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

block includes
  include ../_util-fns
  - var _thisDot = 'this.';

: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 and why we want it.
  Then we'll learn [how to use it](#angular-di) in an Angular app.

  - [Why dependency injection?](#why-dependency-injection)
  - [Angular dependency injection](#angular-dependency-injection)
  - [Injector providers](#injector-providers)
  - [Dependency injection tokens](#dependency-injection-tokens)
  - [Summary](#summary)

  Run the <live-example></live-example>.

.l-main-section#why-di
: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 (without 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 instantiates its own copies 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
  `#{_thisDot}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 definition 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, such as 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 becomes difficult to test.

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

  <a id="ctor-injection"></a>
  That's super easy. We change our `Car` constructor to a version with DI:

+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 (excerpt with DI), app/car/car.ts (excerpt without DI)')(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.

block ctor-syntax
  .l-sub-section
    :marked
      We also leveraged 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', '')(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 this:

  - var stylePattern = { otl: /(new Car.*$)/gm };
  +makeExample('dependency-injection/ts/app/car/car-creations.ts', 'car-ctor-instantiation-with-param', '', stylePattern)(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:

- var stylePattern = { otl: /(new Car.*$)/gm };
+makeExample('dependency-injection/ts/app/car/car-creations.ts', 'car-ctor-instantiation-with-mocks', '', stylePattern)(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 factory is going to become a huge spiderweb 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.

.l-main-section#angular-di
: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.
  Our stripped down version has only one child, `HeroListComponent`,
  which displays a list of heroes.

:marked
  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 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
    Given that the service is a
    [separate concern](https://en.wikipedia.org/wiki/Separation_of_concerns),
    we suggest that you
    write the service code in its own file.
  +ifDocsFor('ts')
    :marked
      See [this note](#one-class-per-file) for details.

+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
    Notice the `@Injectable()` #{_decorator} above the service class.
    We'll discuss its purpose [shortly](#injectable).

- var _perhaps = _docsFor == 'dart' ? '' : 'perhaps';
.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 a !{_PromiseLinked}.
    We'd also have to rewrite the way components consume our service.
    This is important in general, but not 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.

#bootstrap
:marked
  ### Configuring the injector

  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 (excerpt)')(format='.')

:marked
  We do have to configure the injector by registering the **providers**
  that create the services our application requires.
  We'll explain what [providers](#providers) are later in this chapter.

  We can either register a provider within an [NgModule](ngmodule.html) or in application components

  ### Registering providers in an NgModule
  Here's our AppModule where we register a `Logger`, an `UserService`, and an `APP_CONFIG` provider.

- var stylePattern = { otl: /(providers)/ };
+makeExample('dependency-injection/ts/app/app.module.ts', 'ngmodule','app/app.module.ts', stylePattern)(format='.')
  <!--The preferred approach is to register application providers in application components.
  Because the `HeroService` is used within the *Heroes* feature area &mdash;
  and nowhere else &mdash; the ideal place to register it is in the top-level `HeroesComponent`.-->

:marked
  ### Registering providers in a component
  Here's a revised `HeroesComponent` that registers the `HeroService`.

+makeExample('dependency-injection/ts/app/heroes/heroes.component.1.ts', 'full','app/heroes/heroes.component.ts', stylePattern)(format='.')

:marked
  ### When to use the NgModule and when an application component?
  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.

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

  We want the `APP_CONFIG` service to be available all across the application, but a `HeroService` is only used within the *Heroes*
  feature area &mdash; and nowhere else. &mdash;

.l-sub-section
  :marked
    Read also **Should I add providers to the root AppModule or the root AppComponent?** at the [NgModule](ngmodule.html#q-root-component-or-module) chapter.

:marked
  ### 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:

+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 (with DI),
   app/heroes/hero-list.component (without DI)`)

.l-sub-section
  :marked
    #### Focus on the constructor

    Adding a parameter to the constructor isn't all that's happening here.

  +makeExample('dependency-injection/ts/app/heroes/hero-list.component.2.ts', 'ctor')(format=".")

  :marked
    Note that the constructor parameter has the type `HeroService`, and that
    the `HeroListComponent` class has an `@Component` #{_decorator}
    (scroll up to confirm that fact).
    Also recall that the parent component (`HeroesComponent`)
    has `providers` information for `HeroService`.

