Dependency injection is an important application design pattern. Angular has its own dependency injection framework, and you really can't build an Angular application without it. It's used so widely that almost everyone just calls it DI.

依赖注入是重要的程序设计模式。 Angular 有自己的依赖注入框架，离开了它，几乎没法构建 Angular 应用。 它使用得非常广泛，以至于几乎每个人都会把它简称为 DI。

This page covers what DI is, why it's so useful, and how to use it in an Angular app.

本章将学习什么是 DI，它有什么用。 然后，将学习在 Angular 应用中该如何使用它。

Contents

• Why dependency injection?

  为什么需要依赖注入？

• Angular dependency injection

  Angular的依赖注入

  • Configuring the injector

    配置注入器

  • Registering providers in an NgModule

    在NgModule中注册提供商

  • Registering providers in a component

    在组件中注册提供商

  • When to use NgModule versus an application component

    什么时候使用NgModule？什么时候使用应用组件？

  • Preparing the HeroListComponent for injection

    为依赖注入准备HeroListComponent

  • Implicit injector creation

    隐式依赖注入

  • Singleton services

    单例服务

  • Testing the component

    测试组件

  • When the service needs a service

    什么时候服务需要另一个服务

  • Why @Injectable()?

    为什么要加@Injectable()？

• Creating and registering a logger service

  创建并注册日志服务

• Injector providers

  注入器提供商

  • The Provider class and provide object literal

  • Provider类和provide对象字面量

  • Alternative class providers

    代用类提供商

  • Class provider with dependencies

    带有依赖的类提供商

  • Aliased class providers

    别名类提供商

  • Value providers

    值提供商

  • Factory providers

    工厂提供商

• Dependency injection tokens

  依赖注入令牌

  • Non-class dependencies

    非“类”依赖

  • InjectionToken

• Optional dependencies

  可选依赖

• Summary

  总结

• Appendix: Working with injectors directly

  附录：直接使用注入器

Run the .

运行.

Why dependency injection?

为什么需要依赖注入？

To understand why dependency injection is so important, consider an example without it. Imagine writing the following code:

要理解为什么依赖注入这么重要，不妨先考虑不使用它的一个例子。想象下列代码：

The Car class creates everything it needs inside its constructor. What's the problem?

Car类会在它的构造函数中创建所需的每样东西。 问题何在？

The problem is that the Car class is brittle, inflexible, and hard to test.

问题在于，这个Car类过于脆弱、缺乏弹性并且难以测试。

This Car needs an engine and tires. Instead of asking for them, the Car constructor instantiates its own copies from the very specific classes Engine and Tires.

Car类需要一个引擎 (engine) 和一些轮胎 (tire)，它没有去请求现成的实例， 而是在构造函数中用具体的Engine和Tires类实例化出自己的副本。

What if the Engine class evolves and its constructor requires a parameter? That would break the Car class and it would stay broken until you rewrote it along the lines of this.engine = new Engine(theNewParameter). The Engine constructor parameters weren't even a consideration when you first wrote Car. You may not anticipate them even now. But you'll have to start caring because when the definition of Engine changes, the Car class must change. That makes Car brittle.

如果Engine类升级了，它的构造函数要求传入一个参数，这该怎么办？ 这个Car类就被破坏了，在把创建引擎的代码重写为this.engine = new Engine(theNewParameter)之前，它都是坏的。 当第一次写Car类时，我们不关心Engine构造函数的参数，现在也不想关心。 但是，当Engine类的定义发生变化时，就不得不在乎了，Car类也不得不跟着改变。 这就会让Car类过于脆弱。

What if you want to put a different brand of tires on your Car? Too bad. You're locked into whatever brand the Tires class creates. That makes the Car class inflexible.

