angular-docs-cn/public/docs/ts/latest/guide/dependency-injection.jade

include ../../../../_includes/_util-fns
:marked
  **Dependency Injection** is an important application design pattern.
  Angular has its own dependency injection framework and
  we really can't build an Angular application without it.
  It's used so widely that almost everyone just calls it "DI".

  In this chapter we'll learn [what DI is, why](#why-di) we want it.
  Then we'll learn [how to use it](#angular-di) in an Angular app.
  
  All code in this chapter is available as a 
  [live example](/resources/live-examples/dependency-injection/ts/plnkr.html)
  on the web.
    
<a id="why-di"></a>
.l-main-section
:marked
  ## Why Dependency Injection?

  Let's start with the following code.

+makeExample('dependency-injection/ts/app/car/car-no-di.ts', 'car', 'app/car/car.ts (no di)')

:marked
  Our `Car` creates everything it needs inside its constructor.
  What's the problem?

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

  Our `Car` needs an engine and tires. Instead of asking for them,
  the `Car` constructor creates its own copies by "new-ing" them from
  the very specific classes, `Engine` and `Tires`.

  What if the `Engine` class evolves and its constructor requires a parameter?
  Our `Car` is broken and stays broken until we rewrite it along the lines of
  `this.engine = new Engine(theNewParameter)`.
  We didn't care about `Engine` constructor parameters when we first wrote `Car`.
  We don't really care about them now.
  But we'll *have* to start caring because
  when the definion of `Engine` changes, our `Car` class must change.
  That makes `Car` brittle.

  What if we want to put a different brand of tires on our `Car`. Too bad.
  We're locked into whatever brand the `Tires` class creates. That makes our `Car` inflexible.

  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,
  we can think of other dependencies that should be shared ... like the onboard
  wireless connection to the manufacturer's service center. Our `Car` lacks the flexibility
  to share services that have been created previously for other consumers.

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

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

  We have no control over the car's hidden dependencies.
  When we can't control the dependencies, a class become difficult to test.

  How can we make `Car` more robust, more flexible, and more testable?

  That's super easy. We probably already know what to do. We change our `Car` constructor to this:

  <a id="ctor-injection"></a>

+makeTabs(
  'dependency-injection/ts/app/car/car.ts, dependency-injection/ts/app/car/car-no-di.ts',
  'car-ctor, car-ctor', 
  'app/car/car.ts DI (constructor), app/car/car.ts No DI (constructor)')(format=".")

:marked
  See what happened? We moved the definition of the dependencies to the constructor.
  Our `Car` class no longer creates an engine or tires.
  It just consumes them.

.l-sub-section
  :marked
    We also leverage TypeScript's constructor syntax for declaring parameters and properties simultaneously.
:marked
  Now we create a car by passing the engine and tires to the constructor.

+makeExample('dependency-injection/ts/app/car/car-creations.ts', 'car-ctor-instantiation', 'Simple Car')(format=".")

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

  If someone extends the `Engine` class, that is not `Car`'s problem.
.l-sub-section
  :marked
    The consumer of `Car` has the problem. The consumer must update the car creation code to
    something like:

  +makeExample('dependency-injection/ts/app/car/car-creations.ts', 'car-ctor-instantiation-with-param', 'Super Car')(format=".")

  :marked
    The critical point is this: `Car` itself did not have to change.
    We'll take care of the consumer's problem soon enough.

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

+makeExample('dependency-injection/ts/app/car/car-creations.ts', 'car-ctor-instantiation-with-mocks', 'Test Car')(format=".")

:marked
  **We just learned what Dependency Injection is**.

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

  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.
  We need something that takes care of assembling these parts for us.

  We could write a giant class to do that:

+makeExample('dependency-injection/ts/app/car/car-factory.ts', null, 'app/car/car-factory.ts')

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

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

  This is where the Dependency Injection Framework comes into play.
  Imagine the framework had something called an `Injector`.
  We register some classes with this `Injector` and it figures out how to create them.

  When we need a `Car`, we simply ask the `Injector` to get it for us and we're good to go.

