MODX Client Preview Plugin

client_preview.plugin.php

<?php
if ($modx->context->get('key') === 'mgr') return;
$key = $modx->getOption('key', $scriptProperties, 'my-secret-key');
$val = $modx->getOption('val', $scriptProperties, 'my-secret-value');

switch ($modx->event->name) {
    case 'OnHandleRequest':
        if (!$modx->getOption('site_status')) {
            if ($_GET[$key] === $val) {
                $modx->config['site_status'] = 1;
            } else {
                $modx->config['site_status'] = 0;
            }
        }
        break;
    case 'OnLoadWebDocument':
    case 'OnLoadWebPageCache':
        if (!$modx->getOption('site_status')) {
            if ($_GET[$key] === $val) {
                if ($modx->resource instanceof modResource) $modx->resource->set('cacheable', false);
            }
        }
        break;
    default:
        break;
}