Overriding URLs with Drupal panel pages

/**
 * Implementation of hook_ctools_plugin_directory() to let the system know
 * where our task plugins are.
 */
function foo_ctools_plugin_directory($owner, $plugin_type) {
  return 'plugins/' . $plugin_type;
}

<?php
// $Id$
 
$plugin = array(
  // This is a 'page' task and will fall under the page admin UI
  'task type' => 'page',
 
  'title' => t('Notifications'),
  'admin title' => t('Notifications'),
  'admin description' => t('When enabled, this overrides the default Drupal behavior for the notifications page at <em>/user/%user/notifications</em>.'),
  'admin path' => 'user/%user/notifications',
 
  // Menu hooks so that we can alter the default entry
  // foo is the name of the module; notifications is the name of the task.
  //   (this is arbitrary and the name-structure is a convention 
  //    to avoid function-name collisions with other modules.)
  'hook menu alter' => 'foo_notifications_menu_alter',
 
  // This task provides the 'user' context to content-types on the panel
  'handler type' => 'context',
  'get arguments' => 'foo_notifications_get_arguments',
  'get context placeholders' => 'foo_notifications_get_contexts',
 
  // Allow this panel-page to be enabled or disabled:
  'disabled' => variable_get('foo_notifications_disabled', TRUE),
  'enable callback' => 'foo_notifications_enable',
);

function foo_notifications_menu_alter(&$items, $task) {
  // the enable/disable callback will set a variable to control whether the panel is enabled.
  if (variable_get('foo_notifications_disabled', TRUE)) {
    // the panel is disabled: don't run the menu-alter, leave the default menu entry.
    return;
  }
 
  // Check whether the callbacks at user/%user/notifications match those
  // provided by the notifications module.  If not, then the URL has been overridden already.
  $page_callback = $items['user/%user/notifications']['page callback'];
  $page_arguments = $items['user/%user/notifications']['page arguments'];
 
  $is_using_default = ($page_callback == 'drupal_get_form' && page_arguments == array('notifications_user_overview', 1));
 
  // check that it either matches the callback provided by notifications,
  // or if not, it's already been overridden by another module, so check
  // whether page-manager is configured to override already-overridden pages.
  if ($is_using_default || variable_get('page_manager_override_anyway', FALSE)) {
    $items['user/%user/notifications']['page callback'] = 'foo_notifications_page';
    $items['user/%user/notifications']['file path'] = $task['path'];
    $items['user/%user/notifications']['file'] = $task['file'];
  }
  else {
    // disable the panel page
    variable_set('foo_notifications_disabled', TRUE);
    // the enable-function sets a global, so the message is only displayed when trying to enable the panel, not on every cache-clear.
    if (!empty($GLOBALS['foo_enabling_notifications'])) {
      drupal_set_message(t('Foo module is unable to enable the notifications panel page because some other module already has overridden with %callback.', array('%callback' => $callback)), 'warning');
    }
    return;
  }
}

function foo_notifications_enable($cache, $status) {
  variable_set('foo_notifications_disabled', $status);
  // Set a global flag so that the menu routine knows it needs
  // to set a message if enabling cannot be done.
  if (!$status) {
    $GLOBALS['foo_enabling_notifications'] = TRUE;
  }
}

function foo_notifications_get_arguments($task, $subtask_id) {
  return array(
    array(
      'keyword' => 'user',
      'identifier' => t('User'),
      'id' => 1,
      'name' => 'uid',
      'settings' => array(),
    ),
  );
}
 
/**
 * Callback to get context placeholders provided by this handler.
 */
function foo_notifications_get_contexts($task, $subtask_id) {
  return ctools_context_get_placeholders_from_argument(foo_notifications_get_arguments($task, $subtask_id));
}

/**
 * Entry point for our overridden notifications URL.
 *
 * This function asks its assigned handlers who, if anyone, would like
 * to run with it. If no one does, it passes through to notifications's
 * handler.
 */
function foo_notifications_page($user) {
  // Load my task plugin
  $task = page_manager_get_task('notifications');
 
  // Load the user into a context.
  ctools_include('context');
  ctools_include('context-task-handler');
  $contexts = ctools_context_handler_get_task_contexts($task, '', array($user));
  $args = array($user->uid);
 
  $output = ctools_context_handler_render($task, '', $contexts, $args);
  if ($output !== FALSE) {
    return $output;
  }
 
  // fallback to the default notifications handler:
 
  module_load_include('inc', 'notifications', 'notifications.pages');
 
  $function = 'drupal_get_form';
  $args = array('notifications_user_overview', $user);
 
  foreach (module_implements('page_manager_override') as $module) {
    $callback = $module . '_page_manager_override';
    if (($rc = $callback('notifications')) && function_exists($rc)) {
      $function = $rc;
      break;
    }
  }
 
  // Otherwise, fall back.
  return $function($args);
}