As the Tour of Heroes app evolves, you'll add more components that need access to hero data.
随着《英雄指南》的成长,我们要添加更多需要访问英雄数据的组件。
Instead of copying and pasting the same code over and over, you'll create a single reusable data service and inject it into the components that need it. Using a separate service keeps components lean and focused on supporting the view, and makes it easy to unit-test components with a mock service.
为了不再把相同的代码复制一遍又一遍,我们要创建一个单一的可复用的数据服务,并且把它注入到需要它的那些组件中。 使用单独的服务可以保持组件精简,使其集中精力为视图提供支持,并且,借助模拟(Mock)服务,可以更容易的对组件进行单元测试。
Because data services are invariably asynchronous, you'll finish the page with a Promise-based version of the data service.
由于数据服务总是异步的,因此我们最终会提供一个基于承诺(Promise)的数据服务。
When you're done with this page, the app should look like this
当我们完成本章的内容是,本应用会变成这样:
Where you left off
延续上一步教程
Before continuing with the Tour of Heroes, verify that you have the following structure. If not, go back to the previous pages.
在继续《英雄指南》之前,先检查一下,是否已经有如下目录结构。如果没有,回上一章,看看错过了哪里。
Keep the app transpiling and running
让应用代码保持转译和运行
Enter the following command in the terminal window:
在终端窗口中输入如下命令:
This command runs the TypeScript compiler in "watch mode", recompiling automatically when the code changes. The command simultaneously launches the app in a browser and refreshes the browser when the code changes.
这个命令会在“监听”模式下运行TypeScript编译器,当代码变化时,它会自动重新编译。 同时,该命令还会在浏览器中启动该应用,并且当代码变化时刷新浏览器。
You can keep building the Tour of Heroes without pausing to recompile or refresh the browser.
在后续构建《英雄指南》过程中,应用能持续运行,而不用中断服务来编译或刷新浏览器。
Creating a hero service
创建英雄服务
The stakeholders want to show the heroes in various ways on different pages. Users can already select a hero from a list. Soon you'll add a dashboard with the top performing heroes and create a separate view for editing hero details. All three views need hero data.
客户向我们描绘了本应用更大的目标:想要在不同的页面中用多种方式显示英雄。 现在我们已经能从列表中选择一个英雄了,但这还不够。 很快,我们将添加一个仪表盘来显示表现最好的英雄,并创建一个独立视图来编辑英雄的详情。 所有这些视图都需要英雄数据。
At the moment, the AppComponent
defines mock heroes for display.
However, defining heroes is not the component's job,
and you can't easily share the list of heroes with other components and views.
In this page, you'll move the hero data acquisition business to a single service that provides the data and
share that service with all components that need the data.
目前,AppComponent
显示的是模拟数据。
不过,定义这些英雄并非组件的任务,否则我们没法与其它组件和视图共享这些英雄列表数据。
在这一章,我们将把获取英雄数据的任务重构为一个单独的服务,它将提供英雄数据,并把服务在所有需要英雄数据的组件间共享。
Create the HeroService
创建 HeroService
Create a file in the app
folder called hero.service.ts
.
在app
目录下创建一个名叫hero.service.ts
的文件。
The naming convention for service files is the service name in lowercase followed by .service
.
For a multi-word service name, use lower dash-case.
For example, the filename for SpecialSuperHeroService
is special-super-hero.service.ts
.
我们遵循的文件命名约定是:服务名称的小写形式(基本名),加上.service
后缀。
如果服务名称包含多个单词,我们就把基本名部分写成中线形式 (dash-case)。
例如,SpecialSuperHeroService
服务应该被定义在special-super-hero.service.ts
文件中。
Name the class HeroService
and export it for others to import.
我们把这个类命名为HeroService
,并导出它,以供别人使用。
src/app/hero.service.ts (starting point)
Injectable services
可注入的服务
Notice that you imported the Angular Injectable
function and applied that function as an @Injectable()
decorator.
注意,我们导入了 Angular 的Injectable
函数,并作为@Injectable()
装饰器使用这个函数。
Don't forget the parentheses. Omitting them leads to an error that's difficult to diagnose.
不要忘了写圆括号!如果忘了写,就会导致一个很难诊断的错误。
The @Injectable()
decorator tells TypeScript to emit metadata about the service.
The metadata specifies that Angular may need to inject other dependencies into this service.
当 TypeScript 看到@Injectable()
装饰器时,就会记下本服务的元数据。
如果 Angular 需要往这个服务中注入其它依赖,就会使用这些元数据。
Although the HeroService
doesn't have any dependencies at the moment,
applying the @Injectable()
decorator from the start ensures
consistency and future-proofing.
