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docs(ngmodule-faq): stronger, clearer advice on where to register providers (#2262)

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Ward Bell 2016-09-03 13:26:19 -07:00 committed by GitHub
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public/docs/ts/latest/cookbook/ngmodule-faq.jade
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@ -33,12 +33,13 @@ block includes
* [Can I re-export imported classes and modules?](#q-re-export)
* [What is the _forRoot_ method?](#q-for-root)
Service Providers
Service Providers
* [Why is a service provided in a feature module visible everywhere?](#q-module-provider-visibility)
* [Why is a service provided in a _lazy loaded_ module visible only to that module?](#q-lazy-loaded-module-provider-visibility)
* [Why is a service provided in a _lazy loaded_ module visible only to that module?](#q-lazy-loaded-module-provider-visibility)
* [What if two modules provide the _same_ service?](#q-module-provider-duplicates)
* [How do I restrict service scope to a module?](#q-component-scoped-providers)
* [Should I add providers to the root _AppModule_ or the root _AppComponent_?](#q-root-component-or-module)
* [Should I add app-wide providers to the root _AppModule_ or the root _AppComponent_?](#q-root-component-or-module)
* [Should I add other providers to a module or a component?](#q-component-or-module)
* [Why is it bad if _SharedModule_ provides a service to a lazy loaded module?](#q-why-bad)
* [Why does lazy loading create a child injector?](#q-why-child-injector)
* [How can I tell if a module or service was previously loaded?](#q-is-it-loaded)
@ -424,14 +425,21 @@ a#q-component-scoped-providers
a#q-root-component-or-module
.l-main-section
:marked
### Should I add providers to the root _AppModule_ or the root _AppComponent_?
### Should I add app-wide providers to the root _AppModule_ or the root _AppComponent_?
Most apps launch with an initial set of service providers.
Should we register those providers on the root `AppModule` (`@NgModule.providers`) or
the root `AppComponent` (`@Component.providers`)?
.alert.is-helpful
:marked
Register application-wide providers in the root `AppModule`, not in the `AppComponent`.
:marked
Lazy-loaded modules and their components can inject `AppModule` services;
they cannot inject `AppComponent` services.
**_List such providers in the root_ `AppModule` _unless you have a compelling reason to do otherwise_**.
Register a service in `AppComponent` providers _only_ if the service must be hidden
from components outside the `AppComponent` tree. This is a rare exceptional use case.
More generally, [prefer registering providers in modules](#q-component-or-module) to registering in components.
#### **_Discussion_:**
Angular registers all startup module providers with the application root injector.
The services created from root injector providers are available to the entire application.
They are _application-scoped_.
@ -439,7 +447,7 @@ a#q-root-component-or-module
Certain services (e.g., the `Router`) only work when registered in the application root injector.
By contrast, Angular registers `AppComponent` providers with the `AppComponent`'s own injector.
`AppComponent`services are available to that component and its component tree.
`AppComponent`services are available only to that component and its component tree.
They are _component-scoped_.
The `AppComponent`'s injector is a _child_ of the root injector, one down in the injector hierarchy.
@ -448,19 +456,33 @@ a#q-root-component-or-module
`AppComponent` services don't exist at the root level where routing operates.
Lazy loaded modules can't reach them.
In this sample applications, if we had registered `UserService` in the `AppComponent`,
In the Angular Module Chapter sample applications, if we had registered `UserService` in the `AppComponent`,
the `HeroComponent` couldn't inject it.
The application would fail the moment a user navigated to "Heroes".
We _can_ register a service in `AppComponent` providers if the app doesn't use routing.
We _should_ register a service in `AppComponent` providers if the service must be hidden
from components outside the `AppComponent` tree.
.l-hr
a#q-component-or-module
.l-main-section
:marked
### Should I add other providers to a module or a component?
In general, prefer registering feature-specific providers in modules (`@NgModule.providers`)
to registering in components (`@Component.providers`).
These are special cases.
When in doubt, register with the `AppModule`.
Register a provider with a component when you _must_ limit the scope of a service instance
to that component and its component tree.
Apply the same reasoning to registering a provider with a directive.
For example, a hero editing component that needs a private copy of a caching hero service should register
the `HeroService` with the `HeroEditorComponent`.
Then each new instance of the `HeroEditorComponent` gets its own cached service instance.
The changes that editor makes to heroes in its service do not touch the hero instances elsewhere in the application.
[Always register _application-wide_ services with the root `AppModule`](q-root-component-or-module),
not the root `AppComponent`.
.l-hr
a#q-why-bad
.l-main-section
:marked