $return = [
                    'fids' => [],
                ];
            }
            // Confirm that the file exists when used as a default value.
            if (!empty($default_fids)) {
                $fids = [];
                foreach ($default_fids as $fid) {
                    if ($file = File::load($fid)) {
                        $fids[] = $file->id();
                    }
                }
            }
        }
        $return['fids'] = $fids;
        return $return;
    }
    
    /**
     * #ajax callback for managed_file upload forms.
     *
     * This ajax callback takes care of the following things:
     *   - Ensures that broken requests due to too big files are caught.
     *   - Adds a class to the response to be able to highlight in the UI, that a
     *     new file got uploaded.
     *
     * @param array $form
     *   The build form.
     * @param \Drupal\Core\Form\FormStateInterface $form_state
     *   The form state.
     * @param \Symfony\Component\HttpFoundation\Request $request
     *   The current request.
     *
     * @return \Drupal\Core\Ajax\AjaxResponse
     *   The ajax response of the ajax upload.
     */
    public static function uploadAjaxCallback(&$form, FormStateInterface &$form_state, Request $request) {
        
        /** @var \Drupal\Core\Render\RendererInterface $renderer */
        $renderer = \Drupal::service('renderer');
        $form_parents = explode('/', $request->query
            ->get('element_parents'));
        // Sanitize form parents before using them.
        $form_parents = array_filter($form_parents, [
            Element::class,
            'child',
        ]);
        // Retrieve the element to be rendered.
        $form = NestedArray::getValue($form, $form_parents);
        // Add the special AJAX class if a new file was added.
        $current_file_count = $form_state->get('file_upload_delta_initial');
        if (isset($form['#file_upload_delta']) && $current_file_count < $form['#file_upload_delta']) {
            $form[$current_file_count]['#attributes']['class'][] = 'ajax-new-content';
        }
        else {
            $form['#suffix'] .= '<span class="ajax-new-content"></span>';
        }
        $status_messages = [
            '#type' => 'status_messages',
        ];
        $form['#prefix'] .= $renderer->renderRoot($status_messages);
        $output = $renderer->renderRoot($form);
        $response = new AjaxResponse();
        $response->setAttachments($form['#attached']);
        return $response->addCommand(new ReplaceCommand(NULL, $output));
    }
    
