Angular RxJS Observable Class

Objective: In this article, you will use the HTTP library in Angular in conjunction with RxJS, HTTP GET request, RxJS Observables to handle an asynchronous web API request.

Pre-requisite Prior to completing this article, you should have already installed all pre-requisite tooling including: Visual Studio Code, Node Package Manager (NPM), Node, Angular CLI.

Setup✨

1. On your local machine, open Visual Studio Code.
2. Go to the File menu and select the Open Folder option.
3. Create a new project for this exercise and select this folder.
4. Create a new project: open terminal by Ctrl + backtic(`) key then run ng new angular-medium command

Bootstrapping Your Environment✌

1. In Visual Studio Code, Ctrl + backtic(`) key press and select the Open in Terminal option.
2. Run the angular-medium project using npm:

npm start

To start the port correctly in your desired port use:

npm start --port 8000 --open

Add the Http Module

1. Within the app folder, open the app.module.ts file.
2. Add an import statement to the top of your file that imports the HttpClientModule module from the @angular/common/http package:

import {HttpClientModule} from '@angular/common/http';

3.Update the NgModule decorator by adding the HttpClientModule module to the values in the import array property of the NgModule decorator:

@NgModule({
    imports:      [ 
        BrowserModule,
        HttpClientModule
    ],
    declarations: [ 
        AppComponent
    ],
    bootstrap: [ AppComponent ]
})
export class AppModule { }

Invoke HTTP Get Method

1. Within the app folder, open the app.component.ts file.
2. Add a new import statement to import the HttpClient from the @angular/common/http library module:

import { HttpClient } from '@angular/common/http';

3.Add a new import statement to import the Observable member (class) from the rxjs library module:

import {Observable} from 'rxjs';

4.Within the AppComponent class, add a new property named dataItems of type string:

export class AppComponent {
    dataItems2:ItemList[]=[];
}

5.Within the AppComponent class, add a new empty constructor:

export class AppComponent {
    dataItems2:ItemList[]=[];
    constructor() {
    }
}

6.Add a model named ItemList. And import into app.component.ts file

import { ItemList } from './model';
export class ItemList{
    public userId: number;
    public id: number;
    public title: string
}

7.Update the constructor by adding a parameter of type httpclient:

constructor(private httpclient : HttpClient) {
}

8.Within the AppComponent class, add a new method named getDummyApiData2:

export class AppComponent {
    dataItems2:ItemList[]=[];
    constructor(private httpclient : HttpClient) { ... }
    private getDummyApiData2() {
    }
}

9.Update the getDummyApiData2 method signature by adding return type of Observable:

private getDummyApiData2() : Observable<ItemList[]> {
}

10.Within the getDummyApiData2 method, return the result of invoking the get method on the httpclient private variable:

• https://jsonplaceholder.typicode.com/posts

private getbiponIPAddress() : Observable<ItemList[]> {
    return this.httpclient.get<ItemList[]>('https://jsonplaceholder.typicode.com/posts');
}

11.Returning to the empty constructor, add a line of code to invoke the getDummyApiData2 method:

constructor(private httpclient : HttpClient) {
    this.getDummyApiData2()
}

12.Subscribe to the data being available by invoking the subscribe method:

constructor(private httpclient : HttpClient) {
    this.getDummyApiData2()
      .subscribe(res => {
         this.dataItems2 = res;
    });
}

13.Update the subscribe method by adding an inline anonymous function to get the result of the IP Address request and save it to the dataItems2 property in the AppComponent class:

constructor(private httpclient : HttpClient) {
    this.getDummyApiData2()
      .subscribe(res => {
         this.dataItems2 = res;
    });
}

14.Your final app.component.ts class should now look like this:👀

import {Component} from '@angular/core';
import { HttpClient } from '@angular/common/http';
import {Observable} from 'rxjs';
@Component({
    selector: 'app',
    templateUrl: './app.component.html',
    styleUrls: './app.component.css',
})
export class AppComponent {
    dataItems2:ItemList[]=[];
constructor(private httpclient : HttpClient) {
    this.getDummyApiData2()
      .subscribe(res => {
         this.dataItems2 = res;
    });
  }
    private getDummyApiData2() : Observable<ItemList[]> {
     return this.httpclient.get<ItemList[]>('https://jsonplaceholder.typicode.com/posts');
    }
}

Another way

Just restructure for smarter way to data access.

app.component.ts

import { SampleService } from './sample.service';
dataItems:ItemList[]=[];
constructor(
      private sampleService:SampleService, 
      private httpclient: HttpClient) {  
        this.getDummyApiData();
    }
private getDummyApiData(){
      this.sampleService.getDataList().subscribe(data=>{
      this.dataItems=data;
})

sample.service.ts file

getDataList():Observable<ItemList[]>{
    return this.http.get<ItemList[]('https://jsonplaceholder.typicode.com/posts');
  }

app.component.html file

<ul>
    <li *ngFor="let item of dataItems">
        {{item.title}}
    </li> 
</ul>

Render HTTP Response

1. Within the app folder, open the app.component.html file.
2. Render the value of the dataItems2 property iterate by adding *ngFor directive and a template expression:

<h1>Dummy App</h1>
<strong> Example List Item::</strong>
<ul>
    <li *ngFor="let item of dataItems2">
        {{item.title}}
    </li> 
</ul>

Output

Github
StackBlitz

Reference

Fake Online REST API for Testing and Prototyping
TypeScript
Angular CLI
Right way to make API calls