udemy.angular/docs/services_dipendencyinjection.md

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# Services and dependency injection
A service prevents code duplication (LogService) or handlich with data (UserService - Data Sotrage).
## Create a logging service from
Naming: `logging.service.ts`
> Instanciate a service manually with method `new` is the wrong way! Angular offers a much better solution.
## Dependency injection
Angular is responsable to instanciate the service which is passed as argument to the constructor. Which tels Angular thet the LoggingService is required.
```ts
import { LoggingService } from '../logging.service.ts';
@Component({
...
providers: [LoggingService]
})
export class NewAccountComponent {
constructor(private loggingService: LoggingService) {}
function(){
// Calling the loggingService
// logStatusChange is a function in the loggingService which accepts a string as argument.
this.loggingService.logStatusChange(accountStatus);
}
}
```
`private loggingService` makes directly a property of type `LoggingService` and with the `provider` in the `@Component`-decorator, angular instanciate the logging service.
## Hierarchical Injector
By instanciate a service, Angular provides the same Instance to the component and all its childs.
| Level | Description |
| ------------------- | --------------------------------------------------------------------------------------------- |
| AppModule | Service is available **Application-wide** |
| AppComponent | Same instance of service is available for **all Components** (but **not for other services**) |
| Any other Component | Same instance of Service is available for **the Component and all its chilf component** |
By declare the service in a lower component, will create a new instance and override the inherited from the parents one.
It is necessary to declare it in the `impot` and in the `constructor`.
`app.module.ts` is the highest level to declare a service. This opens the possibility to inject services into an other service.
To the service who will be injected a service, is necessaty to provide metadata. Its keyword is `@Injectable()`
Services can also be used to avoiding data/event/listener chaining.
By Implementing the event in the service, and the listener in the component, can mitigate the chaining.