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Angular Dependency Injection

Dependency Injection (DI) is a way to create objects that depend upon other objects. A Dependency Injection system supplies the dependent objects (called the dependencies) when it creates an instance of an object.

The Dependency Injection pattern page describes this general approach. The guide you're reading now explains how Angular's own Dependency Injection system works.

DI by example

You'll learn Angular Dependency Injection through a discussion of the sample app that accompanies this guide. Run the anytime.

Start by reviewing this simplified version of the heroes feature from the The Tour of Heroes.

The HeroesComponent is the top-level heroes component. It's only purpose is to display the HeroListComponent which displays a list of hero names.

This version of the HeroListComponent gets its heroes from the HEROES array, an in-memory collection defined in a separate mock-heroes file.

That may suffice in the early stages of development, but it's far from ideal. As soon as you try to test this component or get heroes from a remote server, you'll have to change the implementation of HerosListComponent and replace every other use of the HEROES mock data.

It's better to hide these details inside a service class, defined in its own file.

Create an injectable HeroService

The Angular CLI can generate a new HeroService class in the src/app/heroes folder with this command.

ng generate service heroes/hero

That command creates the following HeroService skeleton.

Assume for now that the @Injectable decorator is an essential ingredient in every Angular service definition. The rest of the class has been rewritten to expose a getHeroes method that returns the same mock data as before.

Of course, this isn't a real data service. If the app were actually getting data from a remote server, the getHeroes method signature would have to be asynchronous.

That's a defect we can safely ignore in this guide where our focus is on injecting the service into the HeroList component.

{@a injector-config} {@a bootstrap}

Register a service provider

A service is just a class in Angular until you register it with an Angular dependency injector.

An Angular injector is responsible for creating service instances and injecting them into classes like the HeroListComponent.

You rarely create an Angular injector yourself. Angular creates injectors for you as it executes the app, starting with the root injector that it creates during the bootstrap process.

You do have to register providers with an injector before the injector can create that service.

Providers tell the injector how to create the service. Without a provider, the injector would not know that it is responsible for injecting the service nor be able to create the service.

You'll learn much more about providers below. For now it is sufficient to know that they create services and must be registered with an injector.

You can register a provider with any Angular decorator that supports the providers array property.

Many Angular decorators accept metadata with a providers property. The two most important examples are @Component and @NgModule.

{@a register-providers-component}

@Component providers

在组件中注册提供商

Here's a revised HeroesComponent that registers the HeroService in its providers array.

下面是修改过的HerosComponent，把HeroService注册到了它的providers数组中。

{@a register-providers-ngmodule}

@NgModule providers

In the following excerpt, the root AppModule registers two providers in its providers array.

The first entry registers the UserService class (not shown) under the UserService injection token. The second registers a value (HERO_DI_CONFIG) under the APP_CONFIG injection token.

Thanks to these registrations, Angular can inject the UserService or the HERO_DI_CONFIG value into any class that it creates.

You'll learn about injection tokens and provider syntax below.

{@a ngmodule-vs-comp}

@NgModule or @Component?

Should you register a service with an Angular module or with a component? The two choices lead to differences in service scope and service lifetime.

Angular module providers (@NgModule.providers) are registered with the application's root injector. Angular can inject the corresponding services in any class it creates. Once created, a service instance lives for the life of the app and Angular injects this one service instance in every class that needs it.

You're likely to inject the UserService in many places throughout the app and will want to inject the same service instance every time. Providing the UserService with an Angular module is a good choice.

To be precise, Angular module providers are registered with the root injector unless the module is lazy loaded. In this sample, all modules are eagerly loaded when the application starts, so all module providers are registered with the app's root injector.

A component's providers (@Component.providers) are registered with each component instance's own injector.

Angular can only inject the corresponding services in that component instance or one of its descendant component instances. Angular cannot inject the same service instance anywhere else.

Note that a component-provided service may have a limited lifetime. Each new instance of the component gets its own instance of the service and, when the component instance is destroyed, so is that service instance.

In this sample app, the HeroComponent is created when the application starts and is never destroyed so the HeroService created for the HeroComponent also live for the life of the app.

If you want to restrict HeroService access to the HeroComponent and its nested HeroListComponent, providing the HeroService in the HeroComponent may be a good choice.

The scope and lifetime of component-provided services is a consequence of the way Angular creates component instances.

Inject a service

The HeroListComponent should get heroes from the HeroService.

