I've been struggling to come up with an Rx strategy for a particular situation. I'm hoping someone could point me in the right direction.
Basically, I have a socket feed that I would like to skip based on a boolean value. When the stream is skipping the socket, I need to keep a running buffer of the latest value sent from socket.
Once I am no longer skipping the socket events, then push down the stream the last value that was emitted when it was skipping, but only under another condition (bool), and re-start listen to socket events
So basically:
Listen to socket feed
takeWhile(bool)
When start listening again, apply last values while skipping
socket, if reapply==true
Didn't get far, but this is what I have:
Rx.Observable.interval(1000)
.skipWhile(()=>isSkipping)
.bufferWhileSkipping??
.applySkippedValuesAfterSkipping(ifisReapply)??
.subscribe(val=>console.log(val));
Perhaps skipWhile is not the right approach but was the only one that kind of made sense…
You could do it similarly to the following (I'm assuming your isSkipping can be an Observable):
const isSkipping = new BehaviorSubject(false);
Observable.interval(100)
.take(20)
.window(isSkipping)
.withLatestFrom(isSkipping)
.switchMap(([observable, skipping]) => skipping
? observable.takeLast(1).map(val => 'last:' + val)
: observable)
.subscribe(console.log);
setTimeout(() => isSkipping.next(true), 500);
setTimeout(() => isSkipping.next(false), 1050);
setTimeout(() => isSkipping.next(true), 1500);
setTimeout(() => isSkipping.next(false), 1850);
Every time isSkipping emits a value the window operator creates a new Observable that just re-emits everything or chains the .takeLast(1) operator when skipping is set to true.
The example above prints the following output to the console:
0
1
2
3
last:9
10
11
12
13
last:16
17
18
19
Related
Consider this Observable:
myObs$ = interval(1000).pipe(shareReplay(2));
And its usage:
myObs$.subscribe(res => console.log('[Subscriber 1]:' + res));
If I subscribe, then unsubscribe a few seconds later, and finally resubscribe again after another few seconds, it seems like the interval kept running and counting:
[Subscriber 1]: 0
[Subscriber 1]: 1
Unsubscribe, resubscribe here
[Subscriber 1]: 3
[Subscriber 1]: 4
[Subscriber 1]: 5
[Subscriber 1]: 6
Unsubscribe, resubscribe here
[Subscriber 1]: 10
[Subscriber 1]: 11
[Subscriber 1]: 12
I understood that this does not happen when using refCount = true. But when it is false, does this count as a potential memory leak ? If not, how do I stop it?
Also, why do I need to recreate my Subscription after unsubscribing?
sub = new Subscription()
sub.add(myObs.subscribe())
sub.unsubscribe()
sub.add(myObs.subscribe()) // <-- this does not work unless I recreate a new Subscription()
Using shareReply without refCount can result in a memory leak because the operator doesn't close automatically the stream after everyone has unsubscribed. You can make a long lived stream complete by using the takeUntil operator which accepts another stream and it's very useful when used with a Subject.
const sub = new Subject();
const stream$ = interval(1000).pipe(
takeUntil(sub),
shareReplay(1)
);
setTimeout(() => stream$.subscribe(console.log), 4000);
setTimeout(() => sub.next(), 6000);
setTimeout(() => stream$.subscribe(undefined, undefined, () => console.log('completed')), 8000); // this will log completed because the stream$ has successfully finished
Another way to complete a stream is with takeWhile which is very handy as well and it uses a predicate function.
If you dispose a subscription it is marked as closed and you need to create a new one, it cannot be reused again.
Take a look on this line where inside the add method there is a check and if closed is true just executes the teardown on the provided subscription.
I want to ensure that a given API call is throttled so that for a given time interval, only a single request is fired, and that the other, throttled requests wait and receive the results of the request that was actively fired
Example
const generateReport = (args) => client.get(...)
const generateReportA = (argsForA) =>
generateReport(argsForA).then(saveReportSomewhere)
const generateReportB = (argsForB) =>
generateReport(argsForB).then(saveReportSomewhere)
const generateReportC = (argsForC) =>
generateReport(argsForC).then(saveReportSomewhere)
If we then run the statements below
generateReportA(...).then(console.log) // should log result of C
generateReportB(...).then(console.log) // should log result of C
generateReportC(...).then(console.log) // should log result
right after each other, I only want to fire the request associated with generateReportC and I'd like both generateReportA and generateReportB to receive and handle the result of generateReportC.
In the end generateReport should have been called once and saveReportSomewhere should have been called 3 times, each with the result from generateReportC
Is this possible?
This will fire C request get result and save it, then trigger A and B at the same time with result from C and save results immediately after every emit.
generateReportC.pipe(
tap(cResult => saveReportSomewhere(cResult)),
mergeMap(cResult => merge(generateReportA(cResult), generateReportB(cResult))),
tap(result => saveReportSomewhere(result))
).subscribe();
I'd like to implement websocket reconnect in webapp if internet connection is lost. In order to detect that internet is lost I use ping-pong approach, which means that I send from client ping-message and server returns me pong-message.