如果想在Car上使用不同品牌的轮胎会怎样？太糟了。 我们被锁定在Tires类创建时使用的那个品牌上。这让Car类缺乏弹性。

Right now each new car gets its own engine. It can't share an engine with other cars. While that makes sense for an automobile engine, surely you can think of other dependencies that should be shared, such as the onboard wireless connection to the manufacturer's service center. This Car lacks the flexibility to share services that have been created previously for other consumers.

现在，每辆车都有它自己的引擎。它不能和其它车辆共享引擎。 虽然这对于汽车来说还算可以理解，但是设想一下那些应该被共享的依赖，比如用来联系厂家服务中心的车载无线电。 我们的车缺乏必要的弹性，无法共享当初给其它消费者创建的车载无线电。

When you write tests for Car you're at the mercy of its hidden dependencies. Is it even possible to create a new Engine in a test environment? What does Engine depend upon? What does that dependency depend on? Will a new instance of Engine make an asynchronous call to the server? You certainly don't want that going on during tests.

当给Car类写测试的时候，我们就会受制于它背后的那些依赖。 能在测试环境中成功创建新的Engine吗？ Engine自己又依赖什么？那些依赖本身又依赖什么？ Engine的新实例会发起到服务器的异步调用吗？ 我们当然不想在测试期间这么一层层追下去。

What if the Car should flash a warning signal when tire pressure is low? How do you confirm that it actually does flash a warning if you can't swap in low-pressure tires during the test?

如果Car应该在轮胎气压低的时候闪动警示灯该怎么办？ 如果没法在测试期间换上一个低气压的轮胎，那该如何确认它能正确的闪警示灯？

You have no control over the car's hidden dependencies. When you can't control the dependencies, a class becomes difficult to test.

我们没法控制这辆车背后隐藏的依赖。 当不能控制依赖时，类就会变得难以测试。

How can you make Car more robust, flexible, and testable?

该如何让Car更强壮、有弹性以及可测试？

That's super easy. Change the Car constructor to a version with DI:

答案非常简单。把Car的构造函数改造成使用 DI 的版本：

See what happened? The definition of the dependencies are now in the constructor. The Car class no longer creates an engine or tires. It just consumes them.

发生了什么？我们把依赖的定义移到了构造函数中。 Car类不再创建引擎engine或者轮胎tires。 它仅仅“消费”它们。

Now you can create a car by passing the engine and tires to the constructor.

现在，通过往构造函数中传入引擎和轮胎来创建一辆车。

How cool is that? The definition of the engine and tire dependencies are decoupled from the Car class. You can pass in any kind of engine or tires you like, as long as they conform to the general API requirements of an engine or tires.

酷！引擎和轮胎这两个依赖的定义与Car类本身解耦了。 只要喜欢，可以传入任何类型的引擎或轮胎，只要它们能满足引擎或轮胎的通用 API 需求。

Now, if someone extends the Engine class, that is not Car's problem.

这样一来，如果有人扩展了Engine类，那就不再是Car类的烦恼了。

The Car class is much easier to test now because you are in complete control of its dependencies. You can pass mocks to the constructor that do exactly what you want them to do during each test:

Car类非常容易测试，因为现在我们对它的依赖有了完全的控制权。 在每个测试期间，我们可以往构造函数中传入 mock 对象，做想让它们做的事：

You just learned what dependency injection is.

刚刚学习了什么是依赖注入

It's a coding pattern in which a class receives its dependencies from external sources rather than creating them itself.

它是一种编程模式，可以让类从外部源中获得它的依赖，而不必亲自创建它们。

Cool! But what about that poor consumer? Anyone who wants a Car must now create all three parts: the Car, Engine, and Tires. The Car class shed its problems at the consumer's expense. You need something that takes care of assembling these parts.

酷！但是，可怜的消费者怎么办？ 那些希望得到一个Car的人们现在必须创建所有这三部分了：Car、Engine和Tires。 Car类把它的快乐建立在了消费者的痛苦之上。 需要某种机制为我们把这三个部分装配好。

You could write a giant class to do that:

可以写一个巨型类来做这件事：

It's not so bad now with only three creation methods. But maintaining it will be hairy as the application grows. This factory is going to become a huge spiderweb of interdependent factory methods!

