You might want developers building projects with this CMS to be able to change the behaviour of some UIs, or to be able to provide new components that can be consumed by the CMS. Those components can't easily be included in the JS bundle for the CMS, as it would require recompiling the shipped code with outside references.

before and after also accept arrays of constraints.

Registering new services to the Injector If you've created a module using React, it's a good idea to afford other developers an API to enhance those components, forms, and state. To do that, simply register them with Injector.

Instead of overriding a service with your own implementation, you enhance an existing service with your own concerns. This pattern is known as middleware.

In general, add providers to the root module so that the same instance of a service is available everywhere.

you get a new instance of the service with each new instance of that component

How is all this different from mainstream constructors?Because an instance is created by sending a message to an object, and not by some special construct like a constructor invocation, we can replace the receiver of that message with any object that responds to that message. It can be another class (say, an implementation based on polar coordinates), or it can be a factory object that isn’t a class at all.