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What Is Angular Services?

What Is Angular Service?
Services are commonly used for storing data and making HTTP calls.

The main idea behind a service is to provide an easy way to share the data between the components and with the help of dependency injection (DI) you can control how the service instances are shared.
Services use to fetch the data from the RESTful API.

How to setup and create services?
Before starting to create a service, you just need to set up a dev environment.
Install Node.js and npm if they are not already on your machine.

Then install the Angular CLI globally.
npm install -g @angular/cli

Angular CLI tool will allow you to easily and quickly generate services, components, pipes, and many more files for your project.

Now, create a new project using the CLI command -
ng new MyProject

And go to your created project directory and launch the server.
ng serve --open

Now come to service - execute below the command for generating service class.
ng g service MyService

It will create the two files in the folder - src/app/
1.      my-service.service.spec.ts
2.      my-service.service.ts

Now, import my service file into the angular module - app.module.ts file. It looks like this.
import { AppComponent } from './app.component';
import {MyService}  from './my-service.service';//Import Service

//AppModule class with @NgModule decorator
@NgModule({
  //Static, This is the compiler configuration
  //declarations is used for configure the selectors.
  declarations: [
    AppComponent
  ],
  //Composability and Grouping
  //imports used for composing NgModules together.
  imports: [
    BrowserModule
  ], 
  //Runtime or injector configuration
  //providers is used for runtime injector configuration.
  providers: [MyService],
  //bootstrapped entry component
  bootstrap: [AppComponent]
})
export class AppModule { }

And let’s see the created service class -my-service.service.ts
import { Injectable } from '@angular/core';

@Injectable()
export class MyService {

  constructor() { }
}

Now you need to create service methods to get, post, put, and delete the users.
import { Injectable } from '@angular/core';

@Injectable()
export class MyService {
  constructor() { }

  //I’m using the static data
  //You can also fetch the data using HttpClient service for backend APIs.
  users = [
     { "id": 1, "name": "Anil Singh", 'age' :32 },
     { "id": 2, "name": "Aradhya" , 'age' :32},
     { "id": 3, "name": "Reena Singh" , 'age' :32}
  ]

  /* (method) MyService.getUsers(): {
    "id": number;
    "name": string;
    'age': number;
    }[] */
  getUsers(){
    return this.users;
  }
}


Now, use this service in the user component for display on UI - my-user.component.ts
import { Component, OnInit } from '@angular/core';
import {MyService} from '../my-service.service';

@Component({
  selector: 'app-my-user',
  templateUrl: './my-user.component.html',
  styleUrls: ['./my-user.component.css']
})
export class MyUserComponent implements OnInit {
  userList =[];

  constructor(private service :MyService) {
    //Get the user list by using the my service.
    this.userList = service.getUsers();
  }

  ngOnInit() { }
}


And my-user.component.html
<h4>User List -</h4>
<table border="1">
  <tr>
    <th>ID</th><th>Name</th><th>Age</th>
  </tr>
  <tr *ngFor="let user of userList">
    <td>{{user.id}}</td><td>{{user.name}}</td><td>{{user.Age}}</td>
  </tr>
</table>

Finally, you got the user list on your components HTML.

What Is Singleton Service?
In Angular, two ways to make a singleton service -
1.      Include the service in the AppModule
2.      Declare that the service should be provided in the application root.

The preferred way to create a singleton service - Form beginning to Angular 6 is –
import { Injectable } from '@angular/core';

@Injectable({
  providedIn: 'root',
})
export class CustomerService {
}

Another way to create a singleton service - Include service in the AppModule
customer.service.ts –
import { Injectable } from '@angular/core';

@Injectable()
export class CustomersService {

  constructor() { }
}

And app.module.ts -
import {CustomerService} from './customers.service';

//AppModule class with @NgModule decorator
@NgModule({
  //Static, this is the compiler configuration
  //declarations is used for configure the selectors.
  declarations: [
    AppComponent
  ],
  //Composability and Grouping
  //imports used for composing NgModules together.
  imports: [
    BrowserModule
  ], 
  //Runtime or injector configuration
  //providers is used for runtime injector configuration.
  providers: [CustomerService],
  //bootstrapped entry component
  bootstrap: [AppComponent]
})
export class AppModule { }

For more detail kindly refer the link.... 
ANIL SINGH

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