Data and Devops practices and tools: How to use Async pipe in Angular applications

Wednesday, August 26, 2020

How to use Async pipe in Angular applications

In this article, we will learn how to use async pipe in Angular 8 application. We start off with defining an Async pipe and listing its advantages, followed by how to use the code. We will then look at how to create an ASP.NET application and how to write the backend related code using SQL server.

What is Async Pipe?

Async Pipe is an impure pipe which automatically subscribes to an observable to emit the latest values. It not only subscribes to an observable but it also subscribes to promise and calls the method. When the components get destroyed, it automatically unsubscribes it to reduce memory leaks.

Advantages of Async Pipe

Async Pipe makes rendering of data from observable and promise easier.
For promises, it automatically calls the method, and
for observables, it automatically calls subscribe and unsubscribe.

Using the Code

Prerequisites

Basic knowledge of Angular
Visual Studio Code must be installed
Angular CLI must be installed
Node JS must be installed

Step 1

Let's create a new Angular project using the following NPM command:

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ng new asyncPipe

Step 2

Now, let's create a new component by using the following command:

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ng g c async-pipe-example

Step 3

Now, open the async-pipe-example.component.html file and add the following code in the file:

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<div class="card">       
<div class="card-body pb-0">         
<h4 style="text-align:  center">{{SampleMessage}}         </h4>

<div class="row">           
<div class="col-12 col-md-12">             
<div class="card">                    
<div class="card-body position-relative">                 
<div class="table-responsive cnstr-record product-tbl">
<table class="table table-bordered heading-hvr">
	<tbody>
		<tr>
			<td>
			</td>
			<th width="50">Art.No   Brand   Price/Unit   Provider   P. Art.  N  S. A/C
			</th>
		</tr>
	</tbody>
	<tbody>
		<tr>
			<td>
			</td>
			<td align="center">{{product.ArtNo}}    {{product.Brand}}    
			{{product.Price }}    {{product.Provider}}    
            {{product.ProviderArtNo}}    {{product.SalesAccount}}
			<p>Here in the below code we can see how to apply async pipe 
               with our structural directive *ngFor</p>

			<pre lang="C++">

Step 4

Now, open the async-pipe-example.component.ts file and add the following code in this file:

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import { Component, OnInit } from '@angular/core';    
import { ProductsService } from '../product.service';    
import { Observable } from 'rxjs';    
    
@Component({    
  selector: 'app-async-pipe-example',    
  templateUrl: './async-pipe-example.component.html',    
  styleUrls: ['./async-pipe-example.component.css']    
})    
export class AsyncPipeExampleComponent implements OnInit {    
  products = [];    
  products$:Observable<any>;    
  SampleMessage="Example of Angular Async Pipe";    
    
  constructor(private _productService:ProductsService) { }    
    
  ngOnInit() {    
    //this.getProductsUsingSubscribeMethod();    
    this.getProductsUsingAsyncPipe();    
  }    
    
  public getProductsUsingSubscribeMethod() {    
    this.SampleMessage="Example of Angular Subscribe Method";    
    this._productService.getAllProducts().subscribe((data: any) => {    
      this.products =data;    
      console.log(data);    
    })    
  }    
    
  public getProductsUsingAsyncPipe() {    
    this.SampleMessage="Example of Angular Async Pipe";    
    this.products$ =this._productService.getAllProducts();    
  }      
}

Here in the below code, we are using observables of any type and to get the asynchronous flow data, either we can use subscribe method to subscribe the observables or we can simply use 'async pipe' which automatically subscribes to an Observable and returns the latest value and it also unsubscribes automatically to avoid memory leaks.

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public getProductsUsingAsyncPipe() {      
  this.SampleMessage="Example of Angular Async Pipe";      
  this.products$ =this._productService.getAllProducts();      
}

Step 5

Now, open the product.service.ts file and add the following code:

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import { Injectable } from '@angular/core';    
import { HttpClient } from '@angular/common/http';    
  
@Injectable()       
export class ProductsService {      
    private url = '';    
    private baseUrl = "http://localhost:49661/";//Replace it with your http address and port
    constructor(public http: HttpClient) {    
    }      
    getAllProducts() {    
        this.url = this.baseUrl+'api/Product/getAllProducts';    
        return this.http.get<any[]>(this.url);    
    }      
}

WEB API - Create an ASP.NET Application

Follow these steps to create an ASP.NET application.

Step 1

In Visual Studio 2019, click on File -> New -> Project.