现在只需要三个创建方法，这还不算太坏。 但是当应用规模变大之后，维护它将变得惊险重重。 这个工厂类将变成由相互依赖的工厂方法构成的巨型蜘蛛网。

Wouldn't it be nice if you could simply list the things you want to build without having to define which dependency gets injected into what?

如果能简单的列出想建造的东西，而不用定义该把哪些依赖注入到哪些对象中，那该多好！

This is where the dependency injection framework comes into play. Imagine the framework had something called an injector. You register some classes with this injector, and it figures out how to create them.

到了依赖注入框架一展身手的时候了！ 想象框架中有一个叫做注入器 (injector) 的东西。 用这个注入器注册一些类，它会弄明白如何创建它们。

When you need a Car, you simply ask the injector to get it for you and you're good to go.

当需要一个Car时，就简单的找注入器取车就可以了。

Everyone wins. The Car knows nothing about creating an Engine or Tires. The consumer knows nothing about creating a Car. You don't have a gigantic factory class to maintain. Both Car and consumer simply ask for what they need and the injector delivers.

皆大欢喜。Car不需要知道如何创建Engine和Tires。 消费者不需要知道如何创建Car。 开发人员不需要维护巨大的工厂类。 Car和消费者只要简单地请求想要什么，注入器就会交付它们。

This is what a dependency injection framework is all about.

这就是“依赖注入框架”存在的原因。

Now that you know what dependency injection is and appreciate its benefits, read on to see how it is implemented in Angular.

现在，我们知道了什么是依赖注入，以及它的优点。再来看看它在 Angular 中是怎么实现的。

Angular dependency injection

Angular 依赖注入

Angular ships with its own dependency injection framework. This framework can also be used as a standalone module by other applications and frameworks.

Angular 附带了自己的依赖注入框架。此框架也能被当做独立模块用于其它应用和框架中。

To see what it can do when building components in Angular, start with a simplified version of the HeroesComponent that from the The Tour of Heroes.

要了解Angular构建组件时注入器做了什么，我们先从英雄指南中构建的HeroesComponent的简化版本开始。

The HeroesComponent is the root component of the Heroes feature area. It governs all the child components of this area. This stripped down version has only one child, HeroListComponent, which displays a list of heroes.

HeroesComponent是英雄特性区域的根组件。它管理区域内所有子组件。 简化后的版本只有一个子组件HeroListComponent，用来显示英雄列表。

Right now HeroListComponent gets heroes from HEROES, an in-memory collection defined in another 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 want to get your heroes data from a remote server, you'll have to change the implementation of heroes and fix every other use of the HEROES mock data.

现在HeroListComponent从HEROES获得英雄数据，是在另一个文件中定义的内存数据集。 它在开发的早期阶段可能还够用，但离完美就差得远了。 一旦开始测试此组件，或者想从远端服务器获得英雄数据，就不得不修改heroes的实现， 还要修改每个用到了HEROES模拟数据的地方。

It's better to make a service that hides how the app gets hero data.

最好用一个服务把获取英雄数据的代码封装起来。

The following HeroService exposes a getHeroes method that returns the same mock data as before, but none of its consumers need to know that.

HeroService暴露了getHeroes方法，返回跟以前一样的模拟数据，但它的消费者不需要知道这一点。

A service is nothing more than a class in Angular. It remains nothing more than a class until you register it with an Angular injector.

服务只是 Angular 中的一个类。 有 Angular 注入器注册它之前，没有任何特别之处。

Configuring the injector

配置注入器

You don't have to create an Angular injector. Angular creates an application-wide injector for you during the bootstrap process.

不需要创建 Angular 注入器。 Angular 在启动过程中自动为我们创建一个应用级注入器。

You do have to configure the injector by registering the providers that create the services the application requires. This guide explains what providers are later.

