silverstripe-userforms/code/model/recipients/UserFormRecipientItemRequest.php

<?php

/**
 * Controller that handles requests to EmailRecipient's
 *
 * @package userforms
 */
class UserFormRecipientItemRequest extends GridFieldDetailForm_ItemRequest {

	private static $allowed_actions = array(
		'edit',
		'view',
		'ItemEditForm',
		'preview'
	);

	/**
	 * Renders a preview of the recipient email.
	 */
	public function preview() {
		return $this->customise(new ArrayData(array(
			'Body' => $this->record->getEmailBodyContent(),
			'HideFormData' => $this->record->HideFormData,
			'Fields' => $this->getPreviewFieldData()
		)))->renderWith($this->record->EmailTemplate);
	}

	/**
	 * Get some placeholder field values to display in the preview
	 * @return ArrayList
	 */
	private function getPreviewFieldData() {
		$data = new ArrayList();

		$fields = $this->record->Form()->Fields()->filter(array(
			'ClassName:not' => 'EditableLiteralField',
			'ClassName:not' => 'EditableFormHeading'
		));

		foreach ($fields as $field) {
			$data->push(new ArrayData(array(
				'Name' => $field->dbObject('Name'),
				'Title' => $field->dbObject('Title'),
				'Value' => DBField::create_field('Varchar', '$' . $field->Name),
				'FormattedValue' => DBField::create_field('Varchar', '$' . $field->Name)
			)));
		}

		return $data;
	}
}