I'm wondering about the best approach when using both takeUntil and catchError operators.
If we approach an error the stream should be completed so takeUntil is omitted no matter where it is placed, right?
If there's no error and we want to trigger takeUntil it should be placed after catchError due to inner observable possibility between takeUntil and catchError.
I've two thoughts which lead me to a conclusion that takeUntil should be placed after catchError. I'm kindly asking for your point of view, thanks :)
Related
when converting a promise to an observable with rxjs,
from(myPromise).subscribe(r => console.log(r))
do we need to unsubscribe to it ? or is it safe to assume it will always complete and never cause memory leak ?
In short: No
You do not need to unsubscribe from a promise. A promise should always complete or error, which causes the observable to complete or error as well.
There's no native way to cancel an in-flight promise, so unsubscribing from an observable that's wrapping a promise has no effect on the underlying promise.
I have not been able to find any normative text to answer this question. I have been using this code pattern (this is in TypeScript in an Angular application):
observeSomethingFun$: Observable<Fun>;
...
async loadsOfFun() {
const fun = await this.observeSomethingFun$.toPromise();
// I now have fun
}
In general, Observables need to be unsubscribed from. This happens automatically when the Angular async pipe is used but what happens in this case? Does toPromise unsubscribe after emitting one value?
If not, how do I unsubscribe manually?
Update:
It turns out #Will Taylor's answer below is correct but my question needs some clarification.
In my case the Observable emits a never-ending stream, unlike for example Angular's HttpClient Observables that complete after emitting one value. So
in my case I would never get past the await statement according to Taylor's answer.
RxJS makes this easy to fix. The correct statement turns out to be:
const fun = await this.observeSomethingFun$.pipe(first()).toPromise();
The RxJS first operator will receive the first value and unsubscribe from the source Observable. it will then send out that value to the toPromise operator
and then complete.
No need to unsubscribe.
Which is convenient, as there is no way to unsubscribe from a promise.
If the Observable completes - the promise will resolve with the last value emitted and all subscribers will automatically be unsubscribed at this point.
If the Observable errors - the promise will reject and all subscribers will automatically be unsubscribed at this point.
However, there are a couple of edge cases with toPromise in which the behavior is not completely clear from the docs.
If the Observable emits one or more values but does not complete or error, the promise will neither resolve or reject. In this case, the promise would hang around in memory, which is worth considering when working with toPromise.
If the Observable completes but does not emit a value, the promise will resolve with undefined.
First of all, thank you for this question and answer, I wrongly assumed toPromise() knew what it was doing in all scenarios and would unsubscribe when that observable completes (even if it is an observable stream)
So I will just say that it doesn't hurt to pipe all of your observables before using .toPromise()
I just went through a big ordeal of stepping through our app for memory leaks and found the above answer by Will to be good. The elaboration on the actual question was exactly the same issue I was running into.
We are stepping through each observable in the app right now and we use either
pipe(take(1)) which is equivalent to pipe(first()).
or we use pipe(takeUntil(this.destroyed)) where this.destroyed.next(true) is called when we destroy our particular component or service.
We use take() to keep our verbiage consistent so we can search for take or takeUntil across various components.
Long story short, yeah you might take a very slight performance hit piping your observables at each instance, but I highly recommend doing so in order to prevent any unwanted app-wide memory leak hunts. Then maybe if you have the time you can step through each one and see where .toPromise() actually unsubscribes correctly for you.
I was playing around with the switchMap operator to clearly understand what was happening to a "switched" inner observable.
At first i thought that switchMap was only "unsubscribing" from the switched inner observable, but then i realize it was in fact "unsubscribing AND completing" the inner observable.
To confirm that i've written this small snippet:
https://codesandbox.io/s/relaxed-meninsky-c5jmw?fontsize=14
As you can see, the finalize() operator is correctly called when the subject emit for the second time, but:
why does the complete handler of the tap operator is not called ?
This somehow make feel only 80% happy with my understanding of this operator.
A related not on that:
I've read and watch numerous sources regarding switchMap, including:
This (great) ngconf sources: https://medium.com/#shairez/a-super-ninja-trick-to-learn-rxjss-switchmap-mergemap-concatmap-and-exhaustmap-forever-88e178a75f1b
The official rxjs doc: https://rxjs-dev.firebaseapp.com/api/operators/switchMap
And none of them clearly state if inner observable is unsubscribed or unsubcribed AND closed ( or at least i did not understand it :) )
I've watched the switchMap operator source code and there is no mention to takeXXX operator, how can he complete the inner operator without that ?
tl;dr
Do you confirm that switchMap complete inner observable when switching ?
Why does tap operator does not work as expected ?
If switchMap effectively complete inner observable how can he do that without using a takeXXX operator internally ?
I think you are confusing the difference between unsubscribe() and complete(). For a hot observable like a Subject you can "stop" it in a few ways. From the 'top->down' with complete() as you did in your example, or from the 'bottom->up' with unsubscribe().
switchMap() does exactly what it says, it switches from the primary observable to a secondary (or 'inner') observable. That is why when you complete() the outer observable, it has no effect on the inner one - the chain has been switched. To affect the chain (as opposed to just affecting the Subject which is the source observable), you need to get a reference to the Subscriber, and then call that Subscriber's unsubscribe() method.
To see this, I've forked your CodeSandbox and produced this new one
As you will see in that CodeSandbox I have added a few more lines to show what is going on:
Note the new tap() in the chain right above the switchMap - this will show what is going on directly from the Subject() before the chain is switched to a different Observable with the switchMap operator.
The Subscription for the chain is now being captured in the variable sub which can be unsubscribed later to affect the chain from the bottom->up.
Note that the s.complete() after 10 seconds is now reflected in the Subject, and note also how it doesn't affect the chain at all.
Now note that the new sub.unsubscribe() after 15 seconds indeed kills the chain.
uncomment the take(5) in the newT() method to see that indeed the tap's complete method will be called if the source above it actually completes (top->down).
finalize() catches the fact that an unsubscribe has happened (bottom->up), note that it occurs both when switchMap() does the automatic unsubscribe upwards when s.next() is called on the Subject source, as well as when unsubscribe() is called on the Subscription, which again causes a bottom->up termination. In no case is your complete() called in the original observer because the chain is never actually completed. You can complete the chain with a take(10) operator if you want, to see how that works as well.
Hopefully this helps clear up the confusion a little. :)
I have a set of RxJs Observable that uses forkJoin. The issue I have is that the forkJoin makes the calls but subscribe does not get calls.
However, if I use zip it works.
I have created an example in StackBlitz. Is this a bug or am I doing it all wrong?
forkJoin only runs once all inner observables have completed. In your example, the inner observables never complete. zip runs once everytime all observables fire in order. If you want something to run anytime any observable fires, use combineLatest
If an observable completes, do I still have to unsubscribe / dispose (in RxJS) the observable to remove the Observer (prevent memory leaks) or is this handled internally by Rxjs once a onComplete or onError event occurs?
No, you don't need to unsubscribe from an observable you know has completed.
If you look at the source code of the RxJS toPromise() function, you'll see a subscribe, but no unsubscribe. That's because it is not necessary, you know the observable is completed.