    /**
     * Render API callback: Expands the managed_file element type.
     *
     * Expands the file type to include Upload and Remove buttons, as well as
     * support for a default value.
     */
    public static function processManagedFile(&$element, FormStateInterface $form_state, &$complete_form) {
        // This is used sometimes so let's implode it just once.
        $parents_prefix = implode('_', $element['#parents']);
        $fids = isset($element['#value']['fids']) ? $element['#value']['fids'] : [];
        // Set some default element properties.
        $element['#progress_indicator'] = empty($element['#progress_indicator']) ? 'none' : $element['#progress_indicator'];
        $element['#files'] = !empty($fids) ? File::loadMultiple($fids) : [];
        $element['#tree'] = TRUE;
        // Generate a unique wrapper HTML ID.
        $ajax_wrapper_id = Html::getUniqueId('ajax-wrapper');
        $ajax_settings = [
            'callback' => [
                get_called_class(),
                'uploadAjaxCallback',
            ],
            'options' => [
                'query' => [
                    'element_parents' => implode('/', $element['#array_parents']),
                ],
            ],
            'wrapper' => $ajax_wrapper_id,
            'effect' => 'fade',
            'progress' => [
                'type' => $element['#progress_indicator'],
                'message' => $element['#progress_message'],
            ],
        ];
        // Set up the buttons first since we need to check if they were clicked.
        $element['upload_button'] = [
            '#name' => $parents_prefix . '_upload_button',
            '#type' => 'submit',
            '#value' => t('Upload'),
            '#attributes' => [
                'class' => [
                    'js-hide',
                ],
            ],
            '#validate' => [],
            '#submit' => [
                'file_managed_file_submit',
            ],
            '#limit_validation_errors' => [
                $element['#parents'],
            ],
            '#ajax' => $ajax_settings,
            '#weight' => -5,
        ];
        // Force the progress indicator for the remove button to be either 'none' or
        // 'throbber', even if the upload button is using something else.
        $ajax_settings['progress']['type'] = $element['#progress_indicator'] == 'none' ? 'none' : 'throbber';
        $ajax_settings['progress']['message'] = NULL;
        $ajax_settings['effect'] = 'none';
        $element['remove_button'] = [
            '#name' => $parents_prefix . '_remove_button',
            '#type' => 'submit',
            '#value' => $element['#multiple'] ? t('Remove selected') : t('Remove'),
            '#validate' => [],
            '#submit' => [
                'file_managed_file_submit',
            ],
            '#limit_validation_errors' => [
                $element['#parents'],
            ],
            '#ajax' => $ajax_settings,
            '#weight' => 1,
        ];
        $element['fids'] = [
            '#type' => 'hidden',
            '#value' => $fids,
        ];
        // Add progress bar support to the upload if possible.
        if ($element['#progress_indicator'] == 'bar' && ($implementation = file_progress_implementation())) {
            $upload_progress_key = mt_rand();
            if ($implementation == 'uploadprogress') {
                $element['UPLOAD_IDENTIFIER'] = [
                    '#type' => 'hidden',
                    '#value' => $upload_progress_key,
                    '#attributes' => [
                        'class' => [
                            'file-progress',
                        ],
                    ],
                    // Uploadprogress extension requires this field to be at the top of
                    // the form.
'#weight' => -20,
                ];
            }
            // Add the upload progress callback.
            $element['upload_button']['#ajax']['progress']['url'] = Url::fromRoute('file.ajax_progress', [
                'key' => $upload_progress_key,
            ]);
            // Set a custom submit event so we can modify the upload progress
            // identifier element before the form gets submitted.
            $element['upload_button']['#ajax']['event'] = 'fileUpload';
        }
        // Use a manually generated ID for the file upload field so the desired
        // field label can be associated with it below. Use the same method for
        // setting the ID that the form API autogenerator does.
        // @see \Drupal\Core\Form\FormBuilder::doBuildForm()
        $id = Html::getUniqueId('edit-' . implode('-', array_merge($element['#parents'], [
            'upload',
        ])));
        // The file upload field itself.
        $element['upload'] = [
            '#name' => 'files[' . $parents_prefix . ']',
            '#type' => 'file',
            // This #title will not actually be used as the upload field's HTML label,
            // since the theme function for upload fields never passes the element
            // through theme('form_element'). Instead the parent element's #title is
            // used as the label (see below). That is usually a more meaningful label
            // anyway.
'#title' => t('Choose a file'),
            '#title_display' => 'invisible',
            '#id' => $id,
            '#size' => $element['#size'],
            '#multiple' => $element['#multiple'],
            '#theme_wrappers' => [],
            '#weight' => -10,
            '#error_no_message' => TRUE,
        ];
        if (!empty($element['#accept'])) {
            $element['upload']['#attributes'] = [
                'accept' => $element['#accept'],
            ];
        }
        // Indicate that $element['#title'] should be used as the HTML label for the
        // file upload field.
        $element['#label_for'] = $element['upload']['#id'];
        if (!empty($fids) && $element['#files']) {
            foreach ($element['#files'] as $delta => $file) {
                $file_link = [
                    '#theme' => 'file_link',
                    '#file' => $file,
                ];
                if ($element['#multiple']) {
                    $element['file_' . $delta]['selected'] = [
                        '#type' => 'checkbox',
                        '#title' => \Drupal::service('renderer')->renderPlain($file_link),
                    ];
                }
                else {
                    $element['file_' . $delta]['filename'] = $file_link + [
                        '#weight' => -10,
                    ];
                }
                // Anonymous users who have uploaded a temporary file need a
                // non-session-based token added so $this->valueCallback() can check
                // that they have permission to use this file on subsequent submissions
                // of the same form (for example, after an Ajax upload or form
                // validation error).
                if ($file->isTemporary() && \Drupal::currentUser()->isAnonymous()) {
                    $element['file_' . $delta]['fid_token'] = [
                        '#type' => 'hidden',
                        '#value' => Crypt::hmacBase64('file-' . $delta, \Drupal::service('private_key')->get() . Settings::getHashSalt()),
                    ];
                }
            }
        }
        // Add the extension list to the page as JavaScript settings.
        if (isset($element['#upload_validators']['file_validate_extensions'][0])) {
            $extension_list = implode(',', array_filter(explode(' ', $element['#upload_validators']['file_validate_extensions'][0])));
            $element['upload']['#attached']['drupalSettings']['file']['elements']['#' . $id] = $extension_list;
        }
        // Prefix and suffix used for Ajax replacement.
        $element['#prefix'] = '<div id="' . $ajax_wrapper_id . '">';
        $element['#suffix'] = '</div>';
        return $element;
    }
    