    The constructor parameter type, the `@Component` #{_decorator},
    and the parent's `providers` information combine to tell the
    Angular injector to inject an instance of
    `HeroService` whenever it creates a new `HeroListComponent`.

#di-metadata
:marked
  ### Implicit injector creation

  When we introduced the idea of an injector above, we showed how to
  use it to create a new `Car`. Here we also show how such an injector
  would be explicitly created:

+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 that [explicitly creates an injector](#explicit-injector) if we *had* to, but we rarely do.
  Angular takes care of creating and calling injectors
  when it creates components for us &mdash; whether through HTML markup, as in `<hero-list></hero-list>`,
  or after navigating to a component with the [router](./router.html).
  If we let Angular do its job, we'll enjoy the benefits of automated dependency injection.

:marked
  ### Singleton services

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

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

:marked
  ### 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 [Testing](../testing/index.html).

: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 (v2),
  app/heroes/hero.service (v1)`)

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

//- FIXME refer to Dart API when that page becomes available.
- var injMetaUrl = 'https://angular.io/docs/ts/latest/api/core/index/InjectableMetadata-class.html';
h3#injectable Why @Injectable()?
:marked
  **<a href="#{injMetaUrl}">@Injectable()</a>** marks a class as available to an
  injector for instantiation. Generally speaking, an injector will report an
  error when trying to instantiate a class that is not marked as
  `@Injectable()`.

block injectable-not-always-needed-in-ts
  .l-sub-section
    :marked
      As it happens, we could have omitted `@Injectable()` from our first
      version of `HeroService` because it had no injected parameters.
      But we must have it now that our service has an injected dependency.
      We need it because Angular requires constructor parameter metadata 
      in order to inject a `Logger`.

  .callout.is-helpful
    header Suggestion: add @Injectable() to every service class
    :marked
      We recommend adding `@Injectable()` to every service class, even those that don't have dependencies
      and, therefore, do not technically require it. Here's why:

    ul(style="font-size:inherit")
      li <b>Future proofing:</b> No need to remember <code>@Injectable()</code> when we add a dependency later.
      li <b>Consistency:</b> All services follow the same rules, and we don't have to wonder why #{_a} #{_decorator} is missing.

:marked
  Injectors are also responsible for instantiating components
  like `HeroesComponent`. Why haven't we marked `HeroesComponent` as
  `@Injectable()`?

  We *can* add it if we really want to. It isn't necessary because the
  `HeroesComponent` is already marked with `@Component`, and this
  !{_decorator} class (like `@Directive` and `@Pipe`, which we'll learn about later)
  is a subtype of <a href="#{injMetaUrl}">InjectableMetadata</a>.  It is in
  fact `InjectableMetadata` #{_decorator}s that
  identify a class as a target for instantiation by an injector.

+ifDocsFor('ts')
  .l-sub-section
    :marked
      At runtime, injectors can read class metadata in the transpiled JavaScript code
      and use the constructor parameter type information
      to determine what things to inject. 

      Not every JavaScript class has metadata.
      The TypeScript compiler discards metadata by default.
      If the `emitDecoratorMetadata` compiler option is true 
      (as it should be in the `tsconfig.json`),
      the compiler adds the metadata to the generated JavaScript 
      for _every class with at least one decorator_.

      While any decorator will trigger this effect, mark the service class with the
      <a href="#{injMetaUrl}">InjectableMetadata</a> #{_decorator}
      to make the intent clear.

.callout.is-critical
  header Always include the parentheses
  block always-include-paren
    :marked
      Always write `@Injectable()`, not just `@Injectable`.
      Our application will fail mysteriously if we forget the parentheses.

.l-main-section#logger-service
:marked
  ## Creating and registering a logger service

  We're injecting a logger into our `HeroService` in two steps:
  1. Create the logger service.
  1. Register it with the application.

  Our logger service is quite simple:

+makeExample('dependency-injection/ts/app/logger.service.ts', null, 'app/logger.service.ts')

block real-logger
  //- N/A

:marked
  We're likely to need the same logger service everywhere in our application,
  so we put it in the project's `#{_appDir}` folder, and
  we register it in the `providers` #{_array} of the metadata for our application module, `AppModule`.

