typescript - ObjectUnsubscribedError when trying to prevent subscribing twice

Question

I have a Service and a component that uses it:

• PagesService
• PagesListComponent

In the PagesService I have an array of Pages. I notify changes in the array via a BehaviorSubject which both of them are subscribed to.

The PagesService are provided at bootstrap, to have just one instance shared. That's because I need to keep the array, instead of downloading the pages everytime they are needed.

The code is the following:

pages.service.ts

import { Injectable } from '@angular/core';
import { BehaviorSubject } from 'rxjs/Rx';
import { Http, Response } from '@angular/http';

import { Page } from './../models/page';

@Injectable() export class PagesService {

    public pages$: BehaviorSubject<Page[]> = new BehaviorSubject<Page[]>([]);
    private pages: Page[] = [];

    constructor(private http: Http) { }

    getPagesListener() {
        return this.pages$;
    }
    getAll() {
        this.http.get('/mockups/pages.json').map((res: Response) => res.json()).subscribe(
            res => { this.resetPagesFromJson(res); },
            err => { console.log('Pages could not be fetched'); }
        );
    }

    private resetPagesFromJson(pagesArr: Array<any>) {
        // Parses de Array<any> and creates an Array<Page>
        this.pages$.next(this.pages);
    }
}

pages_list.component.ts

import { Component, OnInit } from '@angular/core';
import { Router } from '@angular/router-deprecated';
import { BehaviorSubject } from 'rxjs/Rx';

import { PagesService } from '../../shared/services/pages.service';
import { GoPage } from '../../shared/models/page';

@Component({
    moduleId: module.id,
    selector: 'go-pages-list',
    templateUrl: 'pages_list.component.html',
    styleUrls: ['pages_list.component.css']
})
export class PagesListComponent implements OnInit {
    pages$: BehaviorSubject<GoPage[]>;
    pages: GoPage[];
    constructor(private pagesService: PagesService, private router: Router) { }

    ngOnInit() {
        this.pages$ = this.pagesService.getPagesListener();
        this.pages$.subscribe((pages) => { this.pages = pages; console.log(pages) });
        this.pagesService.getAll();
    }
    ngOnDestroy() {
        this.pages$.unsubscribe();
    }
}

This works fine the first time, both the subscription onInit and de unsubscription onDestroy. But when I return to the list and try to subscribe again (to fetch the current value of pages[] and listen for future changes), it fires the error EXCEPTION: ObjectUnsubscribedError.

If I don't unsubscribe, everytime I enter to the list, a new subscription is stacked, and all of them are fired when a next() is received.

Answer 1

I would get the subscription and unsubscribe on it this way and not on the subject directly:

ngOnInit() {
  this.pages$ = this.pagesService.getPagesListener();
  this.subscription = this.pages$.subscribe((pages) => { // <-------
    this.pages = pages; console.log(pages);
  });
  this.pagesService.getAll();
}

ngOnDestroy() {
    this.subscription.unsubscribe(); // <-------
}