title: build drupal 8 custom classes for shared functionality between modules (shared library of php classes for drupal)
body: <p>Background:<br>
  I built multiple custom modules in Drupal 7, that use shared classes from a base collection.  Originally I designed the base collection as a single module with an empty form (which is never displayed), and each form module referenced the needed class(es) and related member method(s) within that shared base module.  I am looking at upgrading the modules to Drupal 8 and am wondering if there is a more logical way to implement the needed "shared" module, rather than using a form module containing no form. </p>
  
  <p>Question: <br>
  Is it possible to create a shared library (of classes) in drupal 8 that custom modules can reference?</p>
  
  <p>To illustrate the question: <br>There is a "Category Manager" module which allows the user to manage a categories for a document collection.  That Category Manager implements a class that provides member methods for adding/updating/deleting/getting category data.  There is also a "Document Manager" module which allows management of documents in the same way.  Because the documents are associated with categories, The Document Manager Module references the same "category" class.  Obviously, this "category" class is defined within the base module (which both manager modules refer to).  There are several other examples of shared classes.</p>