@Injectable decorator

What Are @Injectable providers? - Injector decorator

What Are @Injectable providers?
The @Injectable decorator identifies services and other classes that are intended to be injected. It can also be used to configure a provider for those services.

To inject the service into a component, Angular provides an Injector decorator: @Injectable().
A provider defines the set of injectable objects that are available in the injector of this module.

The @Injectable decorator marks a class as available to an injector for instantiation. An injector reports an error when trying to instantiate a class that is not marked as @Injectable.

Injectors are also responsible for instantiating components. At the run-time the injectors can read class metadata in the JavaScript code and use the constructor parameter type information to determine what things to inject.

Injectable decorator and metadata -
  providedIn?: Type<any> | 'root' | null
  factory: () => any

To inject the service into a component, Angular provides an Injector decorator: @Injectable().
Here we configure a provider for CustomerService using the @Injectable decorator on the class.

We have the following steps to create a Service-
1.      Create the service class
2.      Define the metadata with a decorator
3.      Import what we need.

In the above example, providedIn tells Angular that the root injector is responsible for creating an instance of the CustomerService.

The Angular CLI sets up provider automatically when you generating a new service.

Why @Inject()?
The @Inject is a special technique for letting Angular knows that a parameter must be injected.

Inject decorator and metadata-
  token: any

When @Inject () is not present, Injector will use the type annotation of the parameter.
import { Component, OnInit, Inject } from '@angular/core';
import { HttpClient } from '@angular/common/http';

  selector: 'app-customer',
  templateUrl: './customer.component.html',
  styleUrls: ['./customer.component.css']

export class CustomerComponent implements OnInit {

  constructor(@Inject(HttpClient) private http) {
    // use this.http which is the Http provider.

  ngOnInit(){ }

At this point, @Inject is a manual way of specifying this lookup token, followed by the lowercase http argument to tell Angular what to assign it against.

What Is Hierarchical Dependency Injectors?
Angular has a Hierarchical Dependency Injection system. There is actually a tree of injectors that parallel an application's component tree. You can reconfigure the injectors at any level of that component tree.

Explore in detail by using above questions -

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