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Services

服务

The Tour of Heroes HeroesComponent is currently getting and displaying fake data.

英雄指南的 HeroesComponent 目前获取和显示的都是模拟数据。

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.

本节课的重构完成之后，HeroesComponent 变得更精简，并且聚焦于为它的视图提供支持。这也让它更容易使用模拟服务进行单元测试。

Why services

为什么需要服务

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.

组件不应该直接获取或保存数据，它们不应该了解是否在展示假数据。 它们应该聚焦于展示数据，而把数据访问的职责委托给某个服务。

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.

本节课，你将创建一个 HeroService，应用中的所有类都可以使用它来获取英雄列表。 不要使用 new 来创建此服务，而要依靠 Angular 的依赖注入机制把它注入到 HeroesComponent 的构造函数中。

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:

服务是在多个“互相不知道”的类之间共享信息的好办法。 你将创建一个 MessageService，并且把它注入到两个地方：

  1. in HeroService which uses the service to send a message.

    HeroService 中，它会使用该服务发送消息。

  2. in MessagesComponent which displays that message.

    MessagesComponent 中，它会显示其中的消息。

Create the HeroService

创建 HeroService

Using the Angular CLI, create a service called hero.

使用 Angular CLI 创建一个名叫 hero 的服务。

ng generate service hero

The command generates skeleton HeroService class in src/app/hero.service.ts The HeroService class should look like the following example.

该命令会在 src/app/hero.service.ts 中生成 HeroService 类的骨架。 HeroService 类的代码如下：

import { Injectable } from '@angular/core'; @Injectable({ providedIn: 'root', }) export class HeroService { constructor() { } }

@Injectable() services

@Injectable() 服务

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.

注意，这个新的服务导入了 Angular 的 Injectable 符号，并且给这个服务类添加了 @Injectable() 装饰器。 它把这个类标记为依赖注入系统的参与者之一。HeroService 类将会提供一个可注入的服务，并且它还可以拥有自己的待注入的依赖。 目前它还没有依赖，但是很快就会有了。

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

@Injectable() 装饰器会接受该服务的元数据对象，就像 @Component() 对组件类的作用一样。

Get hero data

获取英雄数据

The HeroService could get hero data from anywhere—a web service, local storage, or a mock data source.

HeroService 可以从任何地方获取数据：Web 服务、本地存储（LocalStorage）或一个模拟的数据源。

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.

从组件中移除数据访问逻辑，意味着将来任何时候你都可以改变目前的实现方式，而不用改动任何组件。 这些组件不需要了解该服务的内部实现。

The implementation in this tutorial will continue to deliver mock heroes.

这节课中的实现仍然会提供模拟的英雄列表。

Import the Hero and HEROES.

导入 Hero 和 HEROES。

import { Hero } from './hero'; import { HEROES } from './mock-heroes';

Add a getHeroes method to return the mock heroes.

添加一个 getHeroes 方法，让它返回模拟的英雄列表。

getHeroes(): Hero[] { return HEROES; }

Provide the HeroService

提供（provide） HeroService

You must make the HeroService available to the dependency injection system before Angular can inject it into the HeroesComponent, as you will do below. You do this 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.

在要求 Angular 把 HeroService 注入到 HeroesComponent 之前，你必须先把这个服务提供给依赖注入系统。稍后你就要这么做。 你可以通过注册提供商来做到这一点。提供商用来创建和交付服务，在这个例子中，它会对 HeroService 类进行实例化，以提供该服务。

Now, you need to make sure that the HeroService is registered as the provider of this service. You are registering it with an injector, which is the object that is responsible for choosing and injecting the provider where it is required.

现在，你需要确保 HeroService 已经作为该服务的提供商进行过注册。 你要用一个注入器注册它。注入器就是一个对象，负责在需要时选取和注入该提供商。

By default, the Angular CLI command ng generate service registers a provider with the root injector for your service by including provider metadata in the @Injectable decorator.

