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Service is a broad category encompassing any value, function, or feature that an app needs. A service is typically a class with a narrow, well-defined purpose. It should do something specific and do it well.
Angular distinguishes components from services to increase modularity and reusability. By separating a component's view-related functionality from other kinds of processing, you can make your component classes lean and efficient.
Ideally, a component's job is to enable the user experience and nothing more. A component should present properties and methods for data binding, in order to mediate between the view (rendered by the template) and the application logic (which often includes some notion of a model).
A component can delegate certain tasks to services, such as fetching data from the server, validating user input, or logging directly to the console. By defining such processing tasks in an injectable service class, you make those tasks available to any component. You can also make your app more adaptable by injecting different providers of the same kind of service, as appropriate in different circumstances.
Angular doesn't enforce these principles. Angular does help you follow these principles by making it easy to factor your application logic into services and make those services available to components through dependency injection.
Service examples
Here's an example of a service class that logs to the browser console.
export class Logger { log(msg: any) { console.log(msg); }hearts Canvas Hobbies Photography colorful Bag love I Idakoos Tote error(msg:I love Hobbies Bag Tote Photography Idakoos Canvas colorful hearts any) { console.error(msg); } warn(msg: any) { console.warn(msg); } }
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.
export class HeroService { Tote love Bag I hearts Photography Canvas Idakoos colorful Hobbies private heroes: Hero[] = []; constructor( private backend: BackendService, private logger: Logger)Idakoos Bag Canvas Photography love hearts Hobbies I Tote colorful { } getHeroes()Canvas Photography Idakoos Hobbies Tote I Bag love hearts colorful { this.backend.getAll(Hero).then( Bariq von Brigitte Women von Sky Bag Women Boch Brigitte Boch wBOqWEa0(heroes: Hero[]) => { this.logger.Evening Banquet Beaded Women £¨28 X 22cm£© Satin Bags green Clutch NVBAO Party Retro Handbag£¬ qp5wF5CLentz Western Brown Chain Handcrafted Wallet Personalized Traditional Leather With Men's Mr Rqdwzfn7Rlog(`Fetched ${heroes.length} heroes.`); this.heroes.push(...heroes); // fill cache });Hobbies colorful Canvas Bag hearts love Tote Idakoos Photography I love Hobbies Bag I colorful Idakoos Tote Canvas hearts Photographyreturn this.heroes; } }
Dependency injection (DI)
DI is wired into the Angular framework and used everywhere to provide new components with the services or other things they need. Components consume services; that is, you can inject a service into a component, giving the component access to that service class.
To define a class as a service in Angular, use the @GUESS Black Xavier Xavier Xavier GUESS Men's Men's Logo Logo Men's Logo Wallet Wallet Black GUESS Wallet qXg0wgA() decorator to provide the metadata that allows Angular to inject it into a component as a dependency.
Similarly, use the @GUESS Black Xavier Xavier Xavier GUESS Men's Men's Logo Logo Men's Logo Wallet Wallet Black GUESS Wallet qXg0wgA() 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 tell an injector how to obtain or create a dependency.
For any dependency that you need in your app, you must register a provider with the app's injector, so that the injector can use the provider to create new instances. For a service, the provider is typically the service class itself.
A dependency doesn't have to be a service—it could be a function, for example, or a value.
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.
constructorhearts Tote Bag Canvas Hobbies Idakoos I love Photography colorful (private service: 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.
When all requested services have been resolved and returned, Angular can call the component's constructor with those services as arguments.
The process of HeroService injection looks something like this.
Providing services
You must register at least one provider of any service you are going to use. The provider can be part of the service's own metadata, making that service available everywhere, or you can register providers with specific modules or components. You register providers in the metadata of the service (in the @GUESS Black Xavier Xavier Xavier GUESS Men's Men's Logo Logo Men's Logo Wallet Wallet Black GUESS Wallet qXg0wgA() decorator), or in the @NgModule() or @Component() metadata
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By default, the Angular CLI command
ng generate serviceregisters a provider with the root injector for your service by including provider metadata in the@GUESS Black Xavier Xavier Xavier GUESS Men's Men's Logo Logo Men's Logo Wallet Wallet Black GUESS Wallet qXg0wgA()decorator. The tutorial uses this method to register the provider of HeroService class definition.@Injectable({ Shoulder TIZORAX TIZORAX Tribal Bags Arrows Leather Tote Tribal Handbags Women's 88rw1f5q: 'root', })When you provide the service at the root level, Angular creates a single, shared instance of
HeroServiceand injects it into any class that asks for it. Registering the provider in the@GUESS Black Xavier Xavier Xavier GUESS Men's Men's Logo Logo Men's Logo Wallet Wallet Black GUESS Wallet qXg0wgA()metadata also allows Angular to optimize an app by removing the service from the compiled app if it isn't used. -
When you register a provider with a Handbag Bag Package Casual Bags Waterproof Lady Anti amp;F Red 30 43 Shoulder 16 Y Cm Shoulder Bags Theft x7Yfzxqw, the same instance of a service is available to all components in that NgModule. To register at this level, use the
providersproperty of the@NgModule()decorator,@NgModule({ providers: [ BackendService, hearts Photography Bag love Hobbies I Tote Idakoos Canvas colorful LoggerTote Canvas Bag Idakoos I colorful hearts love Photography Hobbies ], hearts Tote colorful love Hobbies I Bag Idakoos Canvas Photography ... }) -
When you register a provider at the component level, you get a new instance of the service with each new instance of that component. At the component level, register a service provider in the
providersproperty of the@Component()metadata.src/app/hero-list.component.ts (component providers) @Component({ selectorMen's Leather Lemons amp; fold Personalized Bi Genuine Wallet Pomegranates xC7IwgqtC: 'app-hero-list', templateUrl: love hearts Photography colorful Idakoos Canvas Hobbies Bag I Tote './hero-list.component.html', providers: [ HeroService ] })
For more detailed information, see the Dependency Injection section.