The component shouldn't create the HeroService with new. It should ask for the HeroService to be injected.

You can tell Angular to inject a dependency in the component's constructor by specifying a constructor parameter with the dependency type. Here's the HeroListComponent constructor, asking for the HeroService to be injected.

Of course, the HeroListComponent should do something with the injected HeroService. Here's the revised component, making use of the injected service, side-by-side with the previous version for comparison.

Notice that the HeroListComponent doesn't know where the HeroService comes from. You know that it comes from the parent HeroesComponent. But if you decided instead to provide the HeroService in the AppModule, the HeroListComponent wouldn't change at all. The only thing that matters is that the HeroService is provided in some parent injector.

{@a singleton-services}

Singleton services

Services are singletons within the scope of an injector. There is at most one instance of a service in a given injector.

There is only one root injector and the UserService is registered with that injector. Therefore, there can be just one UserService instance in the entire app and every class that injects UserService get this service instance.

However, Angular DI is a hierarchical injection system, which means that nested injectors can create their own service instances. Angular creates nested injectors all the time.

{@a component-child-injectors}

Component child injectors

For example, when Angular creates a new instance of a component that has @Component.providers, it also creates a new child injector for that instance.

Component injectors are independent of each other and each of them creates its own instances of the component-provided services.

When Angular destroys one of these component instance, it also destroys the component's injector and that injector's service instances.

Thanks to injector inheritance, you can still inject application-wide services into these components. A component's injector is a child of its parent component's injector, and a descendent of its parent's parent's injector, and so on all the way back to the application's root injector. Angular can inject a service provided by any injector in that lineage.

For example, Angular could inject a HeroListComponent with both the HeroService provided in HeroComponent and the UserService provided in AppModule.

{@a testing-the-component}

Testing the component

测试组件

Earlier you saw that designing a class for dependency injection makes the class easier to test. Listing dependencies as constructor parameters may be all you need to test application parts effectively.

前面强调过，设计一个适合依赖注入的类，可以让这个类更容易测试。要有效的测试应用中的一部分，只需要在构造函数的参数中列出依赖。

For example, you can create a new HeroListComponent with a mock service that you can manipulate under test:

例如，新建的HeroListComponent实例使用一个模拟 (mock) 服务，以便可以在测试中操纵它：

Learn more in the Testing guide.

要学习更多知识，参见测试一章。

{@a service-needs-service}

When the service needs a service

当服务需要别的服务时

The HeroService is very simple. It doesn't have any dependencies of its own.

这个HeroService非常简单。它本身不需要任何依赖。

What if it had a dependency? What if it reported its activities through a logging service? You'd apply the same constructor injection pattern, adding a constructor that takes a Logger parameter.

如果它也有依赖，该怎么办呢？例如，它需要通过日志服务来汇报自己的活动。我们同样用构造函数注入模式，来添加一个带有Logger参数的构造函数。

Here is the revised HeroService that injects the Logger, side-by-side with the previous service for comparison.

The constructor asks for an injected instance of a Logger and stores it in a private field called logger. The getHeroes() method logs a message when asked to fetch heroes.

这个构造函数要求注入一个Logger类的实例，并把它存到名为logger的私有字段中。当请求英雄数据时，getHeroes()中就会记录一个消息。

{@a logger-service}

The dependent Logger service

The sample app's Logger service is quite simple:

If the app didn't provide this Logger, Angular would throw an exception when it looked for a Logger to inject into the HeroService.

ERROR Error: No provider for Logger!

Because a singleton logger service is useful everywhere, it's provided in the root AppModule.

{@a injectable}

@Injectable()

The @Injectable() decorator identifies a service class that might require injected dependencies.

The HeroService must be annotated with @Injectable() because it requires an injected Logger.

Always write @Injectable() with parentheses, not just @Injectable.

When Angular creates a class whose constructor has parameters, it looks for type and injection metadata about those parameters so that it can inject the right service.

If Angular can't find that parameter information, it throws an error.

Angular can only find the parameter information if the class has a decorator of some kind. While any decorator will do, the @Injectable() decorator is the standard decorator for service classes.

The decorator requirement is imposed by TypeScript.

TypeScript normally discards parameter type information when it transpiles the code to JavaScript. It preserves this information if the class has a decorator and the emitDecoratorMetadata compiler option is set true in TypeScript's tsconfig.json configuration file, .

The CLI configures tsconfig.json with emitDecoratorMetadata: true It's your job to put @Injectable() on your service classes.

