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This commit prepares the documentation directories for future tutorials organized by directory. Also, it moves the Angular Libraries topic from the Tutorials section to Developer Guides in TOC PR Close #48162
173 lines
9 KiB
Markdown
173 lines
9 KiB
Markdown
# Singleton services
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A singleton service is a service for which only one instance exists in an application.
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For a sample application using the app-wide singleton service that this page describes, see the <live-example name="ngmodules"></live-example> showcasing all the documented features of NgModules.
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## Providing a singleton service
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There are two ways to make a service a singleton in Angular:
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* Set the `providedIn` property of the `@Injectable()` to `"root"`
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* Include the service in the `AppModule` or in a module that is only imported by the `AppModule`
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<a id="providedIn"></a>
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### Using `providedIn`
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Beginning with Angular 6.0, the preferred way to create a singleton service is to set `providedIn` to `root` on the service's `@Injectable()` decorator.
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This tells Angular to provide the service in the application root.
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<code-example path="providers/src/app/user.service.0.ts" header="src/app/user.service.ts"></code-example>
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For more detailed information on services, see the [Services](tutorial/tour-of-heroes/toh-pt4) chapter of the [Tour of Heroes tutorial](tutorial/tour-of-heroes).
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### NgModule `providers` array
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In applications built with Angular versions prior to 6.0, services are registered NgModule `providers` arrays as follows:
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<code-example format="typescript" language="typescript">
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@NgModule({
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…
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providers: [UserService],
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…
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})
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</code-example>
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If this NgModule were the root `AppModule`, the `UserService` would be a singleton and available throughout the application.
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Though you may see it coded this way, using the `providedIn` property of the `@Injectable()` decorator on the service itself is preferable as of Angular 6.0 as it makes your services tree-shakable.
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<a id="forRoot"></a>
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## The `forRoot()` pattern
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Generally, you'll only need `providedIn` for providing services and `forRoot()`/`forChild()` for routing.
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However, understanding how `forRoot()` works to make sure a service is a singleton will inform your development at a deeper level.
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If a module defines both providers and declarations \(components, directives, pipes\), then loading the module in multiple feature modules would duplicate the registration of the service.
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This could result in multiple service instances and the service would no longer behave as a singleton.
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There are multiple ways to prevent this:
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* Use the [`providedIn` syntax](guide/singleton-services#providedIn) instead of registering the service in the module.
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* Separate your services into their own module.
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* Define `forRoot()` and `forChild()` methods in the module.
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<div class="alert is-helpful">
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**NOTE**: <br />
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There are two example applications where you can see this scenario; the more advanced <live-example noDownload name="ngmodules">NgModules live example</live-example>, which contains `forRoot()` and `forChild()` in the routing modules and the `GreetingModule`, and the simpler <live-example name="lazy-loading-ngmodules" noDownload>Lazy Loading live example</live-example>.
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For an introductory explanation see the [Lazy Loading Feature Modules](guide/lazy-loading-ngmodules) guide.
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</div>
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Use `forRoot()` to separate providers from a module so you can import that module into the root module with `providers` and child modules without `providers`.
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1. Create a static method `forRoot()` on the module.
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1. Place the providers into the `forRoot()` method.
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<code-example header="src/app/greeting/greeting.module.ts" path="ngmodules/src/app/greeting/greeting.module.ts" region="for-root"></code-example>
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<a id="forRoot-router"></a>
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### `forRoot()` and the `Router`
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`RouterModule` provides the `Router` service, as well as router directives, such as `RouterOutlet` and `routerLink`.
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The root application module imports `RouterModule` so that the application has a `Router` and the root application components can access the router directives.
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Any feature modules must also import `RouterModule` so that their components can place router directives into their templates.
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If the `RouterModule` didn't have `forRoot()` then each feature module would instantiate a new `Router` instance, which would break the application as there can only be one `Router`.
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By using the `forRoot()` method, the root application module imports `RouterModule.forRoot(...)` and gets a `Router`, and all feature modules import `RouterModule.forChild(...)` which does not instantiate another `Router`.
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<div class="alert is-helpful">
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**NOTE**: <br />
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If you have a module which has both providers and declarations, you *can* use this technique to separate them out and you may see this pattern in legacy applications.
