作者：老葛，北京亚艾元软件有限责任公司，http://www.yaiyuan.com
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。