我们必须通过注册提供商 (provider) 来配置注入器，这些提供商为应用创建所需服务。 在本章的稍后部分会解释什么是提供商。

You can either register a provider within an NgModule or in application components.

或者在 NgModule 中注册提供商，或者在应用组件中。

Registering providers in an NgModule

在NgModule中注册提供商

Here's the AppModule that registers two providers, UserService and an APP_CONFIG provider, in its providers array.

在AppModule的providers中注册了两个提供商，UserService和APP_CONFIG。

Because the HeroService is used only within the HeroesComponent and its subcomponents, the top-level HeroesComponent is the ideal place to register it.

由于HeroService只会在HeroesComponent及其子组件中使用，所以顶层组件HeroesComponent是一个合理的注册位置。

Registering providers in a component

在组件中注册提供商

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

下面是更新的HerosComponent，把HeroService注册到了它的providers数组中。

When to use NgModule versus an application component

该用 NgModule 还是应用组件？

On the one hand, a provider in an NgModule is registered in the root injector. That means that every provider registered within an NgModule will be accessible in the entire application.

一方面，NgModule 中的提供商是被注册到根注入器。这意味着在 NgModule 中注册的提供商可以被整个应用访问。

On the other hand, a provider registered in an application component is available only on that component and all its children.

另一方面，在应用组件中注册的提供商只在该组件及其子组件中可用。

Here, the APP_CONFIG service needs to be available all across the application, so it's registered in the AppModule @NgModule providers array. But since the HeroService is only used within the Heroes feature area and nowhere else, it makes sense to register it in the HeroesComponent.

这里，APP_CONFIG服务需要在应用中到处可用，所以它被注册到了AppModule @NgModule的providers数组。 但是，由于HeroService只在英雄特性区用到了，其它地方没有用过，因此在HeroesComponent中注册它是有道理的。

Preparing the HeroListComponent for injection

为注入准备HeroListComponent

The HeroListComponent should get heroes from the injected HeroService. Per the dependency injection pattern, the component must ask for the service in its constructor, as discussed earlier. It's a small change:

HeroListComponent应该从注入的HeroService获取英雄数据。 遵照依赖注入模式的要求，组件必须在它的构造函数中请求这些服务，就像以前解释过的那样。 只是个小改动：

Implicit injector creation

隐式注入器的创建

You saw how to use an injector to create a new Car earlier in this guide. You could create such an injector explicitly:

本章前面的部分我们看到了如何使用注入器来创建一个新Car。 你可以像这样显式创建注入器：

You won't find code like that in the Tour of Heroes or any of the other documentation samples. You could write code that explicitly creates an injector if you had to, but it's not always the best choice. Angular takes care of creating and calling injectors when it creates components for you—whether through HTML markup, as in <hero-list></hero-list>, or after navigating to a component with the router. If you let Angular do its job, you'll enjoy the benefits of automated dependency injection.

无论在《英雄指南》还是其它范例中，都没有出现这样的代码。 在必要时，可以写使用显式注入器的代码，但却很少这样做。 当 Angular 创建组件时 —— 无论通过像<hero-list></hero-list>这样的 HTML 标签还是通过路由导航到组件 —— 它都会自己管理好注入器的创建和调用。 只要让 Angular 做好它自己的工作，我们就能安心享受“自动依赖注入”带来的好处。

Singleton services

单例服务

Dependencies are singletons within the scope of an injector. In this guide's example, a single HeroService instance is shared among the HeroesComponent and its HeroListComponent children.

在一个注入器的范围内，依赖都是单例的。 在这个例子中，HeroesComponent和它的子组件HeroListComponent共享同一个HeroService实例。

However, Angular DI is a hierarchical injection system, which means that nested injectors can create their own service instances. For more information, see Hierarchical Injectors.

然而，Angular DI 是一个分层的依赖注入系统，这意味着嵌套的注入器可以创建它们自己的服务实例。 要了解更多知识，参见多级依赖注入器一章。

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：

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 revision compared to the original.

