119 lines
6.5 KiB
Markdown
119 lines
6.5 KiB
Markdown
# Introduction to services and dependency injection
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*Service* is a broad category encompassing any value, function, or feature that an app needs.
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A service is typically a class with a narrow, well-defined purpose.
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It should do something specific and do it well.
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Angular distinguishes components from services to increase modularity and reusability.
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By separating a component's view-related functionality from other kinds of processing,
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you can make your component classes lean and efficient.
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Ideally, a component's job is to enable the user experience and nothing more.
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A component should present properties and methods for data binding,
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in order to mediate between the view (rendered by the template)
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and the application logic (which often includes some notion of a *model*).
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A component can delegate certain tasks to services, such as fetching data from the server,
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validating user input, or logging directly to the console.
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By defining such processing tasks in an *injectable service class*, you make those tasks
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available to any component.
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You can also make your app more adaptable by injecting different providers of the same kind of service,
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as appropriate in different circumstances.
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Angular doesn't *enforce* these principles. Angular does help you *follow* these principles
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by making it easy to factor your application logic into services and make those services
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available to components through *dependency injection*.
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## Service examples
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Here's an example of a service class that logs to the browser console.
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<code-example path="architecture/src/app/logger.service.ts" header="src/app/logger.service.ts (class)" region="class"></code-example>
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Services can depend on other services. For example, here's a `HeroService` that depends on the `Logger` service, and also uses `BackendService` to get heroes. That service in turn might depend on the `HttpClient` service to fetch heroes asynchronously from a server.
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<code-example path="architecture/src/app/hero.service.ts" header="src/app/hero.service.ts (class)" region="class"></code-example>
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## Dependency injection (DI)
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<img src="generated/images/guide/architecture/dependency-injection.png" alt="Service" class="left">
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DI is wired into the Angular framework and used everywhere to provide new components with the services or other things they need.
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Components consume services; that is, you can *inject* a service into a component, giving the component access to that service class.
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To define a class as a service in Angular, use the `@Injectable()` decorator to provide the metadata that allows Angular to inject it into a component as a *dependency*.
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Similarly, use the `@Injectable()` decorator to indicate that a component or other class (such as another service, a pipe, or an NgModule) *has* a dependency.
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* The *injector* is the main mechanism. Angular creates an application-wide injector for you during the bootstrap process, and additional injectors as needed. You don't have to create injectors.
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* An injector creates dependencies, and maintains a *container* of dependency instances that it reuses if possible.
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* A *provider* is an object that tells an injector how to obtain or create a dependency.
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For any dependency that you need in your app, you must register a provider with the app's injector,
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so that the injector can use the provider to create new instances.
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For a service, the provider is typically the service class itself.
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<div class="alert is-helpful">
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A dependency doesn't have to be a service—it could be a function, for example, or a value.
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</div>
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When Angular creates a new instance of a component class, it determines which services or other dependencies that component needs by looking at the constructor parameter types. For example, the constructor of `HeroListComponent` needs `HeroService`.
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<code-example path="architecture/src/app/hero-list.component.ts" header="src/app/hero-list.component.ts (constructor)" region="ctor"></code-example>
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When Angular discovers that a component depends on a service, it first checks if the injector has any existing instances of that service. If a requested service instance doesn't yet exist, the injector makes one using the registered provider, and adds it to the injector before returning the service to Angular.
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When all requested services have been resolved and returned, Angular can call the component's constructor with those services as arguments.
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The process of `HeroService` injection looks something like this.
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<figure class="lightbox">
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<div class="card">
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<img src="generated/images/guide/architecture/injector-injects.png" alt="Service" class="left">
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</div>
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</figure>
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### Providing services
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You must register at least one *provider* of any service you are going to use.
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The provider can be part of the service's own metadata, making that service available everywhere,
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or you can register providers with specific modules or components.
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You register providers in the metadata of the service (in the `@Injectable()` decorator),
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or in the `@NgModule()` or `@Component()` metadata
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* By default, the Angular CLI command [`ng generate service`](cli/generate) registers a provider with the root injector for your service by including provider metadata in the `@Injectable()` decorator. The tutorial uses this method to register the provider of HeroService class definition.
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```
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@Injectable({
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providedIn: 'root',
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})
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```
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When you provide the service at the root level, Angular creates a single, shared instance of `HeroService`
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and injects it into any class that asks for it.
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Registering the provider in the `@Injectable()` metadata also allows Angular to optimize an app
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by removing the service from the compiled app if it isn't used.
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* When you register a provider with a [specific NgModule](guide/architecture-modules), the same instance of a service is available to all components in that NgModule. To register at this level, use the `providers` property of the `@NgModule()` decorator,
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```
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@NgModule({
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providers: [
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BackendService,
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Logger
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],
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...
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})
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```
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* When you register a provider at the component level, you get a new instance of the
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service with each new instance of that component.
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At the component level, register a service provider in the `providers` property of the `@Component()` metadata.
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<code-example path="architecture/src/app/hero-list.component.ts" header="src/app/hero-list.component.ts (component providers)" region="providers"></code-example>
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For more detailed information, see the [Dependency Injection](guide/dependency-injection) section.
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