docs: def register(self, app, *args, **kwargs):
          " Activate loginmanager and principal. "
  
          if not self._login_manager or self.app != app:
              self._login_manager = LoginManager()
              self._login_manager.user_callback = self.user_loader
              self._login_manager.setup_app(app)
              self._login_manager.login_view = 'urls.index'
              self._login_manager.login_message = u'You need to be signed in for this page.'
  
          self.app = app
  
          if not self._principal:
              self._principal = Principal(app)
              identity_loaded.connect(self.identity_loaded)
  
          super(UserManager, self).register(app, *args, **kwargs)