How to Inject Services into Other Services in Angular 2?

Service Injection in Angular 2: A Comprehensive Guide

Injecting services into components is a fundamental task in Angular 2. However, the question arises: how can we inject services into other services outside of components? Let's explore the effective way to achieve this.

To inject services into services, we need to employ the @Injectable decorator on each service intended for injection. This decorator enables the creation or retrieval of corresponding type instances in the injector for the current execution chain.

The Angular 2 application employs a hierarchical tree of injectors, where each injector is linked to a specific component. When an @Injectable decorator is applied to a class, Angular 2 attempts to create or fetch instances of the class in the injector tree.

Consider the following application structure:

• AppComponent: The root component of the application
• ChildComponent: A component nested within AppComponent that interacts with Service1
• Service1: A service used by ChildComponent, and injects Service2
• Service2: A service injected into Service1

In this scenario, when ChildComponent is initialized, Angular 2 locates Service1 in the ChildComponent injector, then in the AppComponent injector, and subsequently in the application injector.

Similarly, when Service1 attempts to inject Service2, the same injector lookup process occurs. Thus, we can inject services at various levels of the application based on the desired sharing scope.

If a provider is defined at the application level, the created instance is shared throughout the application, including all components and services. By defining a provider at the component level, the instance is limited to the component itself, its subcomponents, and the services involved in the dependency chain.

This hierarchical injection mechanism offers flexibility in organizing and managing dependencies. For a more comprehensive understanding, refer to the Angular 2 documentation on hierarchical dependency injection.

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