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However, since Angular 6.0, the best practice for providing services is with the `@Injectable()` `providedIn` property.
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</div>
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### How `forRoot()` works
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`forRoot()` takes a service configuration object and returns a [ModuleWithProviders](api/core/ModuleWithProviders), which is a simple object with the following properties:
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| Properties | Details |
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|:--- |:--- |
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| `ngModule` | In this example, the `GreetingModule` class |
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| `providers` | The configured providers |
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In the <live-example name="ngmodules">live example</live-example> the root `AppModule` imports the `GreetingModule` and adds the `providers` to the `AppModule` providers.
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Specifically, Angular accumulates all imported providers before appending the items listed in `@NgModule.providers`.
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This sequence ensures that whatever you add explicitly to the `AppModule` providers takes precedence over the providers of imported modules.
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The sample application imports `GreetingModule` and uses its `forRoot()` method one time, in `AppModule`.
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Registering it once like this prevents multiple instances.
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You can also add a `forRoot()` method in the `GreetingModule` that configures the greeting `UserService`.
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In the following example, the optional, injected `UserServiceConfig` extends the greeting `UserService`.
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If a `UserServiceConfig` exists, the `UserService` sets the user name from that config.
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<code-example header="src/app/greeting/user.service.ts (constructor)" path="ngmodules/src/app/greeting/user.service.ts" region="ctor"></code-example>
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Here's `forRoot()` that takes a `UserServiceConfig` object:
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<code-example header="src/app/greeting/greeting.module.ts (forRoot)" path="ngmodules/src/app/greeting/greeting.module.ts" region="for-root"></code-example>
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Lastly, call it within the `imports` list of the `AppModule`.
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In the following snippet, other parts of the file are left out.
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For the complete file, see the <live-example name="ngmodules"></live-example>, or continue to the next section of this document.
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<code-example header="src/app/app.module.ts (imports)" path="ngmodules/src/app/app.module.ts" region="import-for-root"></code-example>
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The application displays "Miss Marple" as the user instead of the default "Sherlock Holmes".
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Remember to import `GreetingModule` as a Javascript import at the top of the file and don't add it to more than one `@NgModule` `imports` list.
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## Prevent reimport of the `GreetingModule`
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Only the root `AppModule` should import the `GreetingModule`.
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If a lazy-loaded module imports it too, the application can generate [multiple instances](guide/ngmodule-faq#q-why-bad) of a service.
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To guard against a lazy loaded module re-importing `GreetingModule`, add the following `GreetingModule` constructor.
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<code-example header="src/app/greeting/greeting.module.ts" path="ngmodules/src/app/greeting/greeting.module.ts" region="ctor"></code-example>
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The constructor tells Angular to inject the `GreetingModule` into itself.
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The injection would be circular if Angular looked for `GreetingModule` in the *current* injector, but the `@SkipSelf()` decorator means "look for `GreetingModule` in an ancestor injector, above me in the injector hierarchy."
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By default, the injector throws an error when it can't find a requested provider.
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The `@Optional()` decorator means not finding the service is OK.
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The injector returns `null`, the `parentModule` parameter is null, and the constructor concludes uneventfully.
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It's a different story if you improperly import `GreetingModule` into a lazy loaded module such as `CustomersModule`.
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Angular creates a lazy loaded module with its own injector, a child of the root injector.
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`@SkipSelf()` causes Angular to look for a `GreetingModule` in the parent injector, which this time is the root injector.
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Of course it finds the instance imported by the root `AppModule`.
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Now `parentModule` exists and the constructor throws the error.
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Here are the two files in their entirety for reference:
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<code-tabs>
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<code-pane header="app.module.ts" path="ngmodules/src/app/app.module.ts"></code-pane>
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<code-pane header="greeting.module.ts" region="whole-greeting-module" path="ngmodules/src/app/greeting/greeting.module.ts"></code-pane>
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</code-tabs>
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## More on NgModules
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You may also be interested in:
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* [Sharing Modules](guide/sharing-ngmodules), which elaborates on the concepts covered on this page
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* [Lazy Loading Modules](guide/lazy-loading-ngmodules)
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* [NgModule FAQ](guide/ngmodule-faq)
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<!-- links -->
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<!-- external links -->
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<!-- end links -->
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@reviewed 2022-02-28
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