The Logger service is annotated with @Injectable() decorator too, although it has no constructor and no dependencies.

In fact, every Angular service class in this app is annotated with the @Injectable() decorator, whether or not it has a constructor and dependencies. @Injectable() is a required coding style for services.

{@a providers}

Providers

服务提供商们

A service provider provides the concrete, runtime version of a dependency value. The injector relies on providers to create instances of the services that the injector injects into components, directives, pipes, and other services.

服务提供商提供依赖值的一个具体的、运行时的版本。注入器依靠提供商来创建服务的实例，注入器再将服务的实例注入组件、管道或其它服务。

You must register a service provider with an injector, or it won't know how to create the service.

必须为注入器注册一个服务的提供商，否则它就不知道该如何创建该服务。

The next few sections explain the many ways you can specify a provider.

Almost all of the accompanying code snippets are extracts from the sample app's providers.component.ts file.

The class as its own provider

There are many ways to provide something that looks and behaves like a Logger. The Logger class itself is an obvious and natural provider.

有很多方式可以提供一些实现 Logger类的东西。 Logger类本身是一个显而易见而且自然而然的提供商。

But it's not the only way.

但它不是唯一的途径。

You can configure the injector with alternative providers that can deliver an object that behaves like a Logger. You could provide a substitute class. You could provide a logger-like object. You could give it a provider that calls a logger factory function. Any of these approaches might be a good choice under the right circumstances.

可以用其它备选提供商来配置注入器，只要它们能交付一个行为类似于Logger的对象就可以了。可以提供一个替代类。你可以提供一个类似日志的对象。可以给它一个提供商，让它调用可以创建日志服务的工厂函数。所有这些方法，只要用在正确的场合，都可能是一个好的选择。

What matters is that the injector has a provider to go to when it needs a Logger.

重点是，当注入器需要一个Logger时，它得先有一个提供商。

{@a provide}

The provide object literal

provide 对象字面量

Here's the class-provider syntax again.

This is actually a shorthand expression for a provider registration using a provider object literal with two properties:

这其实是用于注册提供商的简写表达式。使用的是一个带有两个属性的_提供商_对象字面量：

The provide property holds the token that serves as the key for both locating a dependency value and registering the provider.

provide属性保存的是令牌 (token)，它作为键值 (key) 使用，用于定位依赖值和注册提供商。

The second property is always a provider definition object, which you can think of as a recipe for creating the dependency value. There are many ways to create dependency values just as there are many ways to write a recipe.

第二个是一个提供商定义对象。可以把它看做是指导如何创建依赖值的配方。有很多方式创建依赖值…… 也有很多方式可以写配方。

{@a class-provider}

Alternative class providers

备选的类提供商

Occasionally you'll ask a different class to provide the service. The following code tells the injector to return a BetterLogger when something asks for the Logger.

某些时候，我们会请求一个不同的类来提供服务。下列代码告诉注入器，当有人请求Logger时，返回BetterLogger。

{@a class-provider-dependencies}

Class provider with dependencies

带依赖的类提供商

Maybe an EvenBetterLogger could display the user name in the log message. This logger gets the user from the injected UserService, which is also injected at the application level.

假设EvenBetterLogger可以在日志消息中显示用户名。这个日志服务从注入的UserService中取得用户， UserService通常也会在应用级注入。

Configure it like BetterLogger.

就像之前在BetterLogger中那样配置它。

{@a aliased-class-providers}

Aliased class providers

别名类提供商

Suppose an old component depends upon an OldLogger class. OldLogger has the same interface as the NewLogger, but for some reason you can't update the old component to use it.

假设某个旧组件依赖一个OldLogger类。 OldLogger和NewLogger具有相同的接口，但是由于某些原因，我们不能升级这个旧组件并使用它。

When the old component logs a message with OldLogger, you'd like the singleton instance of NewLogger to handle it instead.

当旧组件想使用OldLogger记录消息时，我们希望改用NewLogger的单例对象来记录。

The dependency injector should inject that singleton instance when a component asks for either the new or the old logger. The OldLogger should be an alias for NewLogger.

不管组件请求的是新的还是旧的日志服务，依赖注入器注入的都应该是同一个单例对象。也就是说，OldLogger应该是NewLogger的别名。

You certainly do not want two different NewLogger instances in your app. Unfortunately, that's what you get if you try to alias OldLogger to NewLogger with useClass.

