Fetching Data in Angular2

One of the most common scenario in any application is client interacting with the server. HTTP is the widely used protocol for this interaction. One can fetch data from the server, update data, create data and delete it using HTTP protocol.

The focus of this blog is to discuss the GET method of HTTP protocol.

In Angular1.x, we used $http service which provided us a get() method to fetch data from server. A simple GET request in Angular1.x was something like:

$http({method: 'GET', url: '/someUrl'})
    .then(function successCallback(response) {}, 
        function errorCallback(response) {}); 

As it can be seen above, $http returns a promise where we register two callbacks for success and error.

Now, let's move on to Angular2, and see how stuff works in it. Angular2 has Http service which is used to make get calls to server. But an important thing to note here is that $http service in Angular1.x returned a promise while Http service in Angular2 returns Observables.

So, before we dive deeper into Http service, let's quickly have a glimpse at Observables:

To start off with, Observables are nothing but a stream of data.These data streams can be of anything - a stream of variables, properties, data structures or even stream of events. One can react to the stream by listening to it. Observables are basically based on Observer Design Pattern. In Observer Design Pattern one-to-many dependency is maintained between the objects, when one object changes its state all other objects/dependents are notified. These dependents are known as Observers.

A stream can emit 3 different things:

  1. Value
  2. Error
  3. Completed signal

Suppose that stream is a stream of events being observed. A function is defined that will be executed when a value is emitted, another function executes when an error is emitted and a third one once the complete signal is emitted. One can capture these events by using these functions. These functions are known as Observers and the stream which is being emitted is the Observable.

Observables can be of two types:

1.Hot - Hot observables are those which produce values even before their subscription gets activated. One can consider Hot Observables as live performance. The hot observable sequence is shared among each subscriber, also each subscriber gets the next value in the sequence.

2.Cold - Cold observables behave like standard iterators. They push values only when we subscribes to them and they reset when we subscribe again. One can consider Cold Observables as a movie.

Angular2 has chosen Rxjs as its core async pattern. Rxjs provides a number of operators attached to a stream such as map, filter, scan, flatMap, toPromise, catch.

Well, the above discussion is not even a tip of the iceberg on a subject such as Observable. You can read out more from here and here.

Let's now move back the original agenda of this blog i.e. fetching data using Http service. Here is a sample use case:

We need to display a list of posts. The list of posts can be fetched through this API - http://jsonplaceholder.typicode.com/posts/.

To achieve the above scenario let's break this small app into parts:

  1. AppComponent - This is parent component for our application.
  2. PostComponent - This is child component inside our AppComponent. It will currently have PostListComponent as its child component. Tomorrow, if we plan to display the detail of a post, we may add PostDetailComponent to display the details.
  3. Post - We make Post interface to define the type of element that we will receive from the GET api.
  4. PostService - This service will actually fetch the data via making GET call on the api for us.

Here is our app.component.ts:

import {Component} from '@angular/core';
import {PostComponent} from './post/post.component'
import './rxjs-operators';

@Component({
    selector: 'my-app',
    template: `
        <h1>Fetching:</h1>
        <post-parent></post-parent>
    `,
    directives: <any>[PostComponent]
})

export class AppComponent {
}

and here is the post.component.ts:

import {Component}  from '@angular/core';
import {PostListComponent} from './post-list.component';
import {PostService} from './post.service';

@Component({
    selector: 'post-parent',
    template: `
        <h2>View Posts</h2>
        <post-list></post-list>
    `,
    directives: <any>[PostListComponent],
    providers: <any>[PostService]
})
export class PostComponent {
}

We have injected PostService. We register it as a provider by doing providers:[PostService] so that its instance is available to all the child components of PostComponent. In case you are not aware about the Angular2 Services, you can have a quick read Services in Angular2.

