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.
Related
I usually see the use of either promise and future in the start of a vert.x verticle. Is there any specific difference between both?
I have read about their differences in Scala language, is it the same in case of Vert.x too?
Also when should I know when to use promise or a future?
The best I've read about:
think on Promise as producer (used by producer on one side of async operation) and Future as consumer (used by consumer on the other side).
Futures vs. Promises
Promise are for defining non-blocking operations, and it's future() method returns the Future associated with a promise, to get notified of the promise completion and retrieve its value. The Future interface is the result of an action that may, or may not, have occurred yet.
A bit late to the game, and the other answers say as much in different words, however this might help. Lets say you were wrapping some older API (e.g. callback based) to use Futures, then you might do something like this :
Future<String> getStringFromLegacyCallbackAPI() {
Promise<String> promise = Promise.promise();
legacyApi.getString(promise::complete);
return promise.future();
}
Note that the person who calls this method gets a Future so they can only specify what should happen in the event of successful completion or failure (they cannot trigger completion or failure). So, I think you should not pass the promise up the stack - rather the Future should be handed back and the Promise should be kept under the control of the code which can resolve or reject it.
A Promise as well is a writable side of an action that may or not have occurred yet.
And according to the wiki :
Given the new Promise / Future APIs the start(Future<Void>) and stop(Future<Void>) methods have been deprecated and will be removed in Vert.x 4.
Please migrate to the start(Promise) and stop(Promise) variants.
As a paraphrase,
A future is a read-only container for a result that does not yet exist, while a promise can be written (normally only once).
More from here
I have created this example to demonstrate the issue: https://stackblitz.com/edit/rxjs-vxvmq1?file=index.ts&devtoolsheight=100
Basically I want to call a function (e.g. cleanup as in the example) when a subject is completed. The function is called by others so not subscribing to the subject's changes, rather, a subscription is there to trigger the function for the completion of the subject. The function checks if it should do extra work when the subject is completed, so it checks if the subject has stopped or not. But it seems that once next() is called, the function is triggered before complete() is called, making the function thinks the subject has not been completed yet.
I wonder if there is any way to resolve this? Calling complete() first then next() didn't help as next() didn't notify the subscription after the subject has completed.
Long story short. It is all wrong.
Fix #1, not rxjs-idiomatic: you're subscribing wrongly: passing onNext callback, while you're interested in onComplete. There are three parameters taken by the subscribe: onNext, onError, onComplete and you are responsible for choosing the one you really need.
Fix #2, rxjs-idiomaric: you have to use pipe(...) along with operators defined in rxjs/operators (AFAIR). Note that it is easy to define your custom operator as well. So you either could use finalize(() => ...your cleanup logic goes here) or Observable.create returning finalizing logic as implementation of unsubscribe. Both nicely documented here or here.
In addition, seems that you're misunderstanding the semantics of rxjs. In terms of regex, it could be defined as next*(error|complete), which literally means: zero or infinitely many next's followed by either error or complete (exclusive or: never both simultaneously) exactly once. So don't expect next to do anything after complete (or equally error) fired.
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've googled the issue expecting that there's been a gazillion curious people before me asking it too. For some reason, most hits are on scan vs reduce (which I clearly understand). So there's a risk that I totally misunderstood the docs.
According to the docs, scan(...) will snatch an emitted value, do stuff to it and then, optionally pass it on to the next person in line. Meanwhile, subscribe(...), although accepting parameters for handling of errors and completion, does the very same thing.
I understand the "difference" between them but it appears to me as rather insignificant from the aspect of development tooling. Is it as simple as that the former only a convenience method for cases where the latter would require mundane coding? Or is there a fundamental difference between them (as in: something I can do with scanning that I can't achieve subscribing)?
Scan() and Subscribe() are quite different concepts in RxJS.
Scan is an operator for combining values coming through the stream with previous values that came through the stream, and then outputting some combination of them (I think scan and reduce are the only operators that does this). Subscribe only works on the current value that comes through the stream.
Subscribe is a special method and one of the most important concepts in RxJS. Subscribe comes at the end of the Observable stream, this is where you can use the resulting value for something. From all other operators you return something that can be passed down the chain, but you do not return from subscribe.
If you are working with cold Observables (which you very often are), you need to subscribe to it in order for the code to run at all. If you have no subscriptions on a cold observable, then none of the code in your Observable stream will run.
The syntax for using them is also different. Scan is an operator that you chain inside the pipe() method like map, reduce, filter, tap, mergeMap, flatMap, etc. It looks like:
myObservable$.pipe(map(...), scan(...), flatMap(...));
Subscribe is a method like pipe that you dot chain, like:
myObservable$.pipe(...).subscribe(...);
So the case is this. Suppose somewhere I am filling a Collection. Each time an element is added, an IObservable calls OnNext for its subscribers.
Now, there will be a point where the collection will be filled. (I was reading something and I finished reading .. whatever). At that point, OnComplete() is called on the subscribers.
The user, however, won't observe this method. He will rather call an async method that he will await for ... he doesn't care much about the things he read, he just cares that he finished reading.
So basically, I want, from an IObservable, a Task that returns when the IObservable calls OnComplete() to its subscribers. I specifically want the user not to use an observer, but just to be happy with knowing that whatever happens after his await call will happen after the collection is filled.
Maybe the ToTask() method does the trick? I can't really tell by the documentation.
If you are using Rx 2.0 or later you can await IObservable which returns the last item in the observable. I.e. after the observable has completed.
var lastItem = await myObservable;
This is possible because in Rx 2.0 a GetAwaiter extension method on IObservable was added making it possible to await observables. There are also some handy extension methods that allow you to specify which element you want to await.
There is a nice blog about it here.