+makeExcerpt('app/providers.component.ts','providers-logger','app/app.module.ts (excerpt)')

:marked
  If we forget to register the logger, Angular throws an exception when it first looks for the logger:
code-example(format="nocode").
  EXCEPTION: No provider for Logger! (HeroListComponent -> HeroService -> Logger)

:marked
  That's Angular telling us 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`.

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

.l-main-section#providers
: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 the injector injects into components and other services.

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

  Earlier we registered the `Logger` service in the `providers` #{_array} of the metadata for the `AppModule` like this:

+makeExample('dependency-injection/ts/app/providers.component.ts','providers-logger')

- var implements = _docsFor == 'dart' ? 'implements' : 'looks and behaves like a '
- var objectlike = _docsFor == 'dart' ? '' : 'an object that behaves like '
- var loggerlike = _docsFor == 'dart' ? '' : 'We could provide a logger-like object. '
:marked
  There are many ways to *provide* something that #{implements} `Logger`.
  The `Logger` class itself is an obvious and natural provider.
  But it's not the only way.

  We can configure the injector with alternative providers that can deliver #{objectlike} a `Logger`.
  We could provide a substitute class. #{loggerlike}
  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`.

//- Dart limitation: the provide function isn't const so it cannot be used in an annotation.
- var _andProvideFn = _docsFor == 'dart' ? '' : 'and <i>provide</i> object literal';
#provide
:marked
  ### The *Provider* class !{_andProvideFn}

:marked
  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
  <span if-docs="ts">
    using a _provider_ object literal with two properties:
  </span>
  <span if-docs="dart">
    that creates a new instance of the
    [Provider](../api/core/index/Provider-class.html) class:
  </span>

+makeExample('dependency-injection/ts/app/providers.component.ts','providers-3')

block provider-ctor-args
  - var _secondParam = 'provider definition object';

:marked
  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 !{_secondParam}, 
  which we can 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.

#class-provider
:marked
  ### Alternative class providers

  Occasionally we'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`.

+makeExample('dependency-injection/ts/app/providers.component.ts','providers-4')

block dart-diff-const-metadata
  //- N/A

: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')(format='.')

:marked
  Configure it like we did `BetterLogger`.

+makeExample('dependency-injection/ts/app/providers.component.ts','providers-5')(format=".")

:marked
  ### Aliased class providers

  Suppose an old component 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 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 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
  The solution: alias with the `useExisting` option.

- var stylePattern = { otl: /(useExisting: \w*)/gm };
+makeExample('dependency-injection/ts/app/providers.component.ts','providers-6b', '', stylePattern)(format=".")

#value-provider
:marked
  ### Value providers

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

block dart-diff-const-metadata-ctor
  //- N/A

+makeExample('dependency-injection/ts/app/providers.component.ts','silent-logger')(format=".")

:marked
  Then we register a provider with the `useValue` option,
  which makes this object play the logger role.

- var stylePattern = { otl: /(useValue: \w*)/gm };
+makeExample('dependency-injection/ts/app/providers.component.ts','providers-7', '', stylePattern)(format=".")

:marked
  See more `useValue` examples in the
  [Non-class dependencies](#non-class-dependencies) and
  [OpaqueToken](#opaquetoken) sections.

#factory-provider
: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 changes repeatedly in the course of the browser session.

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

  This situation calls for a **factory provider**.

  Let's illustrate by adding a new business requirement:
  the HeroService must hide *secret* heroes from normal users.
  Only authorized users should see secret heroes.

  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`.
  The `HeroService` won't have direct access to the user information to decide
  who is authorized and who is not.

.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 (excerpt)')(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 (excerpt)')(format='.')

:marked
  Although the `HeroService` has no access to the `UserService`, our factory function does.

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

+makeExample('dependency-injection/ts/app/heroes/hero.service.provider.ts','provider', 'app/heroes/hero.service.provider.ts (excerpt)')(format='.')

.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.
    The injector resolves these tokens and injects the corresponding services into the matching factory function parameters.

- var exportedvar = _docsFor == 'dart' ? 'constant' : 'exported variable'
- var variable = _docsFor == 'dart' ? 'constant' : 'variable'
:marked
  Notice that we captured the factory provider in #{_an} #{exportedvar}, `heroServiceProvider`.
  This extra step makes the factory provider reusable.
  We can register our `HeroService` with this #{variable} wherever 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}.
  Here we see the new and the old implementation side-by-side:

- var stylePattern = { otl: /(providers.*),$/gm };
+makeTabs(
  `dependency-injection/ts/app/heroes/heroes.component.ts,
  dependency-injection/ts/app/heroes/heroes.component.1.ts`,
  ',full',
  `app/heroes/heroes.component (v3),
  app/heroes/heroes.component (v2)`,
  stylePattern)

.l-main-section#token
: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. The token is the key to the map.