Let's see the post.ts, where we define the Post:

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

Now, let's have a look at our post-list.component.ts which exports the PostListComponent:

import {Component} from '@angular/core';
import {PostService} from './post.service';
import {Post} from './post';

@Component({
    selector: 'post-list',
    template: `
        <div>
        </div>
    `
})

export class PostListComponent {
    constructor(private _postDataService:PostService) {
        this.getPosts();
    }

    private posts:Post[] = [];
    private errorMessage:any = '';

    getPosts() {
        //To Do: Fetch Posts here using PostsDataService
    }
}

Couple of most important tasks are still pending in the above component:

  1. We haven't yet written any code to display the list of post.
  2. We still need to fetch the data from server using the PostService i.e. the definition part of the getPosts() function.

So, let's move to the post.service.ts where a lot of action will actually take place:

import {Injectable} from "@angular/core";
import {Post} from './post';

@Injectable()
export class PostService {
}

Now, let's start one by one:

  1. First, we need to import Http and Response from @angular/http and also need to import Observable from rxjs/Observable. So our post.service.ts would now be:

    import {Injectable} from "@angular/core";
    import {Post} from './post';
    import { Http, Response } from '@angular/http';
    import { Observable } from 'rxjs/Observable';
    
    @Injectable()
    export class PostService {
    }
  2. We need to use a few operators in our getData() function so we need to import them. Instead of importing all the operators let's import the required ones in rxjs-operators.ts and then import this into our app.component.ts.

    rxjs-operators.ts:

    import 'rxjs/add/operator/catch';
    import 'rxjs/add/operator/map';
    import 'rxjs/add/operator/toPromise';

    app.components.ts:

    import {Component} from '@angular/core';
    import {PostComponent} from './post/post.component'
    import './rxjs-operators';
    
    @Component({
        selector: 'my-app',
        template: `
          <h1>Fetching:</h1>
          <post-parent></post-parent>
        `,
        directives:[PostComponent]
    })
    
    export class AppComponent {
    }
  3. Now, we need to have a getData() function which will get posts from the api. So here is what our getData() function should be like:

    getData():Observable<Post[]> {
        return this.http.get('http://jsonplaceholder.typicode.com/posts/')
            .map(this.extractData)
            .catch(this.handleError);
    }

The api http://jsonplaceholder.typicode.com/posts/ returns us an array of post whereas our http.get would return us an Observable. We then use the map operator which transforms the response emitted by Observable by applying a function to it. So in case of success, our flow would now move to extractData() function, which is:

private extractData(res:Response) {
    let body = res.json();
    return body || [];
}

In the above snippet we are transforming response to the json format by doing res.json().

But in case had we encountered an error, our flow would have moved to catch operator. The catch operator intercepts an onError notification from Observable and continues the sequence without error. handleError() function would have come into play in that case:

private handleError(error:any) {
    // In a real world app, we might use a remote logging infrastructure
    // We'd also dig deeper into the error to get a better message
    let errMsg = (error.message) ? error.message :
        error.status ? `${error.status} - ${error.statusText}` : 'Server error';
    console.error(errMsg); // log to console instead
    return Observable.throw(errMsg);
}

After joining all the parts, our post.service.ts would look like:

import {Injectable} from "@angular/core";
import {Http, Response} from '@angular/http';
import {Observable} from 'rxjs/Observable';
import {Post} from './post';

@Injectable()
export class PostService {
    constructor(private http:Http) {
    }

    getData():Observable<Post[]> {
        return this.http.get('http://jsonplaceholder.typicode.com/posts/')
            .map(this.extractData)
            .catch(this.handleError);
    }

    private extractData(res:Response) {
        let body = res.json();
        return body || [];
    }

    private handleError(error:any) {
        // In a real world app, we might use a remote logging infrastructure
        // We'd also dig deeper into the error to get a better message
        let errMsg = (error.message) ? error.message :
            error.status ? `${error.status} - ${error.statusText}` : 'Server error';
        console.error(errMsg); // log to console instead
        return Observable.throw(errMsg);
    }
}

We should note that the above Observable is a cold observable. So one has to subscribe to it.

