--- Edit 4 - Additional Resources (2018/09/01)
In a recent 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
--- Edit 3 - The 'Official' Solution (2017/04/09)
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this answer which he said was correct) but he told me the docs team for Angular had a solution to this question that is unpublished (though they are working on getting it approved).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 Observable
s 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.
--- Edit 2 (2016/12/28)
Source 5
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." - Mark Rajcok
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.
--- Edit 1
Source 4
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.
--- Original Answer
TLDR:
For this question there are (2) kinds of Observables
- finite value and infinite value.
http
Observables
produce finite (1) values and something like a DOM event listener
Observables
produce 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.
Source 1
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 1 value and ending - we never worried about unsubscribing from Promises to make sure they clean up xhr
event listeners, right?).
Source 2
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.
Source 3
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
.