  In all previous examples, the dependency value has been a class *instance*, and
  the class *type* served as its own lookup key.
  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 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/heroes/hero-list.component.ts', 'ctor-signature')

:marked
  This is especially convenient when we consider that most dependency values are provided by classes.

//- TODO: if function injection is useful explain or illustrate why.
:marked
  ### Non-class dependencies
p
  | What if the dependency value isn't a class? Sometimes the thing we want to inject is a 
  block non-class-dep-eg
    span string, function, or object.
p
  | Applications often define configuration objects with lots of small facts 
  | (like the title of the application or the address of a web API endpoint)
  block config-obj-maps
    | &nbsp;but these configuration objects aren't always instances of a class.
    | They can be object literals 
  | &nbsp;such as this one:

+makeExample('dependency-injection/ts/app/app.config.ts','config','app/app-config.ts (excerpt)')(format='.')

:marked
  We'd like to make this configuration object available for injection.
  We know we can register an object with a [value provider](#value-provider).

block what-should-we-use-as-token
  :marked
    But what should we use as the token?
    We don't have a class to serve as a token.
    There is no `AppConfig` class.

  .l-sub-section#interface
    :marked
      ### TypeScript interfaces aren't valid tokens

      The `HERO_DI_CONFIG` constant has an interface, `AppConfig`. Unfortunately, we
      cannot use a TypeScript interface as a token:
    +makeExample('dependency-injection/ts/app/providers.component.ts','providers-9-interface')(format=".")
    +makeExample('dependency-injection/ts/app/providers.component.ts','provider-9-ctor-interface')(format=".")
    :marked
      That seems strange if we're used to dependency injection in strongly typed languages, where
      an interface is the preferred dependency lookup key.

      It's not Angular's fault. 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.

//- FIXME simplify once APIs are defined for Dart.
- var opaquetoken = _docsFor == 'dart' ? '<b>OpaqueToken</b>' : '<a href="../api/core/index/OpaqueToken-class.html"><b>OpaqueToken</b></a>'
:marked
  ### OpaqueToken

  One solution to choosing a provider token for non-class dependencies is
  to define and use an !{opaquetoken}.
  The definition looks like this:

+makeExample('dependency-injection/ts/app/app.config.ts','token')(format='.')

:marked
  We register the dependency provider using the `OpaqueToken` object:

+makeExample('dependency-injection/ts/app/providers.component.ts','providers-9')(format=".")

:marked
  Now we can inject the configuration object into any constructor that needs it, with
  the help of an `@Inject` #{_decorator}:

+makeExample('dependency-injection/ts/app/app.component.2.ts','ctor')(format=".")

- var configType = _docsFor == 'dart' ? '<code>Map</code>' : '<code>AppConfig</code>'
.l-sub-section
  :marked
    Although the !{configType} interface plays no role in dependency injection,
    it supports typing of the configuration object within the class.

block dart-map-alternative
  :marked
    Or we can provide and inject the configuration object in an ngModule like `AppModule`.

  +makeExcerpt('app/app.module.ts','ngmodule-providers')

#optional
:marked
  ## Optional dependencies

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

+ifDocsFor('ts')
  +makeExample('dependency-injection/ts/app/providers.component.ts','import-optional', '')
+makeExample('dependency-injection/ts/app/providers.component.ts','provider-10-ctor', '')(format='.')

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

.l-main-section
:marked
  ## Summary

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

  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#explicit-injector
:marked
  ## Appendix: Working with injectors directly

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

+makeExample('dependency-injection/ts/app/injector.component.ts', 'injector', 'app/injector.component.ts')

:marked
  An `Injector` is itself an injectable service.

  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.

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

  The `get` method throws an error if it can't resolve the requested service.
  We can call `get` with a second parameter (the value to return if the service is not found) 
  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
    The technique we just described 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 to discover what it does.

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

+ifDocsFor('ts')
  .l-main-section#one-class-per-file
  :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 by defining components and services in separate files.