RxJS polling by interval and when manually called - notifications

Question

In my Angular app I'm working on notifications and I have an REST API to call for latest user's notifications. I need to call this API on few minutes since it's not really important that user gets notifications in real time (they probably won't even appear that fast). However the idea to refresh notifications on the client side is next:
When user logs in start refreshing notifications - here is the first manual call to start refreshing the API on few minutes
If user leaves app opened or is just navigating through the app then don't change timer and wait for the rest of the time
If user opens subpage where it can perform actions related to notifications and does it, then refresh notifications and reset timer
Refresh notifications until logout
I already have working code for the described procedure, but I'm somehow unsure that it's correct for what I need. Here is the code for performing calls (for manual check there is just a Subject and for stop checking there is a subscription to observable - code below is actually separated, but here is in one place because of readability):
// Subject for manual triggering
this.checkFeed = new Subject<void>();
// Call for refresh in own method
this.checkFeed.next();
// Waiting for manual refresh or triggering it on some interval after it was last triggered
this.feedSub = this.checkFeed.asObservable()
.switchMap(() => Observable.timer(0, this.interval))
.mergeMap(() => this.fetchChanges())
.distinctUntilChanged(this.compareFeed)
.subscribe(res => this.notify(res));
// Unsubscription when logging out
if (this.feedSub) this.feedSub.unsubscribe();
The part which I'm most unsure about is .switchMap(() => Observable.timer(0, this.interval)) since it needs 0 to start right away (which is ok, but still doesn't look correct at all?). So is there any better way to achieve what I described?
I also have another question how to start check for notifications from another observable - which operator should I use. As I mentioned I have call to the Subject's next in own method like this:
refreshFeed(): void {
this.checkFeed.next();
}
So when there is some other observable performing (the action when notifications should be refreshed) I need to call this one. What's the correct way to call void method when other observable has response from API? I was thinking of something like this:
someActionThatCanChangeNotifications(): Observable<any> {
return this.api.get('path/to/endpoint')
.do(() => this.feedService.refreshFeed());
}
Is this ok, or is there also any better way?
Thanks in advance for help!

Answer 1

So basically you have two observable.
One that you call manually:
this.checkFeed
and the interval(let's callit intervalObs):
this.intervalObs = Observable.timer(0, this.interval);
If you see it like this the easyest way is to merge you'r two source stream and then do whatever you want.
var mergedSource = Observable.merge(
this.checkFeed,
this.intervalObs)
subscription = mergedSource.subscribe(this.fetchChanges());
Maybe you need to do some more operation in between but this should give you a more readable alternative.
You can try this working plunker if you want something to play arround https://plnkr.co/edit/n4nNFEMa4YOh2KSjDpSJ?p=preview

Answer 2

From what I can see you've pretty much done it "correctly". As with programming in general, there are many possible (and correct) solutions to a single problem. Personally, I'd do this the same way.
I can give you some commentary on the two points you mentioned too:
.switchMap(() => Observable.timer(0, this.interval))
Observable.timer pretty much an Observable.interval with a custom timeout before the first value. Observable.timer(0, this.interval) is the correct usage.
An alternative could be Observable.just(0).concat(Observable.interval(this.interval)), which returns a value immediately and then starts the interval. I prefer the way you put however; I think it clearly states your intention: "Produce a value after 0 milliseconds, and then an interval of this.interval".
.do(() => this.feedService.refreshFeed())
I'd say this is the totally correct way of doing it. do is meant for side effects, eg. stuff that happening outside the observable.
I can say though, I wouldn't expect someActionThatCanChangeNotifications to kick off a refresh of the feed. When a function returns an observable, I would expect to return an observable that doesn't have any side effects. However, as we live in a non-perfect world, we can't always have what we want.
You can't expect every subscriber to remember to do .do(() => this.feedService.refreshFeed()), instead I'd add a notice in the doc comment for the function: "Note: The returned observable will refresh the feed on every next signal", or something of that kind.