服务link

Add serviceslink

英雄之旅的 HeroesComponent 目前获取和显示的都是模拟数据。

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

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。

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.

为什么需要服务link

Why serviceslink

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。

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.

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 关键字来创建此服务，而要依靠 Angular 的依赖注入机制把它注入到 HeroesComponent 的构造函数中。

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.

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：

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. 注入到 HeroService 中，它会使用该服务发送消息

   Inject in HeroService, which uses the service to send a message.

2. 注入到 MessagesComponent 中，它会显示其中的消息。当用户点击某个英雄时，它还会显示该英雄的 ID。

   Inject in MessagesComponent, which displays that message, and also displays the ID when the user clicks a hero.

创建 HeroServicelink

Create the HeroServicelink

使用 Angular CLI 创建一个名叫 hero 的服务。

Using the Angular CLI, create a service called hero.

      
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      ng generate service hero
    

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

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

      
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      import { Injectable } from '@angular/core';

@Injectable({
  providedIn: 'root',
})
export class HeroService {

  constructor() { }

}
    

@Injectable() 服务link

@Injectable() serviceslink

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。

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.

@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。

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

获取英雄数据link

Get hero datalink

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。

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

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。

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.

这节课中的实现仍然会提供模拟的英雄列表。

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

导入 Hero 和 HEROES。

Import the Hero and HEROES.

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

添加一个 getHeroes 方法，让它返回模拟的英雄列表。

Add a getHeroes method to return the mock heroes.

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

提供（provide） HeroServicelink

Provide the HeroServicelink

你必须先注册一个服务提供者，来让 HeroService 在依赖注入系统中可用，Angular 才能把它注入到 HeroesComponent 中。所谓服务提供者就是某种可用来创建或交付一个服务的东西；在这里，它通过实例化 HeroService 类，来提供该服务。

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.

为了确保 HeroService 可以提供该服务，就要使用注入器来注册它。注入器是一个对象，负责当应用要求获取它的实例时选择和注入该提供者。

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.

默认情况下，Angular CLI 命令 ng generate service 会通过给 @Injectable() 装饰器添加 providedIn: 'root' 元数据的形式，用根注入器将你的服务注册成为提供者。

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.

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

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者，还能允许 Angular 通过移除那些完全没有用过的服务来进行优化。

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.

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

The HeroService is now ready to plug into the HeroesComponent.

修改 HeroesComponentlink

Update HeroesComponentlink

打开 HeroesComponent 类文件。

Open the HeroesComponent class file.

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

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

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

把 heroes 属性的定义改为一句简单的声明。

Replace the definition of the heroes property with a declaration.

      
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      heroes: Hero[] = [];
    

注入 HeroServicelink

Inject the HeroServicelink

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

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

      
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      constructor(private heroService: HeroService) {}
    

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。

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

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。

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

添加 getHeroes()link

Add getHeroes()link

创建一个方法，以从服务中获取这些英雄数据。

Create a method to retrieve the heroes from the service.

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

在 ngOnInit() 中调用它link

Call it in ngOnInit()link

你固然可以在构造函数中调用 getHeroes()，但那不是最佳实践。

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

让构造函数保持简单，只做最小化的初始化操作，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。 它当然不应该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。

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.

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

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

      
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      ngOnInit() {
  this.getHeroes();
}
    

查看运行效果link

See it runlink

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。

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.

可观察（Observable）的数据link

Observable datalink

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。

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.

      
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      this.heroes = this.heroService.getHeroes();
    

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。

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 application will fetch heroes from a remote server, which is an inherently asynchronous operation.

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。

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() 必须具有某种形式的异步函数签名。

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

这节课，HeroService.getHeroes() 将会返回 Observable，部分原因在于它最终会使用 Angular 的 HttpClient.get 方法来获取英雄数据，而 HttpClient.get() 会返回 Observable。

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.

可观察对象版本的 HeroServicelink

Observable HeroServicelink

Observable 是 RxJS 库中的一个关键类。

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

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。

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.

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

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

      
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      import { Observable, of } from 'rxjs';
    

把 getHeroes() 方法改成这样：

Replace the getHeroes() method with the following:

      
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      getHeroes(): Observable<Hero[]> {
  const heroes = of(HEROES);
  return 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。

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

在 HeroesComponent 中订阅link

Subscribe in HeroesComponentlink

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

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

你必须在 HeroesComponent 中也向本服务中的这种形式看齐。

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

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）：

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

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

Observable.subscribe() 是关键的差异点。

Observable.subscribe() is the critical difference.

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。

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.

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

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

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() 方法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。

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.