默认情况下，Angular CLI 命令 ng generate service 会通过给 @Injectable 装饰器添加元数据的形式，为该服务把提供商注册到根注入器上。

If you look at the @Injectable() statement right before the HeroService class definition, you can see that the providedIn metadata value is 'root':

如果你看看 HeroService 紧前面的 @Injectable() 语句定义，就会发现 providedIn 元数据的值是 'root'：

@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 app by removing the service if it turns out not to be used after all.

当你在顶层提供该服务时，Angular 就会为 HeroService 创建一个单一的、共享的实例，并把它注入到任何想要它的类上。 在 @Injectable 元数据中注册该提供商，还能让 Angular 可以通过移除那些完全没有用过的服务，来进行优化。

If you need to, you can register providers at different levels: in the HeroesComponent, in the AppComponent, in the AppModule. For instance, you could have told the CLI to provide the service at the module level automatically by appending --module=app.

如果需要，你也可以在不同的层次上注册提供商 —— 在 HeroesComponent 中、在 AppComponent 中，或在 AppModule 中。 比如，你可以通过附加 --module=app 参数来告诉 CLI 要自动在模块级提供该服务。

ng generate service hero --module=app

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

要了解更多，参见依赖注入指南。

The HeroService is now ready to plug into the HeroesComponent.

现在 HeroService 已经准备好插入到 HeroesComponent 中了。

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".

这是一个过渡性的代码范例，它将会允许你提供并使用 HeroService。此刻的代码和最终代码相差很大。

Update HeroesComponent

修改 HeroesComponent

Open the HeroesComponent class file.

打开 HeroesComponent 类文件。

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

删除 HEROES 的导入语句，因为你以后不会再用它了。 转而导入 HeroService。

import { HeroService } from '../hero.service';

Replace the definition of the heroes property with a simple declaration.

把 heroes 属性的定义改为一句简单的声明。

heroes: Hero[];

Inject the HeroService

注入 HeroService

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

往构造函数中添加一个私有的 heroService，其类型为 HeroService。

constructor(private heroService: HeroService) { }

The parameter simultaneously defines a private heroService property and identifies it as a HeroService injection site.

这个参数同时做了两件事：1. 声明了一个私有 heroService 属性，2. 把它标记为一个 HeroService 的注入点。

When Angular creates a HeroesComponent, the Dependency Injection system sets the heroService parameter to the singleton instance of HeroService.

当 Angular 创建 HeroesComponent 时，依赖注入系统就会把这个 heroService 参数设置为 HeroService 的单例对象。

Add getHeroes()

添加 getHeroes()

Create a function to retrieve the heroes from the service.

创建一个函数，以从服务中获取这些英雄数据。

getHeroes(): void { this.heroes = this.heroService.getHeroes(); }

Call it in ngOnInit

在 ngOnInit 中调用它

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

你固然可以在构造函数中调用 getHeroes()，但那不是最佳实践。

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.

让构造函数保持简单，只做初始化操作，比如把构造函数的参数赋值给属性。 构造函数不应该做任何事。 它肯定不能调用某个函数来向远端服务（比如真实的数据服务）发起 HTTP 请求。

Instead, call getHeroes() inside the ngOnInit lifecycle hook and let Angular call ngOnInit at an appropriate time after constructing a HeroesComponent instance.

而是选择在 ngOnInit 生命周期钩子中调用 getHeroes()，之后交由 Angular 处理，它会在构造出 HeroesComponent 的实例之后的某个合适的时机调用 ngOnInit。

ngOnInit() { this.getHeroes(); }

See it run

查看运行效果

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.

刷新浏览器，该应用仍运行的一如既往。 显示英雄列表，并且当你点击某个英雄的名字时显示出英雄详情视图。

Observable data

可观察（Observable）的数据

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.

HeroService.getHeroes() 的函数签名是同步的，它所隐含的假设是 HeroService 总是能同步获取英雄列表数据。 而 HeroesComponent 也同样假设能同步取到 getHeroes() 的结果。

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.