虽然此时HeroService
还没有任何依赖,但我们还是得加上这个装饰器。
作为一项最佳实践,无论是出于提高统一性还是减少变更的目的,
都应该从一开始就加上@Injectable()
装饰器。
Getting hero data
获取英雄数据
Add a getHeroes()
method stub.
添加一个名叫getHeros
的桩方法。
src/app/hero.service.ts (getHeroes stub)
The HeroService
could get Hero
data from anywhere—a
web service, local storage, or a mock data source.
Removing data access from the component means
you can change your mind about the implementation anytime,
without touching the components that need hero data.
HeroService
可以从任何地方获取Hero
数据 —— Web服务、本地存储或模拟数据源。
从组件中移除数据访问逻辑意味着你可以随时更改这些实现方式,而不影响需要这些英雄数据的组件。
Move the mock hero data
移动模拟的英雄数据
Cut the HEROES
array from app.component.ts
and paste it to a new file in the app
folder named mock-heroes.ts
.
Additionally, copy the import {Hero} ...
statement because the heroes array uses the Hero
class.
从app.component.ts
文件中剪切HEROS
数组,把它粘贴到app
目录下一个名叫mock-heroes.ts
的文件中。
还要复制import {Hero}...
语句,因为我们的英雄数组用到了Hero
类。
src/app/mock-heroes.ts
The HEROES
constant is exported so it can be imported elsewhere, such as the HeroService
.
我们导出了HEROES
常量,以便可以在其它地方导入它 — 例如HeroService
服务。
In app.component.ts
, where you cut the HEROES
array,
add an uninitialized heroes
property:
在刚刚剪切出HEROES
数组的app.component.ts
文件中,添加一个尚未初始化的heroes
属性:
src/app/app.component.ts (heroes property)
Return mocked hero data
返回模拟的英雄数据
Back in the HeroService
, import the mock HEROES
and return it from the getHeroes()
method.
The HeroService
looks like this:
回到HeroService
,我们导入HEROES
常量,并在getHeroes
方法中返回它。
我们的HeroService
服务现在是这样的:
src/app/hero.service.ts
Import the hero service
导入HeroService
You're ready to use the HeroService
in other components, starting with AppComponent
.
我们可以在多个组件中使用 HeroService 服务了,先从 AppComponent 开始。
Import the HeroService
so that you can reference it in the code.
先导入HeroService
,以便我们可以在代码中引用它。
src/app/app.component.ts (hero-service-import)
Don't use new with the HeroService
不要new
出HeroService
How should the AppComponent
acquire a runtime concrete HeroService
instance?
该如何在运行中获得一个具体的HeroService
实例呢?
You could create a new instance of the HeroService
with new
like this:
你可能想用new
来创建HeroService
的实例,就像这样:
However, this option isn't ideal for the following reasons:
但这不是个好主意,有很多理由,例如:
The component has to know how to create a
HeroService
. If you change theHeroService
constructor, you must find and update every place you created the service. Patching code in multiple places is error prone and adds to the test burden.我们的组件得弄清楚该如何创建
HeroService
。 如果有一天我们修改了HeroService
的构造函数,我们不得不找出创建过此服务的每一处代码,并修改它。 围着补丁代码转圈很容易导致错误,还会增加测试负担。You create a service each time you use
new
. What if the service caches heroes and shares that cache with others? You couldn't do that.我们每次使用
new
都会创建一个新的服务实例。 如果这个服务需要缓存英雄列表,并把这个缓存共享给别人呢?怎么办? 没办法,做不到。With the
AppComponent
locked into a specific implementation of theHeroService
, switching implementations for different scenarios, such as operating offline or using different mocked versions for testing, would be difficult.我们把
AppComponent
锁定到HeroService
的一个特定实现。 我们很难在不同的场景中切换实现。 例如,能离线操作吗?能在测试时使用不同的模拟版本吗?这可不容易。
Inject the HeroService
注入 HeroService
Instead of using the new line, you'll add two lines.
你可以用两行代码代替用new
时的一行:
Add a constructor that also defines a private property.
添加一个构造函数,并定义一个私有属性。
Add to the component's
providers
metadata.添加组件的
providers
元数据。
Add the constructor:
添加构造函数:
src/app/app.component.ts (constructor)
The constructor itself does nothing. The parameter simultaneously
defines a private heroService
property and identifies it as a HeroService
injection site.
构造函数自己什么也不用做,它在参数中定义了一个私有的heroService
属性,并把它标记为注入HeroService
的靶点。
Now Angular knows to supply an instance of the HeroService
when it creates an AppComponent
.
现在,当创建AppComponent
实例时,Angular 知道需要先提供一个HeroService
的实例。
Read more about dependency injection in the Dependency Injection page.
更多依赖注入的信息,见依赖注入。
The injector doesn't know yet how to create a HeroService
.
