Understand Drupal's render and theme

You may say that Drupal does not follow the MVC way. In my opinion, Drupal have the implicit MVC pattern like these: 

• Model: database layer and entity module;
• Controller: menu system;
• View: Drupal render and theme system.

What i want to talk is view layer. Let us still begin the journey with menu_execute_active_handler() in index.php. This function maps the request path with menu and call the function "page callback " defined in the menu array. Usually, the page callback function returns the array data with theme entry. In the following steps, Drupal use drupal_deliver_page and drupal_deliver_html_page to render the theme array-type data.

print drupal_render_page($page_callback_result);

Behind this code statement's scene, drupal use render and theme to supply the look and feel.

• Render system: drupal_render_page() and drupal_render
• Theme system: theme functions or template files

Drupal render methods help to add the page element and invoke the theme to display html markup.

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);}

In the drupal_render, you can add pre_render, wrapper and post_render to decorate your content, but the core is theme element.

if (isset($elements['#theme'])) {    $elements['#children'] = theme($elements['#theme'], $elements);  }

Drupal theme can render the html content by theme function.

 $output = '';  if (isset($info['function'])) {    if (function_exists($info['function'])) {      $output = $info['function']($variables);    }  }  else {    // Default render function and extension.    $render_function = 'theme_render_template';    $extension = '.tpl.php';        }

Function theme_render_template follow the PHP internal way to render file.

function theme_render_template($template_file, $variables) {  extract($variables, EXTR_SKIP);               // Extract the variables to a local namespace  ob_start();                                   // Start output buffering  include DRUPAL_ROOT . '/' . $template_file;   // Include the template file  return ob_get_clean();                        // End buffering and return its contents}

In Summary, this is Drupal default workflow, you can alter them by hooking your extension points.