这在真实的应用中几乎是不可能的。 现在能这么做，只是因为目前该服务返回的是模拟数据。 不过很快，该应用就要从远端服务器获取英雄数据了，而那天生就是异步操作。

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 必须等服务器给出响应， 而 getHeroes() 不能立即返回英雄数据， 浏览器也不会在该服务等待期间停止响应。

HeroService.getHeroes() must have an asynchronous signature of some kind.

HeroService.getHeroes() 必须具有某种形式的异步函数签名。

It can take a callback. It could return a Promise. It could return an Observable.

它可以使用回调函数，可以返回 Promise（承诺），也可以返回 Observable（可观察对象）。

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.

这节课，HeroService.getHeroes() 将会返回 Observable，因为它最终会使用 Angular 的 HttpClient.get 方法来获取英雄数据，而 HttpClient.get() 会返回 Observable。

Observable HeroService

可观察对象版本的 HeroService

Observable is one of the key classes in the RxJS library.

Observable 是 RxJS 库中的一个关键类。

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.

在稍后的 HTTP 教程中，你就会知道 Angular HttpClient 的方法会返回 RxJS 的 Observable。 这节课，你将使用 RxJS 的 of() 函数来模拟从服务器返回数据。

Open the HeroService file and import the Observable and of symbols from RxJS.

打开 HeroService 文件，并从 RxJS 中导入 Observable 和 of 符号。

import { Observable, of } from 'rxjs';

Replace the getHeroes method with this one.

把 getHeroes 方法改成这样：

getHeroes(): Observable<Hero[]> { return of(HEROES); }

of(HEROES) returns an Observable<Hero[]> that emits a single value, the array of mock heroes.

of(HEROES) 会返回一个 Observable<Hero[]>，它会发出单个值，这个值就是这些模拟英雄的数组。

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.

在 HTTP 教程中，你将会调用 HttpClient.get<Hero[]>() 它也同样返回一个 Observable<Hero[]>，它也会发出单个值，这个值就是来自 HTTP 响应体中的英雄数组。

Subscribe in HeroesComponent

在 HeroesComponent 中订阅

The HeroService.getHeroes method used to return a Hero[]. Now it returns an Observable<Hero[]>.

HeroService.getHeroes 方法之前返回一个 Hero[]， 现在它返回的是 Observable<Hero[]>。

You'll have to adjust to that difference in HeroesComponent.

你必须在 HeroesComponent 中也向本服务中的这种形式看齐。

Find the getHeroes method and replace it with the following code (shown side-by-side with the previous version for comparison)

找到 getHeroes 方法，并且把它替换为如下代码（和前一个版本对比显示）：

getHeroes(): void { this.heroService.getHeroes() .subscribe(heroes => this.heroes = heroes); }getHeroes(): void { this.heroes = this.heroService.getHeroes(); }

Observable.subscribe() is the critical difference.

Observable.subscribe() 是关键的差异点。

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.

上一个版本把英雄的数组赋值给了该组件的 heroes 属性。 这种赋值是同步的，这里包含的假设是服务器能立即返回英雄数组或者浏览器能在等待服务器响应时冻结界面。

That won't work when the HeroService is actually making requests of a remote server.

当 HeroService 真的向远端服务器发起请求时，这种方式就行不通了。

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.

新的版本等待 Observable 发出这个英雄数组，这可能立即发生，也可能会在几分钟之后。 然后，subscribe 函数把这个英雄数组传给这个回调函数，该函数把英雄数组赋值给组件的 heroes 属性。

This asynchronous approach will work when the HeroService requests heroes from the server.

使用这种异步方式，当 HeroService 从远端服务器获取英雄数据时，就可以工作了。

Show messages

显示消息

In this section you will

在这一节，你将

  • add a MessagesComponent that displays app messages at the bottom of the screen.

    添加一个 MessagesComponent，它在屏幕的底部显示应用中的消息。

  • create an injectable, app-wide MessageService for sending messages to be displayed

    创建一个可注入的、全应用级别的 MessageService，用于发送要显示的消息。

  • inject MessageService into the HeroService

    把 MessageService 注入到 HeroService 中。

  • display a message when HeroService fetches heroes successfully.

    当 HeroService 成功获取了英雄数据时显示一条消息。

Create MessagesComponent

创建 MessagesComponent

Use the CLI to create the MessagesComponent.

使用 CLI 创建 MessagesComponent。

ng generate component messages

The CLI creates the component files in the src/app/messages folder and declare MessagesComponent in AppModule.

CLI 在 src/app/messages 中创建了组件文件，并且把 MessagesComponent 声明在了 AppModule 中。

Modify the AppComponent template to display the generated MessagesComponent

修改 AppComponent 的模板来显示所生成的 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.

你可以在页面的底部看到来自的 MessagesComponent 的默认内容。

Create the MessageService

创建 MessageService

Use the CLI to create the MessageService in src/app.

使用 CLI 在 src/app 中创建 MessageService。

ng generate service message

Open MessageService and replace its contents with the following.

打开 MessageService，并把它的内容改成这样：

import { Injectable } from '@angular/core'; @Injectable({ providedIn: 'root', }) 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.

该服务对外暴露了它的 messages 缓存，以及两个方法：add() 方法往缓存中添加一条消息，clear() 方法用于清空缓存。

Inject it into the HeroService

把它注入到 HeroService 中

Re-open the HeroService and import the MessageService.

重新打开 HeroService，并且导入 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.

修改这个构造函数，添加一个私有的 messageService 属性参数。 Angular 将会在创建 HeroService 时把 MessageService 的单例注入到这个属性中。

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.

这是一个典型的“服务中的服务”场景： 你把 MessageService 注入到了 HeroService 中，而 HeroService 又被注入到了 HeroesComponent 中。

Send a message from HeroService

从 HeroService 中发送一条消息

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

修改 getHeroes 方法，在获取到英雄数组时发送一条消息。

getHeroes(): Observable<Hero[]> { // TODO: send the message _after_ fetching the heroes this.messageService.add('HeroService: fetched heroes'); return of(HEROES); }

Display the message from HeroService

从 HeroService 中显示消息

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

MessagesComponent 可以显示所有消息， 包括当 HeroService 获取到英雄数据时发送的那条。

Open MessagesComponent and import the MessageService.

打开 MessagesComponent，并且导入 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 MessagesComponent.

修改构造函数，添加一个 public 的 messageService 属性。 Angular 将会在创建 MessagesComponent 的实例时 把 MessageService 的实例注入到这个属性中。

constructor(public messageService: MessageService) {}

The messageService property must be public because you're about to bind to it in the template.

这个 messageService 属性必须是公共属性，因为你将会在模板中绑定到它。

Angular only binds to public component properties.

Angular 只会绑定到组件的公共属性。

Bind to the MessageService

绑定到 MessageService

Replace the CLI-generated MessagesComponent template with the following.

把 CLI 生成的 MessagesComponent 的模板改成这样：

<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.

这个模板直接绑定到了组件的 messageService 属性上。

  • The *ngIf only displays the messages area if there are messages to show.

    *ngIf 只有在有消息时才会显示消息区。

  • An *ngFor presents the list of messages in repeated <div> elements.

    *ngFor 用来在一系列 <div> 元素中展示消息列表。

  • An Angular event binding binds the button's click event to MessageService.clear().

    Angular 的事件绑定把按钮的 click 事件绑定到了 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.

当你把 最终代码 某一页的内容添加到 messages.component.css 中时，这些消息会变得好看一些。

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.

刷新浏览器，页面显示出了英雄列表。 滚动到底部，就会在消息区看到来自 HeroService 的消息。 点击“清空”按钮，消息区不见了。

Final code review

查看最终代码

Here are the code files discussed on this page and your app should look like this.

