Introduction to services and dependency injectionlink
\nService is a broad category encompassing any value, function, or feature that an app needs.\nA service is typically a class with a narrow, well-defined purpose.\nIt should do something specific and do it well.
\nAngular distinguishes components from services to increase modularity and reusability.\nBy separating a component's view-related functionality from other kinds of processing,\nyou can make your component classes lean and efficient.
\nIdeally, a component's job is to enable the user experience and nothing more.\nA component should present properties and methods for data binding,\nin order to mediate between the view (rendered by the template)\nand the application logic (which often includes some notion of a model).
\nA component can delegate certain tasks to services, such as fetching data from the server,\nvalidating user input, or logging directly to the console.\nBy defining such processing tasks in an injectable service class, you make those tasks\navailable to any component.\nYou can also make your app more adaptable by injecting different providers of the same kind of service,\nas appropriate in different circumstances.
\nAngular doesn't enforce these principles. Angular does help you follow these principles\nby making it easy to factor your application logic into services and make those services\navailable to components through dependency injection.
\nService exampleslink
\nHere's an example of a service class that logs to the browser console.
\nServices 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.
Dependency injection (DI)link
\n![\"Service\"](\"generated/images/guide/architecture/dependency-injection.png\")
\nDI is wired into the Angular framework and used everywhere to provide new components with the services or other things they need.\nComponents consume services; that is, you can inject a service into a component, giving the component access to that service class.
\nTo 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.\nSimilarly, 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,\nso that the injector can use the provider to create new instances.\nFor a service, the provider is typically the service class itself.
\nA dependency doesn't have to be a service—it could be a function, for example, or a value.
\nWhen 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.
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.
\nWhen all requested services have been resolved and returned, Angular can call the component's constructor with those services as arguments.
\nThe process of HeroService injection looks something like this.
![\"Service\"](\"generated/images/guide/architecture/injector-injects.png\")
\nProviding serviceslink
\nYou must register at least one provider of any service you are going to use.\nThe provider can be part of the service's own metadata, making that service available everywhere,\nor you can register providers with specific modules or components.\nYou register providers in the metadata of the service (in the @Injectable() decorator),\nor in the @NgModule() or @Component() metadata
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By default, the Angular CLI command
\nng generate serviceregisters 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.\n@Injectable({\n providedIn: 'root',\n})\n \nWhen you provide the service at the root level, Angular creates a single, shared instance of
\nHeroService\nand injects it into any class that asks for it.\nRegistering the provider in the@Injectable()metadata also allows Angular to optimize an app\nby removing the service from the compiled app if it isn't used, a process known as tree-shaking. \n - \n
When you register a provider with a specific NgModule, the same instance of a service is available to all components in that NgModule. To register at this level, use the
\nprovidersproperty of the@NgModule()decorator.\n@NgModule({\n providers: [\n BackendService,\n Logger\n ],\n ...\n})\n \n \n - \n
When you register a provider at the component level, you get a new instance of the\nservice with each new instance of that component.\nAt the component level, register a service provider in the
\nprovidersproperty of the@Component()metadata.
\n\n@Component({\n selector: 'app-hero-list',\n templateUrl: './hero-list.component.html',\n providers: [ HeroService ]\n})\n\n \n
For more detailed information, see the Dependency Injection section.
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