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Building a simple application that fetches data from an API and displays it in a component. We'll use RxJS operators like Observable, BehaviorSubject, and catchError to handle data streams and errors.

The tutorial covers:

1. Introduction to RxJS in simple terms

2. Setting up a basic Angular app

3. Creating a simple API service using HttpClient

4. Handling API responses with RxJS Observables

5. Handling errors using RxJS

Introduction to RxJS

RxJS (Reactive Extensions for JavaScript) is a library used with Angular to handle asynchronous events and data streams. Think of it as a way to manage and respond to things happening in your app, like API responses, user inputs, or other async tasks.

• Observable: This is like a stream of data. It can emit multiple values over time (such as API data).

• BehaviorSubject: It's like an Observable but has an initial value and can emit new values when updated.

• catchError: It is used to catch and handle errors in the Observable stream.

File Structure

Step-by-Step Code Explanation

1. Post Model

This model represents the structure of a Post object that we will receive from the API.

export interface Post {
  userId: number;
  id: number;
  title: string;
  body: string;
}

2. Post Service

In this service, we will fetch the posts from a fake online API (jsonplaceholder.typicode.com) using HttpClient.

• getPosts() method fetches data from the API as an Observable of an array of Post[].

import { Injectable } from '@angular/core';
import { HttpClient } from "@angular/common/http";
import { Observable } from "rxjs";
import { Post } from "../models/post.model";

@Injectable({
  providedIn: 'root'
})
export class PostService {
  private apiUrl = 'https://jsonplaceholder.typicode.com/posts';

  constructor(private http: HttpClient) {}

  getPosts(): Observable<Post[]> {
    return this.http.get<Post[]>(this.apiUrl);
  }
}

3. App Component

In this component, we will:

• Fetch the posts using the PostService.

• Use RxJS to handle the data and errors.

• Display the posts in the template.

app.component.ts

In the ngOnInit lifecycle hook, we subscribe to the getPosts() method and handle any errors using catchError.

import { Component, OnInit } from '@angular/core';
import { BehaviorSubject, catchError, Observable } from "rxjs";
import { Post } from "./models/post.model";
import { PostService } from "./services/post.service";

@Component({
  selector: 'app-root',
  templateUrl: './app.component.html',
  styleUrls: ['./app.component.css']
})
export class AppComponent implements OnInit {
  posts$: Observable<Post[]> | undefined;

  private errorSubject = new BehaviorSubject<string | null>(null);
  error$ = this.errorSubject.asObservable();

  constructor(private postService: PostService) {}

  ngOnInit(): void {
    this.posts$ = this.postService.getPosts().pipe(
      catchError(err => {
        this.errorSubject.next(err.message);
        return [];
      })
    );
  }
}

4. Template (HTML)

The template will display:

• A list of posts from the posts$ observable.

• An error message if something goes wrong.

<div *ngIf="error$ | async as error">
  <div>
    {{ error }}
  </div>
</div>

<ul *ngIf="posts$ | async as posts">
  <li *ngFor="let post of posts">
    <h3> {{ post.title }} </h3>
    <p> {{ post.body }} </p>
  </li>
</ul>

5. App Module

The module is where we import required Angular libraries like BrowserModule, RouterModule, and HttpClientModule.

import { NgModule } from '@angular/core';
import { BrowserModule } from '@angular/platform-browser';
import {RouterModule, Routes} from '@angular/router';
import {CommonModule} from "@angular/common";
import {AppComponent} from "./app.component";
import {provideAnimationsAsync} from "@angular/platform-browser/animations/async";
import {PostService} from "./services/post.service";
import { provideHttpClient} from "@angular/common/http";

const routes: Routes = [
  { path: '', component: AppComponent },
];

@NgModule({
  imports: [
    CommonModule,
    BrowserModule,
    RouterModule.forRoot(routes, {enableTracing: true}),
  ],

  exports: [RouterModule],

  declarations: [
  AppComponent,
],
  providers: [
  provideAnimationsAsync(),
    PostService,
    provideHttpClient()
  ],

  bootstrap: [
  AppComponent
  ]
  
})
export class AppModule { }

How the App Works

1. Service: The PostService fetches posts from the API using HttpClient.

2. Component: In AppComponent, we subscribe to the posts observable and display them in the HTML.

3. Template: The template displays the posts and also shows an error if something goes wrong using BehaviorSubject and catchError.

OUTPUT

A list of posts, each consisting of a title and body, fetched from an API jsonplaceholder.typicode.com, which provides mock data for testing purposes.

GitHub Link: