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

  [Run the live example](/resources/live-examples/dependency-injection/ts/plnkr.html)

<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 (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 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, 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 become difficult to test.

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

  That's super easy. We probably already know what to do. We change our `Car` constructor to a version with DI:

  <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 (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.

.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.

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

<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.
  Our stripped down version has only one child, `HeroListComponent`,
  which displays 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 work well in real applications.
:marked
  Right now `HeroListComponent` gets heroes from `HEROES`, 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 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.

  ### 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 (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.
  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
  preregistered services, such as its routing support.

  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`.

  ### 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 `providers` part 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:
+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)`)
:marked

.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
    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 with an explicit 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.

  ### 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.

  ### 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 (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 of `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. 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.
.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
  ## 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.

  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-logger', 'app/app.component.ts (excerpt)')
:marked
  If we forget to register the logger, Angular throws an exception when it first looks for the logger:
code-example(format, language="html").
  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. 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. We can do that.

  First import the `@Optional()` decorator.
+makeExample('dependency-injection/ts/app/providers.component.ts','import-optional')(format='.')
:marked
  Then rewrite the constructor with `@Optional()` decorator preceding the private `_logger` parameter.
  That tells the injector that `_logger` is 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 to avoid a null reference exception?

  We could substitute a *do-nothing* logger stub so that calling 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
  in multiple locations.

  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 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 `AppComponent` like this:
+makeExample('dependency-injection/ts/app/providers.component.ts','providers-logger')
:marked
  The `providers` array appears to hold a service class.
  In reality it holds an instance of the [Provider](/docs/ts/latest/api/core/Provider-class.html) class that can create that service.

  There are many ways to *provide* something that looks and behaves like a `Logger`.
  The `Logger` class itself is an obvious and natural provider &mdash; 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](/docs/ts/latest/api/core/provide-function.html) function is the more common, 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 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.

  <a id="class-provider"></a>
  ### 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')
: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 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 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.
+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 the `useValue` option,
  which makes this object play the logger role.
+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 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='.')
: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.
    The injector resolves these tokens and injects the corresponding services into the matching factory function parameters.
:marked
  Notice that we captured the factory provider in an exported variable, `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:
+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. 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/providers.component.ts','provider-8-ctor')(format=".")
:marked
  This is 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 tend to be object hashes like this one:

+makeExample('dependency-injection/ts/app/app.config.ts','config','app/app-config.ts (excerpt)')(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
  ### Interfaces aren't valid tokens

  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
  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.
// 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 that tells 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 strong 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](/docs/ts/latest/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 when registering the provider and defining the constructor parameter:
+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 itself uses `OpaqueToken` objects to register all of its own non-class dependencies. For example,
    [HTTP_PROVIDERS](/docs/ts/latest/api/http/HTTP_PROVIDERS-let.html)
    is the `OpaqueToken` associated with an array of providers for persisting data with the Angular `Http` client.

    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
  ## 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
<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
  The `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 `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
    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.

.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 by defining components and services in separate files.