下面是在原来的基础上所做的修改：

The constructor now asks for an injected instance of a Logger and stores it in a private property called logger. You call that property within the getHeroes() method when anyone asks for heroes.

现在，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的私有属性中。 当别人请求英雄数据时，在getHeroes()方法中调用这个属性的方法。

Why @Injectable()?

为什么要用 @Injectable()?

@Injectable() marks a class as available to an injector for instantiation. Generally speaking, an injector reports an error when trying to instantiate a class that is not marked as @Injectable().

@Injectable() 标识一个类可以被注入器实例化。 通常，在试图实例化没有被标识为@Injectable()的类时，注入器会报错。

Injectors are also responsible for instantiating components like HeroesComponent. So why doesn't HeroesComponent have @Injectable()?

注入器同时负责实例化像HerosComponent这样的组件。为什么不标记HerosComponent为@Injectable()呢？

You can add it if you really want to. It isn't necessary because the HeroesComponent is already marked with @Component, and this decorator class (like @Directive and @Pipe, which you learn about later) is a subtype of @Injectable(). It is in fact @Injectable() decorators that identify a class as a target for instantiation by an injector.

我们可以添加它。但是没有必要，因为HerosComponent已经有@Component装饰器了， @Component（和随后将会学到的@Directive和@Pipe一样）是 Injectable 的子类型。 实际上，正是这些@Injectable()装饰器是把一个类标识为注入器实例化的目标。

Creating and registering a logger service

创建和注册日志服务

Inject a logger into HeroService in two steps:

要把日志服务注入到HeroService中需要两步：

1. Create the logger service.

   创建日志服务。

2. Register it with the application.

   把它注册到应用中。

The logger service is quite simple:

这个日志服务很简单：

You're likely to need the same logger service everywhere in your application, so put it in the project's app folder and register it in the providers array of the application module, AppModule.

应用的每个角落都可能需要日志服务，所以把它放到项目的app目录， 并在应用模块AppModule的元数据providers数组里注册它。

If you forget to register the logger, Angular throws an exception when it first looks for the logger:

如果忘了注册这个日志服务，Angular 会在首次查找这个日志服务时，抛出一个异常。

That's Angular telling you that the dependency injector couldn't find the provider for the logger. It needed that provider to create a Logger to inject into a new HeroService, which it needed to create and inject into a new HeroListComponent.

Angular 告诉我们，依赖注入器找不到日志服务的提供商。 在创建HeroListComponent的新实例时需要创建并注入HeroService， 而HeroService需要创建并注入一个Logger实例， Angular 需要这个Logger实例的提供商来。

The chain of creations started with the Logger provider. Providers are the subject of the next section.

这个“创建链”始于Logger的提供商。这个提供商就是下一节的主题 。

Injector providers

注入器的提供商们

A 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 and other services.

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

You must register a service provider with the 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，否则它不知道该如何创建该服务。

Earlier you registered the Logger service in the providers array of the metadata for the AppModule like this:

我们在前面通过AppModule元数据中的providers数组注册过Logger服务，就像这样：

There are many ways to provide something that looks and behaves like a Logger. The Logger class itself is an obvious and natural provider. But it's not the only way.

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。 但它不是唯一的选项。

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.

最重要的是，当注入器需要一个Logger时，它得先有一个提供商。

The Provider class and provide object literal

Provider类和一个提供商的字面量

You wrote the providers array like this:

像下面一样写providers数组：

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 first is the token that serves as the key for both locating a dependency value and registering the provider.

第一个是令牌 (token)，它作为键值 (key) 使用，用于定位依赖值和注册提供商。

The second is 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。

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。

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。

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。

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。

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：

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：

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。

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。

InjectionToken

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。

Register the dependency provider using the InjectionToken object:

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：

Aternatively, 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。

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。

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。

Appendix: Why have 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。