The D7 class registry has been replaced with a static class map, built through the hook system. This introduces hook_autoload_info().

Syntax Changes

Taking system.module as an example.

Before (in system.info file):

files[] = system.archiver.inc
files[] = system.mail.inc
files[] = system.queue.inc
files[] = system.tar.inc
files[] = system.updater.inc

After (in system.module file):

/**
 * Implements hook_autoload_info().
 */
function system_autoload_info() {
  return array(
    'ArchiverTar' => 'system.archiver.inc',
    'ArchiverZip' => 'system.archiver.inc',
    'DefaultMailSystem' => 'system.mail.inc',
    'TestingMailSystem' => 'system.mail.inc',
    'DrupalQueue' => 'system.queue.inc',
    'DrupalQueueInterface' => 'system.queue.inc',
    'DrupalReliableQueueInterface' => 'system.queue.inc',
    'SystemQueue' => 'system.queue.inc',
    'MemoryQueue' => 'system.queue.inc',
    'Archive_Tar' => 'system.tar.inc',
    'ModuleUpdater' => 'system.updater.inc',
    'ThemeUpdater' => 'system.updater.inc',
  );
}

This change also requires no mixing of procedural and OO code in the same files. Previously we had a few random classes defined in places like node.module and entity.module. These classes have to be in their own files so that they don't trigger the autoloader just by extending a class.

Introduced in branch: 
1.0.x
Introduced in version: 
1.0.0
Impacts: 
Module developers
Related Github Issues: 
https://github.com/backdrop/backdrop-issues/issues/77