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。

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

显示消息link

Show messageslink

这一节将指导你：

This section guides you through the following:

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

  adding a MessagesComponent that displays application messages at the bottom of the screen

• 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。

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

• 把 MessageService 注入到 HeroService 中。

  injecting MessageService into the HeroService

• 当 HeroService 成功获取了英雄数据时显示一条消息。

  displaying a message when HeroService fetches heroes successfully

创建 MessagesComponentlink

Create MessagesComponentlink

使用 CLI 创建 MessagesComponent。

Use the CLI to create the MessagesComponent.

      
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      ng generate component messages
    

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

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

修改 AppComponent 的模板来显示所生成的 MessagesComponent：

Modify the AppComponent template to display the generated MessagesComponent.

      
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      <h1>{{title}}</h1>
<app-heroes></app-heroes>
<app-messages></app-messages>
    

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

You should see the default paragraph from MessagesComponent at the bottom of the page.

创建 MessageServicelink

Create the MessageServicelink

使用 CLI 在 src/app 中创建 MessageService。

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

      
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      ng generate service message
    

打开 MessageService，并把它的内容改成这样：

Open MessageService and replace its contents with the following.

      
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      import { Injectable } from '@angular/core';

@Injectable({
  providedIn: 'root',
})
export class MessageService {
  messages: string[] = [];

  add(message: string) {
    this.messages.push(message);
  }

  clear() {
    this.messages = [];
  }
}
    

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。

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

把它注入到 HeroService 中link

Inject it into the HeroServicelink

在 HeroService 中导入 MessageService。

In HeroService, import the MessageService.

      
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      import { MessageService } from './message.service';
    

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。

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.

      
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      constructor(private messageService: MessageService) { }
    

从 HeroService 中发送一条消息link

Send a message from HeroServicelink

修改 getHeroes() 方法，在获取到英雄数组时发送一条消息。

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

      
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      getHeroes(): Observable<Hero[]> {
  const heroes = of(HEROES);
  this.messageService.add('HeroService: fetched heroes');
  return heroes;
}
    

从 HeroService 中显示消息link

Display the message from HeroServicelink

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。

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

打开 MessagesComponent，并且导入 MessageService。

Open MessagesComponent and import the MessageService.

      
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      import { MessageService } from '../message.service';
    

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。

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.

      
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      constructor(public messageService: MessageService) {}
    

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。

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

绑定到 MessageServicelink

Bind to the MessageServicelink

把 CLI 生成的 MessagesComponent 的模板改成这样：

Replace the CLI-generated MessagesComponent template with the following.

      
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      <div *ngIf="messageService.messages.length">

  <h2>Messages</h2>
  <button class="clear"
          (click)="messageService.clear()">Clear messages</button>
  <div *ngFor='let message of messageService.messages'> {{message}} </div>

</div>
    

这个模板直接绑定到了组件的 messageService 属性上。

This template binds directly to the component's messageService.

• *ngIf 只有在有消息时才会显示消息区。

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

• *ngFor 用来在一系列 <div> 元素中展示消息列表。

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

• Angular 的事件绑定把按钮的 click 事件绑定到了 MessageService.clear()。

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

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 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.

为 hero 服务添加额外的消息link

Add additional messages to hero servicelink

下面的例子展示了当用户点击某个英雄时，如何发送和显示一条消息，以及如何显示该用户的选取历史。当你学到后面的路由一章时，这会很有帮助。

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.

      
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      import { Component, OnInit } from '@angular/core';

import { Hero } from '../hero';
import { HeroService } from '../hero.service';
import { MessageService } from '../message.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, private messageService: MessageService) { }

  ngOnInit() {
    this.getHeroes();
  }

  onSelect(hero: Hero): void {
    this.selectedHero = hero;
    this.messageService.add(`HeroesComponent: Selected hero id=${hero.id}`);
  }

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

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 Clear messages 按钮，消息区不见了。

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.

查看最终代码link

Final code reviewlink

本页讨论的代码文件如下。

Here are the code files discussed on this page.

      
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      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[]> {
    const heroes = of(HEROES);
    this.messageService.add('HeroService: fetched heroes');
    return heroes;
  }
}
    

小结link

Summarylink

• 你把数据访问逻辑重构到了 HeroService 类中。

  You refactored data access to the HeroService class.

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

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

• 你使用 Angular 依赖注入机制把它注入到了组件中。

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

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

  You gave the HeroService get data method an asynchronous signature.

• 你发现了 Observable 以及 RxJS 库。

  You discovered Observable and the RxJS Observable library.

• 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[]>)。

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

• 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。

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

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

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

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

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