现在，包含节点对象的可呈现数组，已经返回来了。这个页面是怎么构件出来的呢？让我们回到函数menu_execute_active_handler上面来。
$page_callback_result = MENU_ACCESS_DENIED;
$page_callback_result = MENU_NOT_FOUND;
这两种特殊情况我们就不分析了。往下看。
if ($deliver) {
$default_delivery_callback = (isset($router_item) && $router_item) ? $router_item['delivery_callback'] : NULL;
drupal_deliver_page($page_callback_result, $default_delivery_callback);
}
else {
return $page_callback_result;
}
我们没有向menu_execute_active_handler传递参数，所以这里的$deliver为TRUE，将会执行if语句里面的代码。
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();
这是我们这里实际执行的语句。用来合成页面的是drupal_render_page函数。这个时候，传递过来的$page_callback_result，只是一个包含节点对象的呈现数组，节点外面的区域、区块是怎么加进来的呢？这是很多初学者的疑问。让我们来看这个函数的定义：
/**
* Renders the page, including all theming.
*
* @param $page
* A string or array representing the content of a page. The array consists of
* the following keys:
* - #type: Value is always 'page'. This pushes the theming through
* page.tpl.php (required).
* - #show_messages: Suppress drupal_get_message() items. Used by Batch
* API (optional).
*
* @see hook_page_alter()
* @see element_info()
*/
function drupal_render_page($page) {
$main_content_display = &drupal_static('system_main_content_added', FALSE);
// Allow menu callbacks to return strings or arbitrary arrays to render.
// If the array returned is not of #type page directly, we need to fill
// in the page with defaults.
if (is_string($page) || (is_array($page) && (!isset($page['#type']) || ($page['#type'] != 'page')))) {
drupal_set_page_content($page);
$page = element_info('page');
}
// Modules can add elements to $page as needed in hook_page_build().
foreach (module_implements('page_build') as $module) {
$function = $module . '_page_build';
$function($page);
}
// Modules alter the $page as needed. Blocks are populated into regions like
// 'sidebar_first', 'footer', etc.
drupal_alter('page', $page);
// If no module has taken care of the main content, add it to the page now.
// This allows the site to still be usable even if no modules that
// control page regions (for example, the Block module) are enabled.
if (!$main_content_display) {
$page['content']['system_main'] = drupal_set_page_content();
}
return drupal_render($page);
}
这个时候会组装$page这个数组，开始的时候，只包含节点部分，注意往下执行的时候，粗体部分的代码，定义了hook_page_build钩子函数，允许其它模块向$page数组追加内容。难道block模块实现了这个钩子函数？
让我们来试一下，打开block.module文件，搜索block_page_build，还真找到了。真聪明，你猜对了。我们来看一下block模块的具体实现。
/**
* Implements hook_page_build().
*
* Renders blocks into their regions.
*/
function block_page_build(&$page) {
global $theme;
// The theme system might not yet be initialized. We need $theme.
drupal_theme_initialize();
// Fetch a list of regions for the current theme.
$all_regions = system_region_list($theme);
$item = menu_get_item();
if ($item['path'] != 'admin/structure/block/demo/' . $theme) {
// Load all region content assigned via blocks.
foreach (array_keys($all_regions) as $region) {
// Assign blocks to region.
if ($blocks = block_get_blocks_by_region($region)) {
$page[$region] = $blocks;
}
}
// Once we've finished attaching all blocks to the page, clear the static
// cache to allow modules to alter the block list differently in different
// contexts. For example, any code that triggers hook_page_build() more
// than once in the same page request may need to alter the block list
// differently each time, so that only certain parts of the page are
// actually built. We do not clear the cache any earlier than this, though,
// because it is used each time block_get_blocks_by_region() gets called
// above.
drupal_static_reset('block_list');
}
else {
// Append region description if we are rendering the regions demo page.
$item = menu_get_item();
if ($item['path'] == 'admin/structure/block/demo/' . $theme) {
$visible_regions = array_keys(system_region_list($theme, REGIONS_VISIBLE));
foreach ($visible_regions as $region) {
$description = '<div class="block-region">' . $all_regions[$region] . '</div>';
$page[$region]['block_description'] = array(
'#markup' => $description,
'#weight' => 15,
);
}
$page['page_top']['backlink'] = array(
'#type' => 'link',
'#title' => t('Exit block region demonstration'),
'#href' => 'admin/structure/block' . (variable_get('theme_default', 'bartik') == $theme ? '' : '/list/' . $theme),
// Add the "overlay-restore" class to indicate this link should restore
// the context in which the region demonstration page was opened.
