How to emit an event from grandchildren to grandparent in modern angular?

Use rxjs/subject, it can be observer and observable in the same time.

Usage:

1. Create Subject property in service:

import { Subject } from 'rxjs';

export class AuthService {
  loginAccures: Subject<boolean> = new Subject<boolean>();
}

2. When event happens in child page/component use:

logout() {
  this.authService.loginAccures.next(false);
}

3. And subscribe to subject in parent page/component:

constructor(private authService: AuthService) {
  this.authService.loginAccures.subscribe((isLoggedIn: boolean) => {
    this.isLoggedIn = isLoggedIn;
  })
}

There could be 2 ways:

1. Using @output:

Grandparent

<parent (notifyGrandParent)="grandmaHandleClick($event)">
<parent>
...
grandmaHandleClick(event) {
  console.log('grandma knows you clicked')
}

Parent:

<child (handleClick)="childEvent($event)">
</child>

@Output() notifyGrandParent= new EventEmitter();
childEvent(event) {
  this.notifyGrandParent.emit('event')
}

Child is implemented properly in the code so it is good to go.

2. Using BehaviorSubject via Service: With this much level of nesting, you can actually create some service like EventService, and then create BehaviorSubject which can directly be subscribed by the GrandParent. Also, to make this service more component specific, you can keep this service in a module which will have other 3 components (GrandParent, Parent and Child)

export class EventService{

 private childClickedEvent = new BehaviorSubject<string>('');

  emitChildEvent(msg: string){
     this.childClickedEvent.next(msg)
  }

  childEventListner(){
     return this.childClickedEvent.asObservable();
   } 

}

and then in components:

ChildComponent

export class ChildComponent{
   constructor(private evtSvc: EventService){}

   onClick(){
     this.evtSvc.emitChildEvent('clicked a button')
   }
}

GrandParent

export class GrandComponent{
   constructor(private evtSvc: EventService){}

   ngOnInit(){
     this.evtSvc.childEventListner().subscribe(info =>{
         console.log(info); // here you get the message from Child component
      })
   }
}

Please note that, with @output event, you create a tight coupling of components and so a strong dependency (parent-child-grandchild) is created. If the component is not reusable and is only created to serve this purpose, then @output will also make sense because it'll convey the message to any new developer that they have parent-child relationship.

Creating a service to pass data also exposes the data to other components which can inject service in constructor.

So, the decision should be taken accordingly.