Now, let's move back to the PostListComponent and complete our pending stuff:

  1. We will first add definition part to our getPosts() function:

    getPosts() {
        this._postDataService.getData()
            .subscribe(
                posts => this.posts = posts,
                error => this.errorMessage = <any>error);
    }

    We can see the subscribe operator in the above snippet. In Rxjs one can subscribe to an Observable by passing 0 to 3 individual functions onNext, onError and onCompleted.

  2. Now, we need to display the fetched post in this PostListComponent. So our template would like:

    <div>
        <ul class="items">
            <li *ngFor="let post of posts">
                <span>{{post.title}}</span>
            </li>
        </ul>
    </div>

In case you are not aware about how to iterate over Arrays, Map, Set you can have a quick read here.

So now our complete PostListComponent would look like:

import {Component} from '@angular/core';
import {PostService} from './post.service';
import {Post} from './post';

@Component({
    selector: 'post-list',
    template: `
        <div>
            <ul class="items">
                <li *ngFor="let post of posts">
                    <span>{{post.title}}</span>
                </li>
            </ul>
        </div>
    `
})

export class PostListComponent {
    constructor(private _postDataService:PostService) {
        //should be moved to ngOnInit lifecycle hook
        this.getPosts();
    }

    private posts:Post[] = [];
    private errorMessage:any = '';

    getPosts() {
        this._postDataService.getData()
            .subscribe(
                posts => this.posts = posts,
                error => this.errorMessage = <any>error);
    }
}

It is important to note that though we have called getPosts function in constructor, it is not a good practice. We should have called it in the ngOnInit lifecycle hook. Our constructors should be simple to enable easy debugging and testing.

We have completed all the pending stuff and now we should be able to see list of post.

But before we end this post, let's have a look at one more operator i.e. toPromise. This operator converts an Observable sequence to a promise. So if we use promises, then our post.service.ts would look like:

import {Injectable} from "@angular/core";
import {Post} from './post';
import { Http, Response } from '@angular/http';
import { Observable } from 'rxjs/Observable';

@Injectable()
export class PostService {
    constructor (private http: Http) {}
    getData (): Promise<Post[]> {
        return this.http.get('http://jsonplaceholder.typicode.com/posts/')
            .toPromise()
            .then(this.extractData)
            .catch(this.handleError);
    }
    private extractData(res: Response) {
        let body = res.json();
        return body || [];
    }
    private handleError (error: any) {
        // In a real world app, we might use a remote logging infrastructure
        // We'd also dig deeper into the error to get a better message
        let errMsg = (error.message) ? error.message :
            error.status ? `${error.status} - ${error.statusText}` : 'Server error';
        console.error(errMsg); // log to console instead
        return Observable.throw(errMsg);
    }
}

If you could notice the difference, we have moved this.extractData which is the success callback as the first parameter whereas this.errorHandler is the second parameter.

Since we are now using promises we will also have to make tweaks in post-list.component.ts. We will have to call then on the returned promise instead of subscribe.

import {Component} from '@angular/core';
import {PostService} from './post.service';
import {Post} from './post';

@Component({
    selector: 'post-list',
    template: `
        <div>
            <ul class="items">
                <li *ngFor="let post of posts">
                    <span>{{post.title}}</span>
                </li>
            </ul>
        </div>
    `
})

export class PostListComponent {
    constructor(private _postDataService:PostService) {
        this.getPosts();
    }

    private posts:Post[] = [];
    private errorMessage:any = '';

    getPosts() {
        this._postDataService.getData()
            .then(
                posts => this.posts = posts,
                error => this.errorMessage = <any>error);
    }
}

As promised this blog educated us on fetching data in Angular2 We are yet to see how to post data to a server in Angular2 so stay tuned! Till then Happy Learning!

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