    /**
     * Render API callback: Hides display of the upload or remove controls.
     *
     * Upload controls are hidden when a file is already uploaded. Remove controls
     * are hidden when there is no file attached. Controls are hidden here instead
     * of in \Drupal\file\Element\ManagedFile::processManagedFile(), because
     * #access for these buttons depends on the managed_file element's #value. See
     * the documentation of \Drupal\Core\Form\FormBuilderInterface::doBuildForm()
     * for more detailed information about the relationship between #process,
     * #value, and #access.
     *
     * Because #access is set here, it affects display only and does not prevent
     * JavaScript or other untrusted code from submitting the form as though
     * access were enabled. The form processing functions for these elements
     * should not assume that the buttons can't be "clicked" just because they are
     * not displayed.
     *
     * @see \Drupal\file\Element\ManagedFile::processManagedFile()
     * @see \Drupal\Core\Form\FormBuilderInterface::doBuildForm()
     */
    public static function preRenderManagedFile($element) {
        // If we already have a file, we don't want to show the upload controls.
        if (!empty($element['#value']['fids'])) {
            if (!$element['#multiple']) {
                $element['upload']['#access'] = FALSE;
                $element['upload_button']['#access'] = FALSE;
            }
        }
        else {
            $element['remove_button']['#access'] = FALSE;
        }
        return $element;
    }
    
    /**
     * Render API callback: Validates the managed_file element.
     */
    public static function validateManagedFile(&$element, FormStateInterface $form_state, &$complete_form) {
        $clicked_button = end($form_state->getTriggeringElement()['#parents']);
        if ($clicked_button != 'remove_button' && !empty($element['fids']['#value'])) {
            $fids = $element['fids']['#value'];
            foreach ($fids as $fid) {
                if ($file = File::load($fid)) {
                    // If referencing an existing file, only allow if there are existing
                    // references. This prevents unmanaged files from being deleted if
                    // this item were to be deleted. When files that are no longer in use
                    // are automatically marked as temporary (now disabled by default),
                    // it is not safe to reference a permanent file without usage. Adding
                    // a usage and then later on removing it again would delete the file,
                    // but it is unknown if and where it is currently referenced. However,
                    // when files are not marked temporary (and then removed)
                    // automatically, it is safe to add and remove usages, as it would
                    // simply return to the current state.
                    // @see https://www.drupal.org/node/2891902
                    if ($file->isPermanent() && \Drupal::config('file.settings')->get('make_unused_managed_files_temporary')) {
                        $references = static::fileUsage()->listUsage($file);
                        if (empty($references)) {
                            // We expect the field name placeholder value to be wrapped in t()
                            // here, so it won't be escaped again as it's already marked safe.
                            $form_state->setError($element, t('The file used in the @name field may not be referenced.', [
                                '@name' => $element['#title'],
                            ]));
                        }
                    }
                }
                else {
                    // We expect the field name placeholder value to be wrapped in t()
                    // here, so it won't be escaped again as it's already marked safe.
                    $form_state->setError($element, t('The file referenced by the @name field does not exist.', [
                        '@name' => $element['#title'],
                    ]));
                }
            }
        }
        // Check required property based on the FID.
        if ($element['#required'] && empty($element['fids']['#value']) && !in_array($clicked_button, [
            'upload_button',
            'remove_button',
        ])) {
            // We expect the field name placeholder value to be wrapped in t()
            // here, so it won't be escaped again as it's already marked safe.
            $form_state->setError($element, t('@name field is required.', [
                '@name' => $element['#title'],
            ]));
        }
        // Consolidate the array value of this field to array of FIDs.
        if (!$element['#extended']) {
            $form_state->setValueForElement($element, $element['fids']['#value']);
        }
    }
    
    /**
     * Wraps the file usage service.
     *
     * @return \Drupal\file\FileUsage\FileUsageInterface
     */
    protected static function fileUsage() {
        return \Drupal::service('file.usage');
    }

}