我们当然不会希望应用中有两个不同的NewLogger实例。不幸的是，如果尝试通过useClass来把OldLogger作为NewLogger的别名，就会导致这样的后果。

The solution: alias with the useExisting option.

解决方案：使用useExisting选项指定别名。

{@a value-provider}

Value providers

值提供商

Sometimes it's easier to provide a ready-made object rather than ask the injector to create it from a class.

有时，提供一个预先做好的对象会比请求注入器从类中创建它更容易。

Then you register a provider with the useValue option, which makes this object play the logger role.

于是可以通过useValue选项来注册提供商，它会让这个对象直接扮演 logger 的角色。

See more useValue examples in the Non-class dependencies and InjectionToken sections.

查看更多useValue的例子，见非类依赖和 InjectionToken部分。

{@a factory-provider}

Factory providers

工厂提供商

Sometimes you need to create the dependent value dynamically, based on information you won't have until the last possible moment. Maybe the information changes repeatedly in the course of the browser session.

有时，我们需要动态创建这个依赖值，因为它所需要的信息直到最后一刻才能确定。也许这个信息会在浏览器的会话中不停地变化。

Suppose also that the injectable service has no independent access to the source of this information.

还假设这个可注入的服务没法通过独立的源访问此信息。

This situation calls for a factory provider.

这种情况下，请调用工厂提供商。

To illustrate the point, add a new business requirement: the HeroService must hide secret heroes from normal users. Only authorized users should see secret heroes.

下面通过添加新的业务需求来说明这一点： HeroService 必须对普通用户隐藏掉秘密英雄。只有授权用户才能看到秘密英雄。

Like the EvenBetterLogger, the HeroService needs a fact about the user. It needs to know if the user is authorized to see secret heroes. That authorization can change during the course of a single application session, as when you log in a different user.

就像EvenBetterLogger那样，HeroService需要了解此用户的身份。它需要知道，这个用户是否有权看到隐藏英雄。这个授权可能在单一的应用会话中被改变，例如，改用另一个用户的身份登录时。

Unlike EvenBetterLogger, you can't inject the UserService into the HeroService. The HeroService won't have direct access to the user information to decide who is authorized and who is not.

与EvenBetterLogger不同，不能把UserService注入到HeroService中。 HeroService无权访问用户信息，来决定谁有授权谁没有授权。

Instead, the HeroService constructor takes a boolean flag to control display of secret heroes.

让HeroService的构造函数带上一个布尔型的标志，来控制是否显示隐藏的英雄。

You can inject the Logger, but you can't inject the boolean isAuthorized. You'll have to take over the creation of new instances of this HeroService with a factory provider.

我们可以注入Logger，但是不能注入逻辑型的isAuthorized。我们不得不通过通过工厂提供商创建这个HeroService的新实例。

A factory provider needs a factory function:

工厂提供商需要一个工厂方法：

Although the HeroService has no access to the UserService, the factory function does.

虽然HeroService不能访问UserService，但是工厂方法可以。

You inject both the Logger and the UserService into the factory provider and let the injector pass them along to the factory function:

同时把Logger和UserService注入到工厂提供商中，并且让注入器把它们传给工厂方法：

The useFactory field tells Angular that the provider is a factory function whose implementation is the heroServiceFactory.

useFactory字段告诉 Angular：这个提供商是一个工厂方法，它的实现是heroServiceFactory。

The deps property is an array of provider tokens. The Logger and UserService classes serve as tokens for their own class providers. The injector resolves these tokens and injects the corresponding services into the matching factory function parameters.

deps属性是提供商令牌数组。 Logger和UserService类作为它们自身类提供商的令牌。注入器解析这些令牌，把相应的服务注入到工厂函数中相应的参数中去。

Notice that you captured the factory provider in an exported variable, heroServiceProvider. This extra step makes the factory provider reusable. You can register the HeroService with this variable wherever you need it.

注意，我们在一个导出的变量中捕获了这个工厂提供商：heroServiceProvider。这个额外的步骤让工厂提供商可被复用。无论哪里需要，都可以使用这个变量注册HeroService。

In this sample, you need it only in the HeroesComponent, where it replaces the previous HeroService registration in the metadata providers array. Here you see the new and the old implementation side-by-side:

在这个例子中，只在HeroesComponent中需要它，这里，它代替了元数据providers数组中原来的HeroService注册。对比一下新的和旧的实现：

{@a token}

Dependency injection tokens

依赖注入令牌

When you register a provider with an injector, you associate that provider with a dependency injection token. The injector maintains an internal token-provider map that it references when asked for a dependency. The token is the key to the map.

