我们在node_menu里面，并没有定义'delivery callback'，所以这里的$default_delivery_callback将会使用Drupal的默认值。$page_callback_result就是刚才返回的包含节点对象的呈现数组。现在，让我们来看一下drupal_deliver_page，是怎么将这个数组转换成整个页面，并返回给浏览器的。
function drupal_deliver_page($page_callback_result, $default_delivery_callback = NULL) {
if (!isset($default_delivery_callback) && ($router_item = menu_get_item())) {
$default_delivery_callback = $router_item['delivery_callback'];
}
$delivery_callback = !empty($default_delivery_callback) ? $default_delivery_callback : 'drupal_deliver_html_page';
// Give modules a chance to alter the delivery callback used, based on
// request-time context (e.g., HTTP request headers).
drupal_alter('page_delivery_callback', $delivery_callback);
if (function_exists($delivery_callback)) {
$delivery_callback($page_callback_result);
}
else {
// If a delivery callback is specified, but doesn't exist as a function,
// something is wrong, but don't print anything, since it's not known
// what format the response needs to be in.
watchdog('delivery callback not found', 'callback %callback not found: %q.', array('%callback' => $delivery_callback, '%q' => $_GET['q']), WATCHDOG_ERROR);
}
}
实际上我们并没有在任何地方设置$delivery_callback，所以将会使用默认的'drupal_deliver_html_page'。这里的
drupal_alter('page_delivery_callback', $delivery_callback);
用来允许第三方模块修改这里的$delivery_callback，但是实际上，我基本上没有见过哪个第三方模块实现了这个钩子函数。Drupal提供了太多的钩子函数，有些可能从来没有被使用过。
function_exists负责检查函数是否存在，drupal_deliver_html_page这个函数是存在的，所以这里实际调用的是：
drupal_deliver_html_page($page_callback_result);
为什么不直接调用drupal_deliver_html_page呢？我们要的就是html页面啊。Drupal核心的开发者是这样考虑的，这个地方留个接口，这样第三方模块就可以实现，比如说：
drupal_deliver_json_page
drupal_deliver_xml_page
也就是说，Drupal不仅仅支持HTML，返回有可能还是json、xml等其它格式。我们这里只用到了html，所以让我们来看一下默认的具体实现。
/**
* Packages and sends the result of a page callback to the browser as HTML.
*
* @param $page_callback_result
* The result of a page callback. Can be one of:
* - NULL: to indicate no content.
* - An integer menu status constant: to indicate an error condition.
* - A string of HTML content.
* - A renderable array of content.
*
* @see drupal_deliver_page()
*/
function drupal_deliver_html_page($page_callback_result) {
// Emit the correct charset HTTP header, but not if the page callback
// result is NULL, since that likely indicates that it printed something
// in which case, no further headers may be sent, and not if code running
// for this page request has already set the content type header.
if (isset($page_callback_result) && is_null(drupal_get_http_header('Content-Type'))) {
drupal_add_http_header('Content-Type', 'text/html; charset=utf-8');
}
// Send appropriate HTTP-Header for browsers and search engines.
global $language;
drupal_add_http_header('Content-Language', $language->language);
// Menu status constants are integers; page content is a string or array.
if (is_int($page_callback_result)) {
// @todo: Break these up into separate functions?
switch ($page_callback_result) {
case MENU_NOT_FOUND:
// Print a 404 page.
drupal_add_http_header('Status', '404 Not Found');
watchdog('page not found', check_plain($_GET['q']), NULL, WATCHDOG_WARNING);
// Check for and return a fast 404 page if configured.
drupal_fast_404();
// Keep old path for reference, and to allow forms to redirect to it.
if (!isset($_GET['destination'])) {
$_GET['destination'] = $_GET['q'];
}
$path = drupal_get_normal_path(variable_get('site_404', ''));
if ($path && $path != $_GET['q']) {
// Custom 404 handler. Set the active item in case there are tabs to
// display, or other dependencies on the path.
menu_set_active_item($path);
$return = menu_execute_active_handler($path, FALSE);
}
if (empty($return) || $return == MENU_NOT_FOUND || $return == MENU_ACCESS_DENIED) {
// Standard 404 handler.
drupal_set_title(t('Page not found'));
$return = t('The requested page "@path" could not be found.', array('@path' => request_uri()));
}
drupal_set_page_content($return);
$page = element_info('page');
print drupal_render_page($page);
break;
case MENU_ACCESS_DENIED:
// Print a 403 page.
drupal_add_http_header('Status', '403 Forbidden');
watchdog('access denied', check_plain($_GET['q']), NULL, WATCHDOG_WARNING);
// Keep old path for reference, and to allow forms to redirect to it.
if (!isset($_GET['destination'])) {
$_GET['destination'] = $_GET['q'];
}
$path = drupal_get_normal_path(variable_get('site_403', ''));
if ($path && $path != $_GET['q']) {
// Custom 403 handler. Set the active item in case there are tabs to
// display or other dependencies on the path.
menu_set_active_item($path);
$return = menu_execute_active_handler($path, FALSE);
}
if (empty($return) || $return == MENU_NOT_FOUND || $return == MENU_ACCESS_DENIED) {
// Standard 403 handler.
drupal_set_title(t('Access denied'));
$return = t('You are not authorized to access this page.');
}
print drupal_render_page($return);
break;
case MENU_SITE_OFFLINE:
// Print a 503 page.
drupal_maintenance_theme();
drupal_add_http_header('Status', '503 Service unavailable');
drupal_set_title(t('Site under maintenance'));
print theme('maintenance_page', array('content' => filter_xss_admin(variable_get('maintenance_mode_message',
t('@site is currently under maintenance. We should be back shortly. Thank you for your patience.', array('@site' => variable_get('site_name', 'Drupal')))))));
break;
}
}
elseif (isset($page_callback_result)) {
// Print anything besides a menu constant, assuming it's not NULL or
// undefined.
print drupal_render_page($page_callback_result);
}
// Perform end-of-request tasks.
drupal_page_footer();
}
最前面的两端代码，是用来添加http header的（drupal_add_http_header），我们不用去管它。
再往下是对$page_callback_result做了整数判断。
if (is_int($page_callback_result)) {
什么时候会是整数呢，当MENU_NOT_FOUND、MENU_ACCESS_DENIED、MENU_SITE_OFFLINE的时候，会是整数。我们这里面返回的是数组，不是整数，我们这里也就跳过了这三种特殊情况，继续往下看代码。
elseif (isset($page_callback_result)) {
// Print anything besides a menu constant, assuming it's not NULL or
// undefined.
print drupal_render_page($page_callback_result);
}
// Perform end-of-request tasks.
drupal_page_footer();