Services and the dependency injection container

Another architecturally important element of Drupal 8 is the Symfony dependency injection component (http://symfony.com/doc/current/components/dependency_injection.html), concretely represented by the service container.

This component is a staple of modern OOP PHP programming and as such has become foundational to Drupal 8. It allows us to create services that can be injected in various places (and receive themselves services as dependencies). They are then used for the heavy business logic of our functionality. Additionally, they are at times also used as an extension point because the service container is able to collect certain services that are marked as serving a specific purpose and use them automatically. In other words, simply by defining a simple service, we can provide our own functionality or even change the existing logic.

We will encounter many services, and we will see how we can declare our own later in this book.