'#options' => array('attributes' => array('class' => array('block-demo-backlink', 'overlay-restore'))),
'#weight' => -10,
);
}
}
}
Block模块，在这里会找到当前主题的所有的区域，对于每个区域，会加载这个区域里面的所有的当前可用的区块。以区域的机读名字为键，把区域里面的内容追加到$page数组上来。
代码里面的:
if ($item['path'] == 'admin/structure/block/demo/' . $theme) {
这是一种非常特殊的情况，只有在区块的管理界面，显示一个主题的演示区域的时候，才会用到，所以里面的代码，我们这里不用深究。
现在，整个$page数组已经构建完成了，Drupal是怎么把它转为HTML页面的呢？注意这里的这个数组的类型是page，这个和表单元素数组，是一样的。Drupal6里面的表单数组，在Drupal7下，概念上做了进一步的扩充，除了表单元素以外，区块、节点、页面都是以呈现数组的形式出现。所有的呈现数组，经过drupal_render函数处理，就会转换成对应HTML形式。我们来看一下drupal_render的定义。
function drupal_render(&$elements) {
// Early-return nothing if user does not have access.
if (empty($elements) || (isset($elements['#access']) && !$elements['#access'])) {
return;
}
// Do not print elements twice.
if (!empty($elements['#printed'])) {
return;
}
// Try to fetch the element's markup from cache and return.
if (isset($elements['#cache'])) {
$cached_output = drupal_render_cache_get($elements);
if ($cached_output !== FALSE) {
return $cached_output;
}
}
// If #markup is set, ensure #type is set. This allows to specify just #markup
// on an element without setting #type.
if (isset($elements['#markup']) && !isset($elements['#type'])) {
$elements['#type'] = 'markup';
}
// If the default values for this element have not been loaded yet, populate
// them.
if (isset($elements['#type']) && empty($elements['#defaults_loaded'])) {
$elements += element_info($elements['#type']);
}
// Make any final changes to the element before it is rendered. This means
// that the $element or the children can be altered or corrected before the
// element is rendered into the final text.
if (isset($elements['#pre_render'])) {
foreach ($elements['#pre_render'] as $function) {
if (function_exists($function)) {
$elements = $function($elements);
}
}
}
// Allow #pre_render to abort rendering.
if (!empty($elements['#printed'])) {
return;
}
// Get the children of the element, sorted by weight.
$children = element_children($elements, TRUE);
// Initialize this element's #children, unless a #pre_render callback already
// preset #children.
if (!isset($elements['#children'])) {
$elements['#children'] = '';
}
// Call the element's #theme function if it is set. Then any children of the
// element have to be rendered there.
if (isset($elements['#theme'])) {
$elements['#children'] = theme($elements['#theme'], $elements);
}
// If #theme was not set and the element has children, render them now.
// This is the same process as drupal_render_children() but is inlined
// for speed.
if ($elements['#children'] == '') {
foreach ($children as $key) {
$elements['#children'] .= drupal_render($elements[$key]);
}
}
// Let the theme functions in #theme_wrappers add markup around the rendered
// children.
if (isset($elements['#theme_wrappers'])) {
foreach ($elements['#theme_wrappers'] as $theme_wrapper) {
$elements['#children'] = theme($theme_wrapper, $elements);
}
}
// Filter the outputted content and make any last changes before the
// content is sent to the browser. The changes are made on $content
// which allows the output'ed text to be filtered.
if (isset($elements['#post_render'])) {
foreach ($elements['#post_render'] as $function) {
if (function_exists($function)) {
$elements['#children'] = $function($elements['#children'], $elements);
}
}
}
// Add any JavaScript state information associated with the element.
if (!empty($elements['#states'])) {
drupal_process_states($elements);
}
// Add additional libraries, CSS, JavaScript an other custom
// attached data associated with this element.
if (!empty($elements['#attached'])) {
drupal_process_attached($elements);
}
$prefix = isset($elements['#prefix']) ? $elements['#prefix'] : '';
$suffix = isset($elements['#suffix']) ? $elements['#suffix'] : '';
$output = $prefix . $elements['#children'] . $suffix;
// Cache the processed element if #cache is set.
if (isset($elements['#cache'])) {
drupal_render_cache_set($output, $elements);
}
$elements['#printed'] = TRUE;
return $output;
}
这里面注意缓存的相关代码，Drupal缓存无处不在。另外需要注意的是，呈现数组，就是一个树状结构，一个枝干下面会有子枝干，子枝干下面又有子枝干，一直递归下去，最后是树叶。将呈现数组转为HTML的时候，也是这样递归调用的，最先转成HTML的，就是最小的单元组成部分，慢慢地合成，最后整个呈现数组彻底转为HTML。
当Drupal执行完HTTP请求后，调用drupal_page_footer。
/**
* Performs end-of-request tasks.
*
* This function sets the page cache if appropriate, and allows modules to
* react to the closing of the page by calling hook_exit().
*/
function drupal_page_footer() {
global $user;
module_invoke_all('exit');
// Commit the user session, if needed.
drupal_session_commit();
if (variable_get('cache', 0) && ($cache = drupal_page_set_cache())) {
drupal_serve_page_from_cache($cache);
}
else {
ob_flush();
}
_registry_check_code(REGISTRY_WRITE_LOOKUP_CACHE);
drupal_cache_system_paths();
module_implements_write_cache();
system_run_automated_cron();
}
这里提供了一个hook_exit钩子函数，允许第三方模块在HTTP请求结束时，与Drupal进行交互。然后就是会话提交，页面缓存设置。这里的ob_flush，用来刷新输出缓冲区，可以看作引导指令页面头部阶段ob_start()的一个回应。
再往下面就是注册表检查、缓存、自动运行定时任务。我们看到，定时任务的运行，是在HTTP请求结束后，运行的，这样就不会影响返回当前页面的性能。
通过阅读Drupal核心的代码，我们弄明白了，Drupal一个普通节点页面的生成过程。如果你还有不明白的地方，可以多看几遍，把每个函数都认真的读一遍，遇到不懂的函数，查一下文档。