+makeExample('dependency-injection/ts/app/car/car-injector.ts','injector-call')(format=".")

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

  This is what a **Dependency Injection Framework** is all about.

  Now that we know what Dependency Injection is and appreciate its benefits,
  let's see how it is implemented in Angular.

<a id="angular-di"></a>
.l-main-section
:marked
  ## Angular Dependency Injection

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

  That sounds nice. What does it do for us when building components in Angular?
  Let's see, one step at a time.

  We'll begin with a simplified version of the `HeroesComponent`
  that we built in the [The Tour of Heroes](../tutorial/).

+makeTabs(
  `dependency-injection/ts/app/heroes/heroes.component.1.ts,
  dependency-injection/ts/app/heroes/hero-list.component.1.ts,
  dependency-injection/ts/app/heroes/hero.ts,
  dependency-injection/ts/app/heroes/mock-heroes.ts`,
  'v1,,,',
  `app/heroes/heroes.component.ts,
  app/heroes/hero-list.component.ts,
  app/heroes/hero.ts,
  app/heroes/mock-heroes.ts`)

:marked
  The `HeroesComponent` is the root component of the *Heroes* feature area.
  It governs all the child components of this area. 
  In our stripped down version there is only one child, `HeroListComponent`,
  dedicated to displaying a list of heroes.
.l-sub-section
  :marked
    Do we really need so many files? Of course not! 
    We're going *beyond* the strictly necessary 
    in order to illustrate patterns that will serve us well in real applications.
    With each file we can make one or two points rather than crowd them all into one file.
:marked
  Right now it gets heroes from `HEREOES`,  an in-memory collection, 
  defined in another file and imported by this component.
  That may suffice in the early stages of development but it's far from ideal.
  As soon as we try to test this component or want to get our heroes data from a remote server,
  we'll have to change the implementation of `heroes` and
  fix every other use of the `HEROES` mock data.

  Let's make a service that hides how we get hero data.
.l-sub-section
  :marked
    Write this service in its own file. See [this note](#forward-ref) to understand why.

+makeExample('dependency-injection/ts/app/heroes/hero.service.1.ts',null, 'app/heroes/hero.service.ts' )
:marked
  Our `HeroService` exposes a `getHeroes()` method that returns
  the same mock data as before but none of its consumers need to know that.
.l-sub-section
  :marked
    We aren't even pretending this is a real service.
    If we were actually getting data from a remote server, the API would have to be asynchronous,
    perhaps returning
    [ES2015 promises](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise)
    We'd also have to rewrite the way components consume our service.
    This is important but not important to our current story.
:marked
  A service is nothing more than a class in Angular 2.
  It remains nothing more than a class until we register it with an Angular injector.

  ### Configuring the Injector

  <a id="bootstrap"></a>
  We don't have to create an Angular injector.
  Angular creates an application-wide injector for us during the bootstrap process.

+makeExample('dependency-injection/ts/app/main.ts', 'bootstrap', 'app/main.ts (bootstrap)')(format='.')
:marked
  We do have to configure the injector by registering the **providers** 
  that create the services we need in our application.
  We'll explain what [providers](#providers) are later in this chapter.
  Before we do, let's see an example of provider registration during bootstrapping:
  
+makeExample('dependency-injection/ts/app/main.1.ts', 'bootstrap')(format='.')
:marked
  The injector now knows about our `HeroService`.
  An instance of our `HeroService` will be available for injection across our entire application.

  Of course we can't help wondering about that comment telling us not to do it this way.
  It *will* work. It's just not a best practice. 
  The bootstrap provider option is intended for configuring and overriding Angular's own
  pre-registered services.
  
  The preferred approach is to register application providers in application components.
  Because the `HeroService` will be used within the *Heroes* feature area &mdash;
  and nowhere else &mdash; the ideal place to register it is in the top-level `HeroesComponent`.
  
  ### Registering providers in a component
  Here's a revised `HeroesComponent` that registers the `HeroService`.
  
+makeExample('dependency-injection/ts/app/heroes/heroes.component.1.ts',null,'app/heroes/heroes.component.ts')
:marked
  Look closely at the bottom of the `@Component` metadata:
  
+makeExample('dependency-injection/ts/app/heroes/heroes.component.1.ts','providers')(format='.')
:marked
  An instance of the `HeroService` is now available for injection in this `HeroesComponent`
  and all of its child components.
   
  The `HeroesComponent` itself doesn't happen to need the `HeroService`.
  But its child `HeroListComponent` does so we head there next.
  
  ### Preparing the *HeroListComponent* for injection

  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 we explained
  earlier](#ctor-injection).
  It's a small change as we see in this comparison:
+makeTabs(
  `dependency-injection/ts/app/heroes/hero-list.component.2.ts,
  dependency-injection/ts/app/heroes/hero-list.component.1.ts`,
  null,
  `app/heroes/hero-list.component (DI), 
   app/heroes/hero-list.component (no DI)`)
:marked

.l-sub-section
  :marked
    ### Focus on the constructor
  +makeExample('dependency-injection/ts/app/heroes/hero-list.component.2.ts', 'ctor')(format=".")
  :marked
    Adding a parameter to the constructor isn't all that's happening here.

    We're writing in TypeScript and have followed the parameter name with a type annotation, `:HeroService`.
    The class is also decorated with the `@Component` decorator (scroll up to confirm that fact).

    When the TypeScript compiler evaluates this class, it sees the `@Component` decorator and adds class metadata
    into the generated JavaScript code. Within that metadata lurks the information that
    associates the `heroService` parameter with the `HeroService` class.

    That's how the Angular injector knows to inject an instance of the `HeroService` when it
    creates a new `HeroListComponent`.
 <a id="di-metadata"></a>
:marked
  ### Creating the injector (implicitly)
  When we introduced the idea of an injector above, we showed how to create
  an injector and use it to create a new `Car`.

+makeExample('dependency-injection/ts/app/car/car-injector.ts','injector-create-and-call')(format=".")

:marked
  We won't find code like that in the Tour of Heroes or any of our other samples.
  We *could* write code [like that](#explicit-injector) if we *had* to which we rarely do.
  Angular takes care of creating and calling injectors
  when it creates components for us whether through HTML markup, as in `<hero-list></hero-list>`,
  or after navigating to a component with the [router](./router.html).
  Let Angular do its job and we'll enjoy the benefits of automated dependency injection.

  ### Singleton services
  Dependencies are singletons within the scope of an injector.
  In our example, there is a single `HeroService` instance shared among the
  `HeroesComponent` and its `HeroListComponent` children.

  However, Angular DI is an hierarchical injection
  system which means nested injectors can create their own service instances.
  Learn more about that in the [Hierarchical Injection](./hierarchical-dependency-injection.html) chapter.

  ### Testing the component
  We emphasized earlier that designing a class for dependency injection makes the class easier to test.
  Listing dependencies as constructor parameters may be all we need to test application parts effectively.
  
  For example, we can create a new `HeroListComponent` with a mock service that we can manipulate
  under test:

+makeExample('dependency-injection/ts/app/test.component.ts', 'spec')(format='.')
.l-sub-section
  :marked
    Learn more in the [Testing](../testing/index.html) chapter.

:marked
  ### When the service needs a service
  Our `HeroService` is very simple. It doesn't have any dependencies of its own.


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

  Here is the revision compared to the original.
  
+makeTabs(
  `dependency-injection/ts/app/heroes/hero.service.2.ts, 
  dependency-injection/ts/app/heroes/hero.service.1.ts`,
  null, 
  `app/heroes/hero.service (v.2),
  app/heroes/hero.service (v.1)`)

:marked
  The constructor now asks for an injected instance of a `Logger` and stores it in a private property called `_logger`.
  We call that property within our `getHeroes()` method when anyone asks for heroes.

  <a id="injectable"></a>
  ### Why *@Injectable*?
  Notice the `@Injectable()` decoration above the service class.
  We haven't seen `@Injectable()` before.
  As it happens, we could have added it to our first version`HeroService`.
  We didn't bother because we didn't need it then.

  We need it now... now that our service has an injected dependency.
  We need it because Angular requires constructor parameter metadata in order to inject a `Logger`.
  As [we mentioned earlier](#di-metadata), TypeScript *only generates metadata for classes that have a decorator*. .
  
.callout.is-helpful
  header Always add @Injectable() 
  :marked
    We recommend adding `@Injectable()` to every service class, even those that don't have dependencies
    and, therefore, do not technically require it. Two reasons:
  ol(style="font-size:inherit")
    li <i>Future proofing</i>: no need to remember <code>@Injectable</code> when we add a dependency later.
    li <i>Consistency</i>: all services follow the same rules and we don't have to wonder why a decorator is missing.
.l-sub-section
  :marked
    The `HeroesComponent` has an injected dependency too. Why don't we add `@Injectable()` to the `HeroesComponent`?
    
    We *can* add it if we really want to. It isn't necessary because the `HeroesComponent` is already decorated with `@Component`.
    TypeScript generates metadata for *any* class with a decorator and *any* decorator will do.

.alert.is-critical
  :marked
    **Always include the parentheses!** Always call `@Injectable()`.
    Our application will fail mysteriously if we forget the parentheses.

.l-main-section
:marked
  ## Create and register the *Logger* service
  We're injecting a Logger into our `HeroService`. 
  We have two remaining steps:
  1. Create the Logger service
  1. Register it with the application

  The logger service implementation is no big deal. 
  
+makeExample(
  'dependency-injection/ts/app/logger.service.ts',null, 'app/logger.service')
:marked
  We're likely to need the same logger service everywhere in our application 
  so we put it at the root level of the application in the `app/` folder and
  we register it in the `providers` array of the metadata for our application root component, `AppComponent`.
+makeExample('dependency-injection/ts/app/providers.component.ts','providers-1', 'app/app.component.ts (providers)')
:marked
  If we forget to register it, Angular will throw an exception when it first needs the logger:
code-example(format).
  EXCEPTION: No provider for Logger! (HeroListComponent -> HeroService -> Logger)
:marked
  That's telling us that the dependency injector couldn't find the *provider* for the logger
  when it first tried to call the logger &mdash; inside the `HeroService` when the `HeroListComponent`
  was calling for heroes.
  The *provider* is the subject of our next section.
  
  But wait!  What if the logger is optional?
  <a id="optional"></a>
  ### Optional dependencies
  
  Our `HeroService` currently requires a `Logger`. What if we could get by without a logger?
  We'd use it if we had it, ignore it if we didn't.
  
  First we import the `@Optional` decorator.
+makeExample('dependency-injection/ts/app/providers.component.ts','import-optional')(format='.')
:marked
  Then rewrite the constructor with that decorator to make the logger optional.
+makeExample('dependency-injection/ts/app/providers.component.ts','provider-10-ctor')(format='.')
:marked
  Be prepared for a null logger. If we don't register one somewhere up the line,
  the injector will inject `null`. We have a method that logs. What can we do?
  
  We could substitute a *do-nothing* logger stub so that our methods continue to work:
+makeExample('dependency-injection/ts/app/providers.component.ts','provider-10-logger')(format='.')
:marked
  Obviously we'd take a more sophisticated approach if the logger were optional
  elsewhere as well. 
  
  But enough about optional loggers. In our sample application, the `Logger` is required. 
  We must register a `Logger` with the application injector using *providers*
  as we learn in the next section.

<a id="providers"></a>
.l-main-section
:marked
  ## Injector Providers
  
  A provider *provides* the concrete, runtime version of a dependency value.
  The injector relies on **providers** to create instances of the services 
  that it injects into components and other services.

  We must register *providers* with the injector or it won't know what to do. 
  
  Earlier we registered the `Logger` service in the `providers` array of the metadata for the `AppComponent` like this:
+makeExample('dependency-injection/ts/app/providers.component.ts','providers-logger')
:marked
  The `providers` array appears to hold service classes (one service class in this example). 
  In reality it holds instances of the [Provider](../api/core/Provider-class.html) class.
  
  In our example, when the `HeroService` constructor specifies `logger:Logger`, it's expecting
  something that has the shape and behavior of the `Logger` class.
  
  There are many ways to *provide* something that has the shape and behavior of a `Logger`.
  The `Logger` class itself is an obvious and natural provider - it has the right shape and it's designed to be created. 
  But it's not the only way.
  
  We can configure the injector with alternative providers that can deliver an object that behaves like a `Logger`.
  We could provide a substitute class. We could provide a logger-like object.
  We could give it a provider that calls a logger factory function.
  Any of these approaches might be a good choice under the right circumstances.

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

  <a id="provide"></a>
  ### The *provide* function

  We wrote the `providers` array like this:
+makeExample('dependency-injection/ts/app/providers.component.ts','providers-1')
:marked
  This is actually a short-hand expression for a provider registration that creates a new instance of the
  [Provider](/docs/ts/latest/api/core/Provider-class.html) class.
+makeExample('dependency-injection/ts/app/providers.component.ts','providers-2')
:marked
  The [provide](../api/core/provide-function.html) function is the more common and friendlier way to create a `Provider`:
+makeExample('dependency-injection/ts/app/providers.component.ts','providers-3')
:marked
  In both approaches &mdash; `Provider` class and `provide` function &mdash; 
  we supply two arguments.

  The first is the [token](#token) that serves as the key for both locating a dependency value
  and registering the provider.

  The second is a provider definition object
  which we think of as a *recipe* for creating the dependency value.
  There are many ways to create dependency values ... and many ways to write a recipe.

  <a id="class-provider"></a>
  ### Alternative Class Providers

  Occasionally we'll ask a different class to provide the service. 
  In this example, we tell the injector
  to return a `BetterLogger` when something asks for the `Logger`.

+makeExample('dependency-injection/ts/app/providers.component.ts','providers-4')
:marked
  ### Class provider with dependencies
  Maybe an `EvenBetterLogger` could display the user name in the log message.
  This logger gets the user from the injected `UserService`
  which happens also to be injected at the application level.  
+makeExample('dependency-injection/ts/app/providers.component.ts','EvenBetterLogger')
:marked
  Configure it like we did `BetterLogger`.
+makeExample('dependency-injection/ts/app/providers.component.ts','providers-5')(format=".")
:marked
  ### Aliased Class Providers

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

  When the *old* component logs a message with `OldLogger`, 
  we want the one singleton instance of `NewLogger` to handle it instead.
  
  The dependency injector should inject that singleton instance
  when a component asks for either the new or the the old logger.
  The `OldLogger` should be an alias for `NewLogger`.
  
  We certainly do not want two different `NewLogger` instances in our app. 
  Unfortunately, that's what we get if we try to alias `OldLogger` to `NewLogger` with `useClass`.
  
+makeExample('dependency-injection/ts/app/providers.component.ts','providers-6a')(format=".")
:marked
  Alias with the `useExisting` option instead.
+makeExample('dependency-injection/ts/app/providers.component.ts','providers-6b')(format=".")

<a id="value-provider"></a>
:marked
  ### Value Providers

  Sometimes it's easier to provide a ready-made object rather than ask the injector to create it from a class.
+makeExample('dependency-injection/ts/app/providers.component.ts','silent-logger')(format=".")
:marked
  Then we register a provider with this object playing the logger role via the `useValue` option.
+makeExample('dependency-injection/ts/app/providers.component.ts','providers-7')(format=".")

<a id="factory-provider"></a>
:marked
  ### Factory Providers

  Sometimes we need to create the dependent value dynamically, 
  based on information we won't have until the last possible moment.
  Maybe the information can change.
  
  Suppose also that the injectable service has no independent access to the source of this information.
  
  This situation calls for a **factory provider** as we illustrate next.
  
  Our HeroService should hide *secret* heroes from normal users. 
  Only authorized users should see them.
  
  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 we log in a different user.
  
