Angular

ViewProviders are similar to Providers except that the dependencies that you define are visible only to its view children. They are not visible to the Content children.

The providedIn allow us to specify where the dependency needs to be provided in the service class itself instead of in the Angular Module. It also helps to make the service tree shakable i.e. remove the service from the final bundle if the app does not use it.

A singleton service is a service for which only one instance exists in an app. In this tutorial, we will show how to create a singleton service in Angular when the service is in the root module, eagerly loaded module, or lazy loaded module.

The DI system in Angular uses tokens to uniquely identify a Provider. There are three types of tokens that you can create in Angular. They are Type Token, String Token, and Injection Token.

In this tutorial, we will look at how Angular dependency injection works. The Angular creates a hierarchical dependency injection system. It creates a hierarchical tree of Injectors. Each Injector gets their own copy of Angular Providers. Together these two form the core of the Angular dependency injection framework. We will learn how Angular creates the injector tree. How injector resolves the dependency.