# 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.