Utilizing Angular 4: Sharing Data through Services and Components

After transitioning my data from an object in a service to a database connection, I'm facing issues where the data is not reaching the component as expected.

To solve this problem, I have set up the service to subscribe to the data retrieved from the database using the following code:

public setPerson(ac: string): void{
    console.log(ac);
    this.generatePerson(ac).subscribe((data) => {
        // this.mapPersonFromInput(data[0]);
        console.dir(data);
    });

}

The mapPersonFrominput() function was originally used for mock data and needs to be updated to work with actual database input.

The generatePerson function retrieves the person data from the API like this:

public generatePerson(id: string):Observable<Person>{
    var datRetUrl: string = '/api/'
    var fullUrl: string = datRetUrl + id;
    return this.http.get(fullUrl)
                .map(this.extractData)
                .catch(this.handleError);
  }

The extractData function assigns values from the input object to the service's structure, while handleError logs errors to the console.

I initialize the data object in the service before the component loads by calling the passCodeToService function from a navigation component:

passCodeToService():void{
    this.psn.setPerson(this.accessCode);
    this.route.navigate(['/name']);
}

In the component that should display the data, I currently use ngOnInit but suspect ngOnChanges might be more appropriate. Here's the code snippet I'm struggling to fix:

ngOnInit() {
  this.name = this.psn.getName();
  console.log(this.name);
}

The getName function simply returns the stored object from the service.

public getName(): Name{
    return this.servicePerson.name;
}

Answer №1

Avoid using ngOnChanges in this scenario as it is not intended for the task you are attempting.

Based on your query, here is what you aim to accomplish:

  • Retrieve data from the database
  • Enable your component to asynchronously receive a portion of that data

You can achieve this by expanding on your existing codebase and utilizing RxJS. In particular:

  • Establish a person subject within your person service
  • Upon receiving data from the DB, utilize personSubject.next(dataFromDB) to append it to the person stream
  • Create a function that will provide the person subject as an observable, facilitating subscription from your component

Adopting this approach ensures that any new data received from the DB will be incorporated into a person stream, allowing components subscribed to the stream to access the information.

Here's a brief example (since I lack your complete code):

import { ReplaySubject } from 'rxjs/ReplaySubject';
import { Observable } from 'rxjs/Observable';

@Injectable()
export class PersonService {
    // The person subject
    personStream: ReplaySubject<Person> = new ReplaySubject();

    // The person observable
    person$(): Observable<Person> {
        return this.personStream.asObservable();
    }

    // Retrieve person from DB and add to stream
    getDataFromDB() {
        this.http.get(url).subscribe(response => {
            this.personStream.next(response.person);
        });
    }
}

@Component({...})
export class MyComponent implements OnInit {
    person: Person;

    constructor(private personService: PersonService) {}

    ngOnInit() {
        // Subscribe to person observable. This will trigger whenever there is a change in person data.
        this.personService.person$().subscribe(person => this.person = person);

        // Alternatively, if `this.http.get(url)` is returned from `getDataFromDB`, you can simplify this line...
        this.personService.getDataFromDB().subscribe(person => this.person = person);
    }
}

In essence, simply subscribing to the getDataFromDB function in your component suffices, eliminating the need for additional complexities.

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