angular-cn/aio/content/tutorial/toh-pt4.md

Add 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.

For the sample application that this page describes, see the .

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 the new keyword, 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.

1. Inject in HeroService, which uses the service to send a message.
2. Inject in MessagesComponent, which displays that message, and also displays the ID when the user clicks a hero.

Create the HeroService

Using the Angular CLI, create a service called hero.

ng generate service hero

The command generates a skeleton HeroService class in src/app/hero.service.ts as follows:

@Injectable() services

Notice that the new service imports the Angular Injectable symbol and annotates the class with the @Injectable() decorator. This marks the class as one that participates in the dependency injection system. The HeroService class is going to provide an injectable service, and it can also have its own injected dependencies. It doesn't have any dependencies yet, but it will soon.

The @Injectable() decorator accepts a metadata object for the service, the same way the @Component() decorator did for your component classes.

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.

Add a getHeroes method to return the mock heroes.

{@a provide}

Provide the HeroService

You must make the HeroService available to the dependency injection system before Angular can inject it into the HeroesComponent by registering a provider. A provider is something that can create or deliver a service; in this case, it instantiates the HeroService class to provide the service.

To make sure that the HeroService can provide this service, register it with the injector, which is the object that is responsible for choosing and injecting the provider where the application requires it.

By default, the Angular CLI command ng generate service registers a provider with the root injector for your service by including provider metadata, that is providedIn: 'root' in the @Injectable() decorator.

@Injectable({
  providedIn: 'root',
})

When you provide the service at the root level, Angular creates a single, shared instance of HeroService and injects into any class that asks for it. Registering the provider in the @Injectable metadata also allows Angular to optimize an application by removing the service if it turns out not to be used after all.

To learn more about providers, see the Providers section. To learn more about injectors, see the Dependency Injection guide.

The HeroService is now ready to plug into the HeroesComponent.

This is an 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".

Update HeroesComponent

Open the HeroesComponent class file.

Delete the HEROES import, because you won't need that anymore. Import the HeroService instead.

Replace the definition of the heroes property with a declaration.

{@a inject}

Inject the HeroService

Add a private heroService parameter of type HeroService to the constructor.

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 method to retrieve the heroes from the service.

{@a oninit}

Call it in ngOnInit()

While you could call getHeroes() in the constructor, that's not the best practice.

Reserve the constructor for minimal 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.

See it run

After the browser refreshes, the application 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 will not work in a real application. You're getting away with it now because the service currently returns mock heroes. But soon the application 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.

In this tutorial, HeroService.getHeroes() will return an Observable 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 Observables. 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.

Replace the getHeroes() method with the following:

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)

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. The subscribe() method 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

This section guides you through the following:

• adding a MessagesComponent that displays application messages at the bottom of the screen
• creating an injectable, app-wide MessageService for sending messages to be displayed
• injecting MessageService into the HeroService
• displaying 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 declares the MessagesComponent in AppModule.

Modify the AppComponent template to display the generated MessagesComponent.

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.

ng generate service message

Open MessageService and replace its contents with the following.

The service exposes its cache of messages and two methods: one to add() a message to the cache and another to clear() the cache.

{@a inject-message-service}

Inject it into the HeroService

In HeroService, import the MessageService.

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.

This is a typical "service-in-service" scenario: you inject the MessageService into the HeroService which is injected into the HeroesComponent.

Send a message from HeroService

Modify the getHeroes() method to send a message when the heroes are fetched.

Display the message from HeroService

The MessagesComponent should display all messages, including the message sent by the HeroService when it fetches heroes.

Open MessagesComponent and import the MessageService.

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 MessagesComponent.

The messageService property must be public because you're going 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.

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.

Add additional messages to hero service

The following example shows how to send and display a message each time the user clicks on a hero, showing a history of the user's selections. This will be helpful when you get to the next section on Routing.

Refresh the browser to see the list of heroes, and scroll to the bottom to see the messages from the HeroService. Each time you click a hero, a new message appears to record the selection. Use the Clear messages button to clear the message history.

{@a final-code-review}

Final code review

Here are the code files discussed on this page.

Summary

• You refactored data access to the HeroService class.
• You registered the HeroService as the provider of its service at the root level so that it can be injected anywhere in the application.
• 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 the HeroService 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.