当向注入器注册提供商时，实际上是把这个提供商和一个 DI 令牌关联起来了。注入器维护一个内部的令牌-提供商映射表，这个映射表会在请求依赖时被引用到。令牌就是这个映射表中的键值。

In all previous examples, the dependency value has been a class instance, and the class type served as its own lookup key. Here you get a HeroService directly from the injector by supplying the HeroService type as the token:

在前面的所有例子中，依赖值都是一个类实例，并且类的类型作为它自己的查找键值。在下面的代码中，HeroService类型作为令牌，直接从注入器中获取HeroService 实例：

You have similar good fortune when you write a constructor that requires an injected class-based dependency. When you define a constructor parameter with the HeroService class type, Angular knows to inject the service associated with that HeroService class token:

编写需要基于类的依赖注入的构造函数对我们来说是很幸运的。只要定义一个HeroService类型的构造函数参数， Angular 就会知道把跟HeroService类令牌关联的服务注入进来：

This is especially convenient when you consider that most dependency values are provided by classes.

这是一个特殊的规约，因为大多数依赖值都是以类的形式提供的。

{@a non-class-dependencies}

Non-class dependencies

非类依赖

What if the dependency value isn't a class? Sometimes the thing you want to inject is a string, function, or object.

如果依赖值不是一个类呢？有时候想要注入的东西是一个字符串，函数或者对象。

Applications often define configuration objects with lots of small facts (like the title of the application or the address of a web API endpoint) but these configuration objects aren't always instances of a class. They can be object literals such as this one:

应用程序经常为很多很小的因素定义配置对象（例如应用程序的标题或网络API终点的地址）。但是这些配置对象不总是类的实例，它们可能是对象，如下面这个：

What if you'd like to make this configuration object available for injection? You know you can register an object with a value provider.

我们想让这个配置对象在注入时可用，而且知道可以使用值提供商来注册一个对象。

But what should you use as the token? You don't have a class to serve as a token. There is no AppConfig class.

但是，这种情况下用什么作令牌呢？我们没办法找一个类来当作令牌，因为没有Config类。

TypeScript interfaces aren't valid tokens

TypeScript 接口不是一个有效的令牌

The HERO_DI_CONFIG constant conforms to the AppConfig interface. Unfortunately, you cannot use a TypeScript interface as a token:

CONFIG常量有一个接口：AppConfig。不幸的是，不能把 TypeScript 接口用作令牌：

That seems strange if you're used to dependency injection in strongly typed languages, where an interface is the preferred dependency lookup key.

对于习惯于在强类型的语言中使用依赖注入的开发人员，这会看起来很奇怪，因为在强类型语言中，接口是首选的用于查找依赖的主键。

It's not Angular's doing. An interface is a TypeScript design-time artifact. JavaScript doesn't have interfaces. The TypeScript interface disappears from the generated JavaScript. There is no interface type information left for Angular to find at runtime.

这不是 Angular 的错。接口只是 TypeScript 设计时 (design-time) 的概念。JavaScript 没有接口。 TypeScript 接口不会出现在生成的 JavaScript 代码中。在运行期，没有接口类型信息可供 Angular 查找。

{@a injection-token}

InjectionToken

InjectionToken 值

One solution to choosing a provider token for non-class dependencies is to define and use an InjectionToken. The definition of such a token looks like this:

解决方案是为非类依赖定义和使用InjectionToken作为提供商令牌。定义方式是这样的：

The type parameter, while optional, conveys the dependency's type to developers and tooling. The token description is another developer aid.

类型参数，虽然是可选的，但可以向开发者和开发工具传达类型信息。而且这个令牌的描述信息也可以为开发者提供帮助。

使用这个InjectionToken对象注册依赖的提供商：

Now you can inject the configuration object into any constructor that needs it, with the help of an @Inject decorator:

现在，在@Inject装饰器的帮助下，这个配置对象可以注入到任何需要它的构造函数中：

Although the AppConfig interface plays no role in dependency injection, it supports typing of the configuration object within the class.

虽然AppConfig接口在依赖注入过程中没有任何作用，但它为该类中的配置对象提供了强类型信息。

Alternatively, you can provide and inject the configuration object in an ngModule like AppModule.