你的应用应该变成了这样。本页所提及的代码文件如下：

import { Injectable } from '@angular/core'; import { Observable, of } from 'rxjs'; import { Hero } from './hero'; import { HEROES } from './mock-heroes'; import { MessageService } from './message.service'; @Injectable({ providedIn: 'root', }) 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); } }import { Injectable } from '@angular/core'; @Injectable({ providedIn: 'root', }) export class MessageService { messages: string[] = []; add(message: string) { this.messages.push(message); } clear() { this.messages = []; } }import { Component, OnInit } from '@angular/core'; import { Hero } from '../hero'; import { HeroService } from '../hero.service'; @Component({ selector: 'app-heroes', templateUrl: './heroes.component.html', styleUrls: ['./heroes.component.css'] }) export class HeroesComponent implements OnInit { selectedHero: Hero; heroes: Hero[]; constructor(private heroService: HeroService) { } ngOnInit() { this.getHeroes(); } onSelect(hero: Hero): void { this.selectedHero = hero; } getHeroes(): void { this.heroService.getHeroes() .subscribe(heroes => this.heroes = heroes); } }import { Component, OnInit } from '@angular/core'; import { MessageService } from '../message.service'; @Component({ selector: 'app-messages', templateUrl: './messages.component.html', styleUrls: ['./messages.component.css'] }) export class MessagesComponent implements OnInit { constructor(public messageService: MessageService) {} ngOnInit() { } }<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>/* MessagesComponent's private CSS styles */ h2 { color: red; font-family: Arial, Helvetica, sans-serif; font-weight: lighter; } body { margin: 2em; } body, input[text], button { color: crimson; font-family: Cambria, Georgia; } button.clear { font-family: Arial; background-color: #eee; border: none; padding: 5px 10px; border-radius: 4px; cursor: pointer; cursor: hand; } button:hover { background-color: #cfd8dc; } button:disabled { background-color: #eee; color: #aaa; cursor: auto; } button.clear { color: #888; margin-bottom: 12px; }import { BrowserModule } from '@angular/platform-browser'; import { NgModule } from '@angular/core'; import { FormsModule } from '@angular/forms'; import { AppComponent } from './app.component'; import { HeroesComponent } from './heroes/heroes.component'; import { HeroDetailComponent } from './hero-detail/hero-detail.component'; import { MessagesComponent } from './messages/messages.component'; @NgModule({ declarations: [ AppComponent, HeroesComponent, HeroDetailComponent, MessagesComponent ], imports: [ BrowserModule, FormsModule ], providers: [ // no need to place any providers due to the `providedIn` flag... ], bootstrap: [ AppComponent ] }) export class AppModule { }<h1>{{title}}</h1> <app-heroes></app-heroes> <app-messages></app-messages>

Summary

小结

  • You refactored data access to the HeroService class.

    你把数据访问逻辑重构到了 HeroService 类中。

  • You registered the HeroService as the provider of its service at the root level so that it can be injected anywhere in the app.

    你在根注入器中把 HeroService 注册为该服务的提供商，以便在别处可以注入它。

  • You used Angular Dependency Injection to inject it into a component.

    你使用 Angular 依赖注入机制把它注入到了组件中。

  • You gave the HeroService get data method an asynchronous signature.

    你给 HeroService 中获取数据的方法提供了一个异步的函数签名。

  • You discovered Observable and the RxJS Observable library.

    你发现了 Observable 以及 RxJS 库。

  • You used RxJS of() to return an observable of mock heroes (Observable<Hero[]>).

    你使用 RxJS 的 of() 方法返回了一个模拟英雄数据的可观察对象 (Observable<Hero[]>)。

  • The component's ngOnInit lifecycle hook calls the HeroService method, not the constructor.

    在组件的 ngOnInit 生命周期钩子中调用 HeroService 方法，而不是构造函数中。

  • You created a MessageService for loosely-coupled communication between classes.

    你创建了一个 MessageService，以便在类之间实现松耦合通讯。

  • The HeroService injected into a component is created with another injected service, MessageService.

    HeroService 连同注入到它的服务 MessageService 一起，注入到了组件中。