Angular/RxJS When should I unsubscribe from `Subscription`

TL;DR

For this question there are two kinds of Observables - finite value and infinite value.

http Observables produce finite (1) values and something like a DOM event listener Observable produces infinite values.

If you manually call subscribe (not using async pipe), then unsubscribe from infinite Observables.

Don't worry about finite ones, RxJs will take care of them.

Sources:

I tracked down an answer from Rob Wormald in Angular's Gitter here.

He states (I reorganized for clarity and emphasis is mine):

if its a single-value-sequence (like an http request) the manual cleanup is unnecessary (assuming you subscribe in the controller manually)

i should say "if its a sequence that completes" (of which single value sequences, a la http, are one)

if its an infinite sequence, you should unsubscribe which the async pipe does for you

Also he mentions in this YouTube video on Observables that "they clean up after themselves..." in the context of Observables that complete (like Promises, which always complete because they are always producing one value and ending - we never worried about unsubscribing from Promises to make sure they clean up XHR event listeners, right?)
Also in the Rangle guide to Angular 2 it reads

In most cases we will not need to explicitly call the unsubscribe method unless we want to cancel early or our Observable has a longer lifespan than our subscription. The default behavior of Observable operators is to dispose of the subscription as soon as .complete() or .error() messages are published. Keep in mind that RxJS was designed to be used in a "fire and forget" fashion most of the time.

When does the phrase "our Observable has a longer lifespan than our subscription" apply?

It applies when a subscription is created inside a component which is destroyed before (or not 'long' before) the Observable completes.

I read this as meaning if we subscribe to an http request or an Observable that emits 10 values and our component is destroyed before that http request returns or the 10 values have been emitted, we are still OK!

When the request does return or the 10th value is finally emitted the Observable will complete and all resources will be cleaned up.
If we look at this example from the same Rangle guide we can see that the subscription to route.params does require an unsubscribe() because we don't know when those params will stop changing (emitting new values).

The component could be destroyed by navigating away in which case the route params will likely still be changing (they could technically change until the app ends) and the resources allocated in subscription would still be allocated because there hasn't been a completion.
In this video from NgEurope Rob Wormald also says you do not need to unsubscribe from Router Observables. He also mentions the http service and ActivatedRoute.params in this video from November 2016.
The Angular tutorial, the Routing chapter now states the following:

The Router manages the observables it provides and localizes the subscriptions. The subscriptions are cleaned up when the component is destroyed, protecting against memory leaks, so we don't need to unsubscribe from the route params Observable.

Here's a discussion on the GitHub Issues for the Angular docs regarding Router Observables where Ward Bell mentions that clarification for all of this is in the works.

I spoke with Ward Bell about this question at NGConf (I even showed him this answer which he said was correct) but he told me the docs team for Angular had a solution to this question that is unpublished (though they are working on getting it approved). He also told me I could update my SO answer with the forthcoming official recommendation.

The solution we should all use going forward is to add a private ngUnsubscribe = new Subject(); field to all components that have .subscribe() calls to Observables within their class code.

We then call this.ngUnsubscribe.next(); this.ngUnsubscribe.complete(); in our ngOnDestroy() methods.

The secret sauce (as noted already by @metamaker) is to call takeUntil(this.ngUnsubscribe) before each of our .subscribe() calls which will guarantee all subscriptions will be cleaned up when the component is destroyed.

Example:

import { Component, OnDestroy, OnInit } from '@angular/core';
// RxJs 6.x+ import paths
import { filter, startWith, takeUntil } from 'rxjs/operators';
import { Subject } from 'rxjs';
import { BookService } from '../books.service';

@Component({
    selector: 'app-books',
    templateUrl: './books.component.html'
})
export class BooksComponent implements OnDestroy, OnInit {
    private ngUnsubscribe = new Subject();

    constructor(private booksService: BookService) { }

    ngOnInit() {
        this.booksService.getBooks()
            .pipe(
               startWith([]),
               filter(books => books.length > 0),
               takeUntil(this.ngUnsubscribe)
            )
            .subscribe(books => console.log(books));

        this.booksService.getArchivedBooks()
            .pipe(takeUntil(this.ngUnsubscribe))
            .subscribe(archivedBooks => console.log(archivedBooks));
    }

    ngOnDestroy() {
        this.ngUnsubscribe.next();
        this.ngUnsubscribe.complete();
    }
}

Note: It's important to add the takeUntil operator as the last one to prevent leaks with intermediate Observables in the operator chain.