或者在 ngModule 中提供并注入这个配置对象，如AppModule。

{@a optional}

Optional dependencies

可选依赖

The HeroService requires a Logger, but what if it could get by without a logger? You can tell Angular that the dependency is optional by annotating the constructor argument with @Optional():

HeroService需要一个Logger，但是如果想不提供 Logger 也能得到它，该怎么办呢？可以把构造函数的参数标记为@Optional()，告诉 Angular 该依赖是可选的：

When using @Optional(), your code must be prepared for a null value. If you don't register a logger somewhere up the line, the injector will set the value of logger to null.

当使用@Optional()时，代码必须准备好如何处理空值。如果其它的代码没有注册一个 logger，注入器会设置该logger的值为空 null。

Summary

总结

You learned the basics of Angular dependency injection in this page. You can register various kinds of providers, and you know how to ask for an injected object (such as a service) by adding a parameter to a constructor.

本章，我们学习了 Angular 依赖注入的基础知识。我们可以注册很多种类的提供商，知道如何通过添加构造函数的参数来请求一个注入对象（例如一个服务）。

Angular dependency injection is more capable than this guide has described. You can learn more about its advanced features, beginning with its support for nested injectors, in Hierarchical Dependency Injection.

Angular 依赖注入比前面描述的更能干。学习更多高级特性，如对嵌套注入器的支持，见多级依赖注入一章。

{@a explicit-injector}

Appendix: Working with injectors directly

附录：直接使用注入器

Developers rarely work directly with an injector, but here's an InjectorComponent that does.

这里的InjectorComponent直接使用了注入器，但我们很少直接使用它。

An Injector is itself an injectable service.

Injector本身是可注入的服务。

In this example, Angular injects the component's own Injector into the component's constructor. The component then asks the injected injector for the services it wants in ngOnInit().

在这个例子中，Angular 把组件自身的Injector注入到了组件的构造函数中。然后，组件在ngOnInit()中向注入的注入器请求它所需的服务。

Note that the services themselves are not injected into the component. They are retrieved by calling injector.get().

注意，这些服务本身没有注入到组件，它们是通过调用injector.get()获得的。

The get() method throws an error if it can't resolve the requested service. You can call get() with a second parameter, which is the value to return if the service is not found. Angular can't find the service if it's not registered with this or any ancestor injector.

get()方法如果不能解析所请求的服务，会抛出异常。调用get()时，还可以使用第二个参数，一旦获取的服务没有在当前或任何祖先注入器中注册过，就把它作为返回值。

The technique is an example of the service locator pattern.

刚描述的这项技术是服务定位器模式的一个范例。

Avoid this technique unless you genuinely need it. It encourages a careless grab-bag approach such as you see here. It's difficult to explain, understand, and test. You can't know by inspecting the constructor what this class requires or what it will do. It could acquire services from any ancestor component, not just its own. You're forced to spelunk the implementation to discover what it does.

要避免使用此技术，除非确实需要它。它会鼓励鲁莽的方式，就像在这里看到的。它难以解释、理解和测试。仅通过阅读构造函数，没法知道这个类需要什么或者它将做什么。它可以从任何祖先组件中获得服务，而不仅仅是它自己。会迫使我们深入它的实现，才可能明白它都做了啥。

Framework developers may take this approach when they must acquire services generically and dynamically.

框架开发人员必须采用通用的或者动态的方式获取服务时，可能采用这个方法。

{@a one-class-per-file}

Appendix: one class per file

附录：为什么建议每个文件只放一个类

Having multiple classes in the same file is confusing and best avoided. Developers expect one class per file. Keep them happy.

在同一个文件中有多个类容易造成混淆，最好避免。开发人员期望每个文件只放一个类。这会让它们开心点。

If you combine the HeroService class with the HeroesComponent in the same file, define the component last. If you define the component before the service, you'll get a runtime null reference error.

如果我们蔑视这个建议，并且 —— 比如说 —— 把HeroService和HeroesComponent组合在同一个文件里， 就得把组件定义放在最后面！ 如果把组件定义在了服务的前面，在运行时抛出空指针错误。

You actually can define the component first with the help of the forwardRef() method as explained in this blog post.

But it's best to avoid the problem altogether by defining components and services in separate files.

在forwardRef()方法的帮助下，实际上也可以先定义组件，具体说明见这篇博客。但是为什么要先给自己找麻烦呢？还是通过在独立的文件中定义组件和服务，完全避免此问题吧。

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