If you ran the code now, Angular would fail with this error:
注入器还不知道该如何创建HeroService
。
如果现在运行我们的代码,Angular 就会失败,并报错:
To teach the injector how to make a HeroService
,
add the following providers
array property to the bottom of the component metadata
in the @Component
call.
我们还得注册一个HeroService
提供商,来告诉注入器如何创建HeroService
。
要做到这一点,我们在@Component
组件的元数据底部添加providers
数组属性如下:
src/app/app.component.ts (providers)
The providers
array tells Angular to create a fresh instance of the HeroService
when it creates an AppComponent
.
The AppComponent
, as well as its child components, can use that service to get hero data.
providers
数组告诉 Angular,当它创建新的AppComponent
组件时,也要创建一个HeroService
的新实例。
AppComponent
会使用那个服务来获取英雄列表,在它组件树中的每一个子组件也同样如此。
getHeroes() in the AppComponent
AppComponent 中的 getHeroes()
The service is in a heroService
private variable.
该服务被存入了一个私有变量heroService
中。
You could call the service and get the data in one line.
我们可以在同一行内调用此服务,并获得数据。
You don't really need a dedicated method to wrap one line. Write it anyway:
在真实的世界中,我们并不需要把一行代码包装成一个专门的方法,但无论如何,我们在演示代码中先这么写:
src/app/app.component.ts (getHeroes)
The ngOnInit lifecycle hook
ngOnInit 生命周期钩子
AppComponent
should fetch and display hero data with no issues.
毫无疑问,AppComponent
应该获取英雄数据并显示它。
You might be tempted to call the getHeroes()
method in a constructor, but
a constructor should not contain complex logic,
especially a constructor that calls a server, such as as a data access method.
The constructor is for simple initializations, like wiring constructor parameters to properties.
你可能想在构造函数中调用getHeroes()
方法,但构造函数不应该包含复杂的逻辑,特别是那些需要从服务器获取数据的逻辑更是如此。构造函数是为了简单的初始化工作而设计的,例如把构造函数的参数赋值给属性。
To have Angular call getHeroes()
, you can implement the Angular ngOnInit lifecycle hook.
Angular offers interfaces for tapping into critical moments in the component lifecycle:
at creation, after each change, and at its eventual destruction.
只要我们实现了 Angular 的 ngOnInit 生命周期钩子,Angular 就会主动调用这个钩子。 Angular提供了一些接口,用来介入组件生命周期的几个关键时间点:刚创建时、每次变化时,以及最终被销毁时。
Each interface has a single method. When the component implements that method, Angular calls it at the appropriate time.
每个接口都有唯一的一个方法。只要组件实现了这个方法,Angular 就会在合适的时机调用它。
Read more about lifecycle hooks in the Lifecycle Hooks page.
更多生命周期钩子信息,见生命周期钩子。
Here's the essential outline for the OnInit
interface (don't copy this into your code):
这是OnInit
接口的基本轮廓(但不要拷贝到你自己的代码中):
Add the implementation for the OnInit
interface to your export statement:
往export语句中添加OnInit
接口的实现:
Write an ngOnInit
method with the initialization logic inside. Angular will call it
at the right time. In this case, initialize by calling getHeroes()
.
我们写了一个带有初始化逻辑的ngOnInit
方法,Angular会在适当的时候调用它。
在这个例子中,我们通过调用getHeroes()
来完成初始化。
app/app.component.ts (ng-on-init)
The app should run as expected, showing a list of heroes and a hero detail view when you click on a hero name.
我们的应用将会像期望的那样运行,显示英雄列表,并且在我们点击英雄的名字时,显示英雄的详情。
Async services and Promises
异步服务与承诺
The HeroService
returns a list of mock heroes immediately;
its getHeroes()
signature is synchronous.
我们的HeroService
立即返回一个模拟的英雄列表,它的getHeroes()
函数签名是同步的。
Eventually, the hero data will come from a remote server. When using a remote server, users don't have to wait for the server to respond; additionally, you aren't able to block the UI during the wait.
但最终,英雄的数据会从远端服务器获取。当使用远端服务器时,用户不会等待服务器的响应。换句话说,你没法在等待期间阻塞浏览器界面。
To coordinate the view with the response,
you can use Promises, which is an asynchronous
technique that changes the signature of the getHeroes()
method.
为了协调视图与响应,我们可以使用承诺(Promise),它是一种异步技术,它会改变getHeroes()
方法的签名。
The hero service makes a Promise
HeroService
会生成一个承诺
A Promise essentially promises to call back when the results are ready. You ask an asynchronous service to do some work and give it a callback function. The service does that work and eventually calls the function with the results or an error.