  Unlike `EvenBetterLogger`, we can't inject the `UserService` into the `HeroService`.
.l-sub-section
  :marked
    Why? We don't know either. Stuff like this happens.
:marked
  Instead the `HeroService` constructor takes a boolean flag to control display of secret heroes.
+makeExample('dependency-injection/ts/app/heroes/hero.service.ts','internals', 'app/heroes/hero.service.ts (internals)')(format='.')
:marked
  We can inject the `Logger` but we can't inject the  boolean `isAuthorized`. 
  We'll have to take over the creation of new instances of this `HeroService` with a factory provider.
  
  A factory provider needs a factory function:
+makeExample('dependency-injection/ts/app/heroes/hero.service.provider.ts','factory', 'app/heroes/hero.service.provider.ts (factory)')(format='.')
:marked
  Although our `HeroService` knows nothing about the `UserService`, our factory function does.
  
  We inject both the `Logger` and the `UserService` into the factory provider:
+makeExample('dependency-injection/ts/app/heroes/hero.service.provider.ts','provider', 'app/heroes/hero.service.provider.ts (provider)')(format='.')
:marked

.l-sub-section
  :marked
    The `useFactory` field tells Angular that the provider is a factory function 
    whose implementation is the `heroServiceFactory`.

    The `deps` property is an array of [provider tokens](#token).
    The `Logger` and `UserService` classes serve as tokens for their own class providers.
:marked
  Notice that we've captured the factory provider in an exported variable, `heroServiceProvider`.
  This extra step makes it easier for us to register our `HeroService` whereever we need it.
  
  In our sample, we need it only in the `HeroesComponent` 
  where it replaces the previous `HeroService` registration in the metadata `providers` array:
+makeTabs(
  `dependency-injection/ts/app/heroes/heroes.component.ts, 
  dependency-injection/ts/app/heroes/heroes.component.1.ts`,
  null, 
  `app/heroes/heroes.component (v.3),
  app/heroes/heroes.component (v.2)`)
  
<a id="token"></a>
.l-main-section
:marked
  ## Dependency Injection Tokens

  When we register a provider with an injector we associate that provider with a dependency injection token.
  The injector maintains an internal *token/provider* map that it references when
  asked for a dependency
  
  In all previous examples, the dependency value has been a class *instance* and 
  the class *type* served as its own lookup token.
  Here we get a `HeroService` directly from the injector by supplying the `HeroService` type as the token.
+makeExample('dependency-injection/ts/app/injector.component.ts','get-hero-service')(format='.')
:marked
  We have similar good fortune (in typescript) when we write a constructor that requires an injected class-based dependency.
  We define a constructor parameter with the `HeroService` class type and Angular knows to inject the
  service associated with that `HeroService` class token:
+makeExample('dependency-injection/ts/app/providers.component.ts','provider-8-ctor')(format=".")
:marked
  This is all especially convenient when we consider that most dependency values are provided by classes.
  
  ### Non-class Dependencies
  
  What if the dependency value isn't a class? 
  Sometimes the thing we want to inject is a string, a function, or an object.

  Applications often define configuration objects with lots of small facts like the title of the application or the address of a web api endpoint.
  These configuration objects aren't always instances of a class. They're just objects ... like this one:

+makeExample('dependency-injection/ts/app/app.config.ts','config','app/app-config.ts')(format='.')
:marked
  We'd like to make this `config` object available for injection.
  We know we can register an object with a [Value Provider](#value-provider).
  But what do we use for the token?
  We don't have a class to serve as a token. There is no `Config` class.
  
