Drupal 8, add an image field from a BuildForm with preview

Question

I created a custom form from a buildForm function. In this form, I would like add an image field.

I can do that via this code :

$form['main']['image'] = array(
    '#type' => 'text_format',
    '#title' => t('image'),
    '#default_value' => array(10),
);

I can upload and remove the image from my form. However, when I upload an image, I haven't this preview. I mean, when I create a content via the Drupal UI. I can add a preconfigured "image" field. When I upload an image via this "image" field, I have a preview of the image.

And here, when I create the field element programmatically, I haven't a preview of the image when I upload her. How use api Drupal for have the preview of the image when I upload her via my "image" field ?

Answer 1

So here is how I got my form to display the thumbnail image. What I basically did was take the code in ImageWidget::process, put it in a theme preprocessor and set the #theme property of the element to image_widget.

Your image element in your form class should look like this:

$form['profile_image'] = [
   '#type' => 'managed_file',
   '#title' => t('Profile Picture'),
   '#upload_validators' => array(
       'file_validate_extensions' => array('gif png jpg jpeg'),
       'file_validate_size' => array(25600000),
   ),
   **'#theme' => 'image_widget',**
   **'#preview_image_style' => 'medium',**
   '#upload_location' => 'public://profile-pictures',
   '#required' => TRUE,
];

In your name_of_default_theme.theme file, you need the following:

function name_of_default_theme_preprocess_image_widget(&$variables) {
    $element = $variables['element'];

    $variables['attributes'] = array('class' => array('image-widget', 'js-form-managed-file', 'form-managed-file', 'clearfix'));

    if (!empty($element['fids']['#value'])) {
        $file = reset($element['#files']);
        $element['file_' . $file->id()]['filename']['#suffix'] = ' <span class="file-size">(' . format_size($file->getSize()) . ')</span> ';
        $file_variables = array(
            'style_name' => $element['#preview_image_style'],
            'uri' => $file->getFileUri(),
        );

        // Determine image dimensions.
        if (isset($element['#value']['width']) && isset($element['#value']['height'])) {
            $file_variables['width'] = $element['#value']['width'];
            $file_variables['height'] = $element['#value']['height'];
        } else {
            $image = \Drupal::service('image.factory')->get($file->getFileUri());
            if ($image->isValid()) {
                $file_variables['width'] = $image->getWidth();
                $file_variables['height'] = $image->getHeight();
            }
            else {
                $file_variables['width'] = $file_variables['height'] = NULL;
            }
        }

        $element['preview'] = array(
            '#weight' => -10,
            '#theme' => 'image_style',
            '#width' => $file_variables['width'],
            '#height' => $file_variables['height'],
            '#style_name' => $file_variables['style_name'],
            '#uri' => $file_variables['uri'],
        );

        // Store the dimensions in the form so the file doesn't have to be
        // accessed again. This is important for remote files.
        $element['width'] = array(
            '#type' => 'hidden',
            '#value' => $file_variables['width'],
        );
        $element['height'] = array(
            '#type' => 'hidden',
            '#value' => $file_variables['height'],
        );
    }

    $variables['data'] = array();
    foreach (Element::children($element) as $child) {
        $variables['data'][$child] = $element[$child];
    }
}

The solution works well, but the image module is missing a class with the @FormElement annotation. That is why the element isn't rendered properly.