承诺 就是 …… 好吧,它就是一个承诺,在有了结果时,它承诺会回调我们。 我们请求一个异步服务去做点什么,并且给它一个回调函数。 它会去做(在某个地方),一旦完成,它就会调用我们的回调函数,并通过参数把工作结果或者错误信息传给我们。
This is a simplified explanation. Read more about ES2015 Promises in the Promises for asynchronous programming page of Exploring ES6.
Update the HeroService
with this Promise-returning getHeroes()
method:
把HeroService
的getHeroes
方法改写为返回承诺的形式:
src/app/hero.service.ts (excerpt)
You're still mocking the data. You're simulating the behavior of an ultra-fast, zero-latency server, by returning an immediately resolved Promise with the mock heroes as the result.
我们继续使用模拟数据。我们通过返回一个 立即解决的承诺 的方式,模拟了一个超快、零延迟的超级服务器。
Act on the Promise
基于承诺的行动
As a result of the change to HeroService
, this.heroes
is now set to a Promise
rather than an array of heroes.
修改HeroService
之后,this.heroes
会被赋值为一个Promise
而不再是英雄数组。
src/app/app.component.ts (getHeroes - old)
You have to change the implementation to act on the Promise
when it resolves.
When the Promise
resolves successfully, you'll have heroes to display.
我们得修改这个实现,把它变成基于承诺的,并在承诺的事情被解决时再行动。 一旦承诺的事情被成功解决(Resolve),我们就会显示英雄数据。
Pass the callback function as an argument to the Promise's then()
method:
我们把回调函数作为参数传给承诺对象的then方法:
src/app/app.component.ts (getHeroes - revised)
As described in Arrow functions,
the ES2015 arrow function
in the callback is more succinct than the equivalent function expression and gracefully handles this
.
回调中所用的 ES2015 箭头函数 比等价的函数表达式更加简洁,能优雅的处理 this 指针。
The callback sets the component's heroes
property to the array of heroes returned by the service.
在回调函数中,我们把服务返回的英雄数组赋值给组件的heroes
属性。
The app is still running, showing a list of heroes, and responding to a name selection with a detail view.
我们的程序仍在运行,仍在显示英雄列表,在选择英雄时,仍然会把它/她显示在详情页面中。
At the end of this page, Appendix: take it slow describes what the app might be like with a poor connection.
查看附录中的“慢!”,来了解在较差的网络连接中这个应用会是什么样的。
Review the app structure
回顾本应用的结构
Verify that you have the following structure after all of your refactoring:
再检查下,经历了本章的所有重构之后,应该有了下列文件结构:
Here are the code files discussed in this page.
下面是本章讨论过的代码文件:
The road you've travelled
走过的路
Here's what you achieved in this page:
来盘点一下我们完成了什么。
You created a service class that can be shared by many components.
我们创建了一个能被多个组件共享的服务类。
You used the
ngOnInit
lifecycle hook to get the hero data when theAppComponent
activates.我们使用了
ngOnInit
生命周期钩子,以便在AppComponent
激活时获取英雄数据。You defined the
HeroService
as a provider for theAppComponent
.我们把
HeroService
定义为AppComponent
的一个提供商。You created mock hero data and imported them into the service.
我们创建了模拟的英雄数据,并把它导入我们的服务中。
You designed the service to return a Promise and the component to get the data from the Promise.
我们把服务设计为返回承诺,组件从承诺中获取数据。
Your app should look like this
现在应用变成了这样:
The road ahead
前方的路
The Tour of Heroes has become more reusable using shared components and services. The next goal is to create a dashboard, add menu links that route between the views, and format data in a template. As the app evolves, you'll discover how to design it to make it easier to grow and maintain.
通过使用共享组件和服务,我们的《英雄指南》更有复用性了。 我们还要创建一个仪表盘,要添加在视图间路由的菜单链接,还要在模板中格式化数据。 随着我们应用的进化,我们还会学到如何进行设计,让它更易于扩展和维护。
Read about the Angular component router and navigation among the views in the next tutorial page.
我们将在下一章学习 Angular 组件路由,以及在视图间导航的知识。
Appendix: Take it slow
附件:慢
To simulate a slow connection,
import the Hero
symbol and add the following getHeroesSlowly()
method to the HeroService
.
我们可以模拟慢速连接。导入Hero
类,并且在HeroService
中添加如下的getHeroesSlowly()
方法:
app/hero.service.ts (getHeroesSlowly)
Like getHeroes()
, it also returns a Promise
.
But this Promise waits two seconds before resolving the Promise with mock heroes.
像getHeroes()
一样,它也返回一个承诺。
但是,这个承诺会在提供模拟数据之前等待两秒钟。
Back in the AppComponent
, replace getHeroes()
with getHeroesSlowly()
and see how the app behaves.
回到AppComponent
,用heroService.getHeroesSlowly()
替换heroService.getHeroes()
,并观察应用的行为。