// begin Typescript only
<a id="interface"></a>
:marked
  The `CONFIG` constant has an interface, `Config`. Unfortunately, we
  **cannot use an interface as a token**
+makeExample('dependency-injection/ts/app/providers.component.ts','providers-9a-interface')(format=".")
+makeExample('dependency-injection/ts/app/providers.component.ts','provider-9a-ctor-interface')(format=".")
:marked
  It's not Angular's fault. An interface is a TypeScript design-time artifact. 
  It disappears from the generated JavaScript so there is no interface type information for Angular to find at runtime.
// end Typescript only
<a id="string-token"></a>
:marked
  ### String tokens
  Fortunately, we can register any dependency provider with a string token.
+makeExample('dependency-injection/ts/app/providers.component.ts','providers-9a')(format=".")
:marked
  Now we inject the configuration object into any constructor that needs it with
  the help of an `@Inject` decorator to tell Angular how to find the configuration dependency value.
+makeExample('dependency-injection/ts/app/providers.component.ts','provider-9a-ctor')(format=".")
:marked
 
// begin Typescript only
.l-sub-section
  :marked
    Although it plays no role in dependency injection, 
    the `Config` interface supports strongly-typing of the configuration object within the class.
:marked  
// end typescript only 

<a id="opaque-token"></a>
:marked
  ### OpaqueToken
  Alternatively, we could define and use an [OpaqueToken](../api/core/OpaqueToken-class.html)
  rather than rely on magic strings that may collide with other developers' string choices.
  
  The definition looks like this:
+makeExample('dependency-injection/ts/app/app.config.ts','token')(format='.')
:marked
  Substitute `APP_CONFIG` for the magic string in provider registration and constructor:
+makeExample('dependency-injection/ts/app/providers.component.ts','providers-9b')(format=".")
+makeExample('dependency-injection/ts/app/providers.component.ts','provider-9b-ctor')(format=".")
:marked
  Here's how we provide and inject the configuration object in our top-level `AppComponent`.
+makeExample('dependency-injection/ts/app/app.component.ts','providers', 'app/app.component.ts (providers)')(format=".")
+makeExample('dependency-injection/ts/app/app.component.ts','ctor', 'app/app.component.ts (constructor)')(format=".")

.l-sub-section
  :marked
    Angular uses `OpaqueTokens` to register all of its non-class dependencies.
    
    Internally, the `Provider` turns both the string and the class type into an `OpaqueToken`
    and keys its *token/provider* map with that.

.l-main-section
:marked
  # Next Steps
  We learned the basics of Angular Dependency Injection in this chapter.

  The Angular Dependency Injection is more capable than we've described.
  We can learn more about its advanced features, beginning with its support for
  nested injectors, in the 
  [Hierarchical Dependency Injection](hierarchical-dependency-injection.html) chapter.
  
.l-main-section
<a id="explicit-injector"></a>
:marked
  ### Appendix: Working with injectors directly
  We rarely work directly with an injector.
  Here's an `InjectorComponent` that does.
  
+makeExample('dependency-injection/ts/app/injector.component.ts', 'injector',
             'app/injector.component.ts')
:marked
  Angular injects the component's own `Injector` which the component uses to acquire services.
  The services themselves are not injected. They're retrieved via the injector.
  
  The `get` method throws an error if it can't resolve the requested service.
  We can call `getOptional` instead, which we do in one case
  to retrieve a service (`ROUS`) that isn't registered with this or any ancestor injector.
  
.l-sub-section
  :marked
    This technique is an example of the [Service Locator Pattern](https://en.wikipedia.org/wiki/Service_locator_pattern).
    
    We **avoid** this technique unless we genuinely need it.
    It encourages a careless grab-bag approach such as we see here.
    It's difficult to explain, understand, and test.
    We 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.    
    We're forced to spelunk the implementation and hope for the best.
    
    Framework developers may take this approach when they 
    must acquire services generically and dynamically.

.l-main-section
<a id="forward-ref"></a>
:marked
  ### Appendix: Why we recommend one class per file
  
  Having multiple classes in the same file is confusing and best avoided.
  Developers expect one class per file. Keep them happy.

  If we scorn this advice and, say, 
  combine our `HeroService` class with the `HeroesComponent` in the same file,
  **define the component last!**
  If we define the component before the service,
  we'll get a runtime null reference error.

.l-sub-section
  :marked
    We actually can define the component first with the help of the `forwardRef()` method as explained
    in this [blog post](http://blog.thoughtram.io/angular/2015/09/03/forward-references-in-angular-2.html).
    But why flirt with trouble?
    Avoid the problem altogether and define components and services in separate files.