Services
The Tour of Heroes HeroesComponent
is currently getting and displaying fake data.
After the refactoring in this tutorial, HeroesComponent
will be lean and focused on supporting the view.
It will also be easier to unit-test with a mock service.
Why services
Components shouldn't fetch or save data directly and they certainly shouldn't knowingly present fake data. They should focus on presenting data and delegate data access to a service.
In this tutorial, you'll create a HeroService
that all application classes can use to get heroes.
Instead of creating that service with new
,
you'll rely on Angular dependency injection
to inject it into the HeroesComponent
constructor.
Services are a great way to share information among classes that don't know each other.
You'll create a MessageService
and inject it in two places:
- in
HeroService
which uses the service to send a message. - in
MessagesComponent
which displays that message.
Create the HeroService
Using the Angular CLI, create a service called hero
.
ng generate service hero
The command generates skeleton HeroService
class in src/app/hero.service.ts
The HeroService
class should look like the below.
import { Injectable } from '@angular/core';
@Injectable()
export class HeroService {
constructor() { }
}
@Injectable() services
Notice that the new service imports the Angular Injectable
symbol and annotates
the class with the @Injectable()
decorator.
The @Injectable()
decorator tells Angular that this service might itself
have injected dependencies.
It doesn't have dependencies now but it will soon.
Whether it does or it doesn't, it's good practice to keep the decorator.
The Angular style guidelines strongly recommend keeping it and the linter enforces this rule.
Get hero data
The HeroService
could get hero data from anywhere—a web service, local storage, or a mock data source.
Removing data access from components means you can change your mind about the implementation anytime, without touching any components. They don't know how the service works.
The implementation in this tutorial will continue to deliver mock heroes.
Import the Hero
and HEROES
.
import { Hero } from './hero';
import { HEROES } from './mock-heroes';
Add a getHeroes
method to return the mock heroes.
getHeroes(): Hero[] {
return HEROES;
}
HeroService
Provide the You must provide the HeroService
in the dependency injection system
before Angular can inject it into the HeroesComponent
,
as you will do below.
There are several ways to provide the HeroService
:
in the HeroesComponent
, in the AppComponent
, in the AppModule
.
Each option has pros and cons.
This tutorial chooses to provide it in the AppModule
.
That's such a popular choice that you could have told the CLI to provide it there automatically
by appending --module=app
.
ng generate service hero --module=app
Since you did not, you'll have to provide it yourself.
Open the AppModule
class, import the HeroService
, and add it to the @NgModule.providers
array.
providers: [
HeroService,
/* . . . */
],
The providers
array tells Angular to create a single, shared instance of HeroService
and inject into any class that asks for it.
The HeroService
is now ready to plug into the HeroesComponent
.
This is a interim code sample that will allow you to provide and use the HeroService
. At this point, the code will differ from the HeroService
in the "final code review".
Learn more about providers in the Providers guide.
HeroesComponent
Update Open the HeroesComponent
class file.
Delete the HEROES
import as you won't need that anymore.
Import the HeroService
instead.
import { HeroService } from '../hero.service';
Replace the definition of the heroes
property with a simple declaration.
heroes: Hero[];
HeroService
Inject the Add a private heroService
parameter of type HeroService
to the constructor.
constructor(private heroService: HeroService) { }
The parameter simultaneously defines a private heroService
property and identifies it as a HeroService
injection site.
When Angular creates a HeroesComponent
, the Dependency Injection system
sets the heroService
parameter to the singleton instance of HeroService
.
Add getHeroes()
Create a function to retrieve the heroes from the service.
getHeroes(): void {
this.heroes = this.heroService.getHeroes();
}
ngOnInit
Call it in While you could call getHeroes()
in the constructor, that's not the best practice.
Reserve the constructor for simple initialization such as wiring constructor parameters to properties. The constructor shouldn't do anything. It certainly shouldn't call a function that makes HTTP requests to a remote server as a real data service would.
Instead, call getHeroes()
inside the ngOnInit lifecycle hook and
let Angular call ngOnInit
at an appropriate time after constructing a HeroesComponent
instance.
ngOnInit() {
this.getHeroes();
}
See it run
After the browser refreshes, the app should run as before, showing a list of heroes and a hero detail view when you click on a hero name.
Observable data
The HeroService.getHeroes()
method has a synchronous signature,
which implies that the HeroService
can fetch heroes synchronously.
The HeroesComponent
consumes the getHeroes()
result
as if heroes could be fetched synchronously.
this.heroes = this.heroService.getHeroes();
This will not work in a real app. You're getting away with it now because the service currently returns mock heroes. But soon the app will fetch heroes from a remote server, which is an inherently asynchronous operation.
The HeroService
must wait for the server to respond,
getHeroes()
cannot return immediately with hero data,
and the browser will not block while the service waits.
HeroService.getHeroes()
must have an asynchronous signature of some kind.
It can take a callback. It could return a Promise
. It could return an Observable
.
In this tutorial, HeroService.getHeroes()
will return an Observable
in part because it will eventually use the Angular HttpClient.get
method to fetch the heroes
and HttpClient.get()
returns an Observable
.
Observable HeroService
Observable
is one of the key classes in the RxJS library.
In a later tutorial on HTTP, you'll learn that Angular's HttpClient
methods return RxJS Observable
s.
In this tutorial, you'll simulate getting data from the server with the RxJS of()
function.
Open the HeroService
file and import the Observable
and of
symbols from RxJS.
import { Observable } from 'rxjs/Observable';
import { of } from 'rxjs/observable/of';
Replace the getHeroes
method with this one.
getHeroes(): Observable<Hero[]> {
return of(HEROES);
}
of(HEROES)
returns an Observable<Hero[]>
that emits a single value, the array of mock heroes.
