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Posted on by Katherine Bailey.

Hi Ryan,
that is a great question. So, one area I didn't touch on at all in this post is the Dependency Injection Container. That's what is used in core to inject dependencies. Briefly, you provide configuration in the form of "service definitions" that tell the container what to actually inject when instantiating the class. What doesn't come through in the example above, because I was keeping it simple for the purpose of making the point about loose coupling, is that the SubPathAuto class would actually be a service itself, with a definition along the lines of

subpathauto:
  class: Drupal\subpathauto\SubPathAuto
  arguments: ['@path_processor_alias']

telling the container to inject the existing 'path_processor_alias' service when intantiating the SubPathAuto class.
And since all service definitions can be altered by any module, you could have another module alter this and specify something else to pass in.
Here are the symfony docs explaining how the DI container works http ://symfony.com/doc/current/book/service_container.html It is also covered in the DrupalCon Portland talk I linked to at the start of this post.

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