Since pretty much the second page of this book, you've been reading about how important plugins are and how widely they are used in Drupal 8. I have backed that claim with references to "this or that" being a plugin in basically every chapter. However, I have not really explained how you can create your own custom plugin type. However, since our importer logic is a perfect candidate for plugins, I will do so here, and to exemplify the theory, we will implement an Importer plugin type.
The very first thing a plugin type needs is a manager service. This is responsible for bringing together two critical aspects of plugins (but not only)--discovery and factory (instantiation). For these two tasks, it delegates to specialized ...
