Unable to call a method that is declared in one class from another singleton class

Question

As the title says I'm trying to access a method that is declared in one class (Post) from another class (Comments) which is following a singleton pattern. Post class is a service class which has some methods to make API calls. So I need to have access to them from inside Comments class so that I can make API calls.

This is how a simplied version of Post class looks like right now:

@Injectable({
  providedIn: 'root'
})
class PostService extends AnotherService {

  constructor( auth: AuthService, http: HttpClient ) {
    super('string', auth, http);
  }

  getPost( id: string ) {
    return this.http.get(`posts/${id}`);
  }

}

This is how the Comments class look like:

class Comments {

    private postService: PostService;
    private static instance;
 
    private constructor() {}

    static createInstance() {
        if ( !Comments.instance ) {
            Comments.instance = new Comments();
        }
        return Comments.instance;
    }

    getComments( id ) {

        // these does not even run
        this.postService.getPost( id )
            .then( post => {
                 console.log( post );
            })
            .catch( error => {
                 console.log( error );
            }); 

    }

}

How can I go about accessing it?

=========UPDATE=======

Creating an instance of Comment class in another class called ClassC.

const instance - Comments.createInstance();
instance.getComments( id );

Answer 1

Use a new service to save your comment object data let say We have a service named SharedDataService.

private _comments: Array<any> =[];// or array<IComment> (a defined interface from your part)
class SharedDataService(){}
get comments():Array<any>{
return this._comments}
set comments(value:Array<any>){
this._comments = value;

}

}

You should init PostService on your Comments Constructor

    private constructor(private postService: PostService,private sharedDataService :SharedDataService) {

}

    getComments() {

        // these does not even run
        this.postService.getPost( '1' )
            .then( post => {
this.sharedDataService.comments = post // if You get an array of comments here
                 console.log( post );
console.log(this.comments)// getter function its new value has been set
            })
            .catch( error => {
                 console.log( error );
            }); 

get comments(){

this.sharedDataService.comments

}

    }