a service is just a class

only components and directives need to be registered

    export class LoggingService{
        logStatusChange(status: string){
            console.log('A server status changed. New status : ' + status);
        }
    }
        

1. import the class
2. add the providers property to the component directive if necessary
3. add the type to the providers property
4. add instance of the type as a c'tor argument

    import { Component, EventEmitter, Output } from '@angular/core';

    import {LoggingService } from '../logging.service';

    @Component({
      ...
      providers: [LoggingService]
    })
    export class NewAccountComponent {
        ...
        constructor(private loggingService: LoggingService){}
        ...
    }    
        

Angular DI creates a type instance for a component and its children will all use the same instance

1. AppModule - same instance of service is available app-wide
2. AppComponent - same instance of service is available for all Components (only components)
3. Any other component - same instance of service is available for the component and its children

the exception is if a child component decorator's provider property declares the service, the child component will get its own instance of the injected service

override behavior by declaring service in another component or module decorator's providers property

for a service to be injected with another service the Injectable decorator is used

    import { Injectable } from '@angular/core';
    ...
    @Injectable()
    export class AService {
        ...
        constructor(private loggingService: LoggingService) { }
        ...
    }
        

    ...
    @Injectable()
    export class AService {
        ...
        statusUpdated = new EventEmitter<string>();
        ...
    } 
        

component uses service methods to fire events

    ...
    export class AComponent {

      constructor(private aService: AService) { }
  
      onSetTo(status: string) {
        this.aService.update(this.id, status);
        this.aService.statusUpdated.emit(status);
      }
    } 
        

in c'tor the component subscribes to the service's event

    ...
    export class BComponent {

      constructor(private aService: AService) {
        // subscribe to the event
        this.aService.statusUpdated.subscribe(
          // display a message box describing the new status
          (status: string) => alert('New status : ' + status)
        );
      }
      ...
    }