udemy.angular/docs/services_dipendencyinjection.md

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.

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.