More recently, in an episode of Adventures in Angular Ben Lesh and Ward Bell discuss the issues around how/when to unsubscribe in a component. The discussion starts at about 1:05:30.

Ward mentions "right now there's an awful takeUntil dance that takes a lot of machinery" and Shai Reznik mentions "Angular handles some of the subscriptions like http and routing".

In response Ben mentions that there are discussions right now to allow Observables to hook into the Angular component lifecycle events and Ward suggests an Observable of lifecycle events that a component could subscribe to as a way of knowing when to complete Observables maintained as component internal state.

That said, we mostly need solutions now so here are some other resources.

A recommendation for the takeUntil() pattern from RxJs core team member Nicholas Jamieson and a TSLint rule to help enforce it: https://ncjamieson.com/avoiding-takeuntil-leaks/
Lightweight npm package that exposes an Observable operator that takes a component instance (this) as a parameter and automatically unsubscribes during ngOnDestroy: https://github.com/NetanelBasal/ngx-take-until-destroy
Another variation of the above with slightly better ergonomics if you are not doing AOT builds (but we should all be doing AOT now): https://github.com/smnbbrv/ngx-rx-collector
Custom directive *ngSubscribe that works like async pipe but creates an embedded view in your template so you can refer to the 'unwrapped' value throughout your template: https://netbasal.com/diy-subscription-handling-directive-in-angular-c8f6e762697f

I mention in a comment to Nicholas' blog that over-use of takeUntil() could be a sign that your component is trying to do too much and that separating your existing components into Feature and Presentational components should be considered. You can then | async the Observable from the Feature component into an Input of the Presentational component, which means no subscriptions are necessary anywhere. Read more about this approach here.

You don't need to have bunch of subscriptions and unsubscribe manually. Use Subject and takeUntil combo to handle subscriptions like a boss:

import { Subject } from "rxjs"
import { takeUntil } from "rxjs/operators"

@Component({
  moduleId: __moduleName,
  selector: "my-view",
  templateUrl: "../views/view-route.view.html"
})
export class ViewRouteComponent implements OnInit, OnDestroy {
  componentDestroyed$: Subject<boolean> = new Subject()

  constructor(private titleService: TitleService) {}

  ngOnInit() {
    this.titleService.emitter1$
      .pipe(takeUntil(this.componentDestroyed$))
      .subscribe((data: any) => { /* ... do something 1 */ })

    this.titleService.emitter2$
      .pipe(takeUntil(this.componentDestroyed$))
      .subscribe((data: any) => { /* ... do something 2 */ })

    //...

    this.titleService.emitterN$
      .pipe(takeUntil(this.componentDestroyed$))
      .subscribe((data: any) => { /* ... do something N */ })
  }

  ngOnDestroy() {
    this.componentDestroyed$.next(true)
    this.componentDestroyed$.complete()
  }
}

Alternative approach, which was proposed by @acumartini in comments, uses takeWhile instead of takeUntil. You may prefer it, but mind that this way your Observable execution will not be cancelled on ngDestroy of your component (e.g. when you make time consuming calculations or wait for data from server). Method, which is based on takeUntil, doesn't have this drawback and leads to immediate cancellation of request. Thanks to @AlexChe for detailed explanation in comments.

So here is the code:

@Component({
  moduleId: __moduleName,
  selector: "my-view",
  templateUrl: "../views/view-route.view.html"
})
export class ViewRouteComponent implements OnInit, OnDestroy {
  alive: boolean = true

  constructor(private titleService: TitleService) {}

  ngOnInit() {
    this.titleService.emitter1$
      .pipe(takeWhile(() => this.alive))
      .subscribe((data: any) => { /* ... do something 1 */ })

    this.titleService.emitter2$
      .pipe(takeWhile(() => this.alive))
      .subscribe((data: any) => { /* ... do something 2 */ })

    // ...

    this.titleService.emitterN$
      .pipe(takeWhile(() => this.alive))
      .subscribe((data: any) => { /* ... do something N */ })
  }

  ngOnDestroy() {
    this.alive = false
  }
}

Angular/RxJS When should I unsubscribe from `Subscription`

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