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Hello, everyone, and welcome back.

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The last lesson we wrote, our own provider for the cause is service dependency, and we have defined

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here a unique injection token.

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Now, notice that normally we don't need to write these functions or provide injection tokens.

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It all works in a much simpler way.

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So let's see exactly what Engelhard is doing under the hood when we are not using the inject decorated

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or manual profiler's.

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So how does Angela know, for example, what dependency to inject here without using here to inject

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liquor there?

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Because our previous version of the program worked fine without it?

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Well, the answer is very simple.

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What's going on is that angular supports as injection tokens, class names.

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So we here in the provide function if instead of creating here a manual injection token, we specify

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here the name of the class.

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These will also work as a unique identifier for a dependency.

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Let's try this out.

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If we reload here, the application, we're going to see that everything is still working as expected.

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So this is one of the key features of the angular dependency injection system.

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It's possible to identify dependencies using class names as a unique identifier.

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It's possible to use a class name as a dependency injection token because the last name exists at runtime

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under the form of a constructor function.

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On the other hand, we would not be able to use an interface as an injection token.

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And that's because an interface does not exist at runtime.

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It's a compile time construct only.

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And the other thing that we don't need to do is to write our own manual provider function.

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And the reason for that is that if to the angular dependency injection system, we pass these use class

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property instead of use factory and the dependencies and we specify here the name of a class in this

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case causes service, then Angular knows that it needs to call the new constructor on this class and

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it needs to provide the dependencies that are needed by the constructor.

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So that's one thing that Angular already knows how to do.

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So we don't need to write this function either.

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And we can remove here the injection token.

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We won't be needing it anymore.

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We don't need our manually written injection token anymore because Angular internally is using the class

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name as a unique identifier for the injectable.

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Notice that this configuration in its current form has some redundancy.

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We are using the courtesy service class on multiple places.

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This can be further simplified into the following notation.

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We are going to remove here the complete configuration object and we're going to pass in here only the

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class name and nothing more.

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With this notation.

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Angular knows that this is the unique identifier of the dependency is being used as an injection token.

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And also this is a constructor function that allows ANGULAR to instantiate the dependency itself.

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So with this, the dependency injection system has all the necessary information for knowing how to

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create here.

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The course is service dependency.

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Now notice the following.

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So far we have been using here this provider's property of the component so each component can specify

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its own provider's list.

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This is not the usual way that we define application wide singletons.

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Usually we define them here at the level of the application module.

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So let's learn exactly what does it mean for dependency to be creative, the level of a given component.

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Let's talk about hierarchical dependency injection.

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This is coming right up in the next lesson.