Next and onNext method from observer not working

Ionic Framework ionic-v3
1

I tried use the tutorial below to implement observer on my code:
https://www.joshmorony.com/building-mobile-apps-with-ionic-2/observables-in-ionic2.html

but there is no next method for observer type:
this.checklistObserver.next(true);

when I put this line in the code I get a blank screen…

After a search I found a explanation about RXJS observer and the new method was named onNext() but no success.

my code:

import { Injectable } from '@angular/core';
import { Http, Response, Headers, RequestOptions  } from '@angular/http';

import {Observable} from 'rxjs/Observable';
//import {Observable} from 'rxjs/Rx';
import {Observer} from 'rxjs/Observer';
import { LoginProvider } from "../../providers/login/login";

import { SQLite, SQLiteObject } from '@ionic-native/sqlite';
import 'rxjs/add/operator/do';
import 'rxjs/add/operator/map';
import 'rxjs/add/operator/catch';
/*
  Generated class for the ItemsProvider provider.

  See https://angular.io/docs/ts/latest/guide/dependency-injection.html
  for more info on providers and Angular DI.
*/

@Injectable()
export class ItemsProvider {
  items: Array<Item>;
  checkitems: any;
  checkitemsObserver: any;

  constructor(public http: Http, private sqlite: SQLite, private login: LoginProvider) {
    console.log('Hello ItemsProvider Provider');
    this.items = [];
    this.checkitems = Observable.create(observer => {
     this.checkitemsObserver = observer;
   });
    this.requestServer();
  }

  checkitemsUpdates(): Observable<any> {
    return this.checkitems;
  }

  }
  requestServer() {
      /* CODE THAT GET DATA FROM SERVER AND UPDATE DATA IN SQLITE DATABASE - was omitted */
      this.checkitemsObserver.onNext(this.items); // the data was new items

  }
}
2

I don’t see a question.

3

checklistitemsObserver is also defined as any. I would try and define it as having a proper type.

4

@AaronSterling the questions is: Why the .next doesn’t work inside a provider?

@SigmundFroyd thank you… I changed to Observer, but the problem is still there… one thing I did not mention is the observer code is in a provider… when I change to a model the .next works. Is there a problem running inside a provider? whether yes… why http.get works?

5

I have yet to see a situation where Observable.create() is the best option. Please describe what you are actually trying to achieve here.

6

The idea is: A list of items can be updated by server or local database… and a page shows the items… for example: User open app and the app request a list of items from server, update the database and shows the list of items updated… when user add a item, the item is inserted in local database, sent to the server and the list of items is updated… when server discover new attribute for the item it send a message to app and the list is updated. There are 3 ways to update the list on screen… 1. a app request for updates, 2. user updates, 3. server send new updates.

what I did: A Item model (to get data from database and make requests to the server, the object responsible for the consistency of the items)… a server provider (a interface to the server)… and the page that observe item model to show the changes.

7

All of that seems reasonable. I think some flavor of Subject would be more straightforward to work with than a hand-crafted Observable, though.

8

@rapropos thank you… I will try a Subject instead Observable :grinning:
I didn’t know the diference between subject and observable… based in this link subject will be a better choice than observable (considering that Items observes/receives/listening states/changes from server and database and is observed by external object. Am I correct?).

@SigmundFroyd You are correct… I removed the old code and add a new one, with explicit Subject type and works… but why in a model/page set variable with <any> works and inside a provider doesn’t work call the method .next()? And Why when I declared Observer<any> (like you said) doesn’t work but Subject<any> works???

9

I would agree with this assessment. In order for your provider to both send and receive updates, to do so with a raw Observable would require you to more or less reinvent what a Subject would already give you. The primary difference between the two main flavors is that BehaviorSubjects have a well-defined initial state, whereas ReplaySubjects can start in a sort of “idk nothing to report yet” limbo state, which is sometimes useful.

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