In the HTTP tutorial, you'll call HttpClient.get<Hero[]>()
which also returns an Observable<Hero[]>
that emits a single value, an array of heroes from the body of the HTTP response.
Subscribe in HeroesComponent
The HeroService.getHeroes
method used to return a Hero[]
.
Now it returns an Observable<Hero[]>
.
You'll have to adjust to that difference in HeroesComponent
.
Find the getHeroes
method and replace it with the following code
(shown side-by-side with the previous version for comparison)
getHeroes(): void {
this.heroService.getHeroes()
.subscribe(heroes => this.heroes = heroes);
}
Observable.subscribe()
is the critical difference.
The previous version assigns an array of heroes to the component's heroes
property.
The assignment occurs synchronously, as if the server could return heroes instantly
or the browser could freeze the UI while it waited for the server's response.
That won't work when the HeroService
is actually making requests of a remote server.
The new version waits for the Observable
to emit the array of heroes—
which could happen now or several minutes from now.
Then subscribe
passes the emitted array to the callback,
which sets the component's heroes
property.
This asynchronous approach will work when
the HeroService
requests heroes from the server.
Show messages
In this section you will
- add a
MessagesComponent
that displays app messages at the bottom of the screen. - create an injectable, app-wide
MessageService
for sending messages to be displayed - inject
MessageService
into theHeroService
- display a message when
HeroService
fetches heroes successfully.
Create MessagesComponent
Use the CLI to create the MessagesComponent
.
ng generate component messages
The CLI creates the component files in the src/app/messages
folder and declare MessagesComponent
in AppModule
.
Modify the AppComponent
template to display the generated MessagesComponent
<h1>{{title}}</h1>
<app-heroes></app-heroes>
<app-messages></app-messages>
You should see the default paragraph from MessagesComponent
at the bottom of the page.
Create the MessageService
Use the CLI to create the MessageService
in src/app
.
The --module=app
option tells the CLI to provide this service in the AppModule
,
ng generate service message --module=app
Open MessageService
and replace its contents with the following.
import { Injectable } from '@angular/core';
@Injectable()
export class MessageService {
messages: string[] = [];
add(message: string) {
this.messages.push(message);
}
clear() {
this.messages = [];
}
}
The service exposes its cache of messages
and two methods: one to add()
a message to the cache and another to clear()
the cache.
HeroService
Inject it into the Re-open the HeroService
and import the MessageService
.
import { MessageService } from './message.service';
Modify the constructor with a parameter that declares a private messageService
property.
Angular will inject the singleton MessageService
into that property
when it creates the HeroService
.
constructor(private messageService: MessageService) { }
This is a typical "service-in-service" scenario:
you inject the MessageService
into the HeroService
which is injected into the HeroesComponent
.
HeroService
Send a message from Modify the getHeroes
method to send a message when the heroes are fetched.
getHeroes(): Observable<Hero[]> {
// TODO: send the message _after_ fetching the heroes
this.messageService.add('HeroService: fetched heroes');
return of(HEROES);
}
HeroService
Display the message from The MessagesComponent
should display all messages,
including the message sent by the HeroService
when it fetches heroes.
Open MessagesComponent
and import the MessageService
.
import { MessageService } from '../message.service';
Modify the constructor with a parameter that declares a public messageService
property.
Angular will inject the singleton MessageService
into that property
when it creates the HeroService
.
constructor(public messageService: MessageService) {}
The messageService
property must be public because you're about to bind to it in the template.
Angular only binds to public component properties.
Bind to the MessageService
Replace the CLI-generated MessagesComponent
template with the following.
<div *ngIf="messageService.messages.length">
<h2>Messages</h2>
<button class="clear"
(click)="messageService.clear()">clear</button>
<div *ngFor='let message of messageService.messages'> {{message}} </div>
</div>
This template binds directly to the component's messageService
.
- The
*ngIf
only displays the messages area if there are messages to show.
- An
*ngFor
presents the list of messages in repeated<div>
elements.
- An Angular event binding binds the button's click event
to
MessageService.clear()
.
The messages will look better when you add the private CSS styles to messages.component.css
as listed in one of the "final code review" tabs below.
The browser refreshes and the page displays the list of heroes.
Scroll to the bottom to see the message from the HeroService
in the message area.
Click the "clear" button and the message area disappears.
Final code review
Here are the code files discussed on this page and your app should look like this
- import { Injectable } from '@angular/core';
-
- import { Observable } from 'rxjs/Observable';
- import { of } from 'rxjs/observable/of';
-
- import { Hero } from './hero';
- import { HEROES } from './mock-heroes';
- import { MessageService } from './message.service';
-
- @Injectable()
- export class HeroService {
-
- constructor(private messageService: MessageService) { }
-
- getHeroes(): Observable<Hero[]> {
- // TODO: send the message _after_ fetching the heroes
- this.messageService.add('HeroService: fetched heroes');
- return of(HEROES);
- }
- }
Summary
- You refactored data access to the
HeroService
class. - You provided the
HeroService
in the rootAppModule
so that it can be injected anywhere. - You used Angular Dependency Injection to inject it into a component.
- You gave the
HeroService
get data method an asynchronous signature. - You discovered
Observable
and the RxJS Observable library. - You used RxJS
of()
to return an Observable of mock heroes (Observable<Hero[]>
). - The component's
ngOnInit
lifecycle hook calls theHeroService
method, not the constructor. - You created a
MessageService
for loosely-coupled communication between classes. - The
HeroService
injected into a component is created with another injected service,MessageService
.