When webapp loaded I send init ping message and start to listen a reply on socket some kind of this:
this.websocket.onmessage = (evt) => {
try {
const websocketPayload: any = JSON.parse(evt.data);
if (websocketPayload.pong !== undefined && websocketPayload.pong == 1) {
this.pingPong$.next('pong');
}
It means that internet connection looks ok and we can continue. Also I have the follow code:
Observable.race(
Observable.of('timeout').delay(5000).repeat(),
this.pingPong$
).subscribe((data) => {
console.log("[ping-pong]:", data);
if (data == 'pong') {
Observable.interval(5000).take(1).subscribe(() => {
console.log("[ping-pong]:sending ping")
this.send({ping:1})
});
} else if (data == 'timeout'){
// show reconnect screen and start reconnect
console.error("It looks like websocket connection lost");
}
});
But!
When this.pingPong$ subject stops to emit events - .next() doesn't happen because of we can't get response when I break connection manually - I considered that in Observable.race this observable will be emitted
Observable.of('timeout').delay(5000).repeat()
But my subscribe never happens if this.pingPong$ stop emitting.
Why ?
Thank you
race picks and keeps subscribed to the first Observable that emits.
So if your this.pingPong$ starts emitting and then stops it makes no difference because race keeps subscribed to this.pingPong$. The other Observables don't matter any more. You might want emit one value from this.pingPong$ and the repeat the whole process. For example like the following:
Observable.race(
Observable.of('timeout').delay(5000).repeat(),
this.pingPong$
)
.pipe(
take(1), // complete the chain immediately
repeat() // resubscribe after take(1) completes the chain
)
.subscribe(...);
Obviously it mostly depends on what you want to do but I hope you get the point.
I'm working on a use case that requires that if an observable has not emitted a value within a certain amount of time then we should do some side effect.
To give a practical use case:
open web socket connection
if no message has been sent/received within X time then close web socket connection and notify user
This requires for a timer to be initiated on every emitted value and upon initial subscription of observable which will then run some function after the allotted time or until a value is emitted in which the timer resets. I'm struggling to do this the Rx way. Any help would be appreciated :)
debounceTime is the operator you're looking for: it only emits a value if no others follow within a specific timeout. Listening for the first message of the debounced stream will let you time out and clean up your websocket connection. If you need to time out starting from the opening of the stream, you can simply startWith. Concretely:
messages$.startWith(null)
.debounceTime(timeout)
.take(1)
.subscribe(() => { /* side effects */ });
Edit: if instead you're looking to end the a message stream entirely when it times out (e.g. you clean up in the onComplete handler), just cram debounceTime into a takeUntil:
messages$.takeUntil(
messages$.startWith(null)
.debounceTime(timeout)
).subscribe(timeout_observer);
With a timeout_observable: Observer<TMessage> that contains your cleanup onComplete.
You can do this with race:
timer(5000).race(someSource$)
.subscribe(notifyUser);
If someSource$ notifies faster than timer(5000) (5 seconds), then someSource$ "wins" and lives on.
If you only want one value from someSource$, you can obviously have a take(1) or first() on someSource$ and that will solve that issue.
I hope that helps.
Might not be the perfect answer but it does what you asked, it depends on how you want to disconnect, there might be some variation to be done
const source = new Rx.Subject();
const duration = 2000;
source.switchMap(value=>{
return Rx.Observable.of(value).combineLatest(Rx.Observable.timer(2000).mapTo('disconnect').startWith('connected'))
}).flatMap(([emit,timer])=>{
if(timer=='disconnect'){
console.log('go disconnect')
return Rx.Observable.throw('disconnected')
}
return Rx.Observable.of(emit)
})
//.catch(e=>Rx.Observable.of('disconnect catch'))
.subscribe(value=>console.log('subscribed->',value),console.log)
setTimeout(() => source.next('normal'), 300);
setTimeout(() => source.next('normal'), 300);
setTimeout(() => source.next('last'), 1800);
setTimeout(() => source.next('ignored'), 4000);
<script src="https://unpkg.com/rxjs#5/bundles/Rx.min.js"></script>
A timer is initiated on each element and if it takes 4 seconds to be shown, then it will timeout and you can execute your function in the catchError
Here an example, it displays aa at T0s, then bb at t3s, then timeout after 4 second because the last one cc takes 10s to be displayed
import './style.css';
screenLog.init()
import { from } from 'rxjs/observable/from';
import { of } from 'rxjs/observable/of';
import { race } from 'rxjs/observable/race';
import { timer } from 'rxjs/observable/timer';
import { groupBy, mergeMap, toArray, map, reduce, concatMap, delay, concat, timeout, catchError, take } from 'rxjs/operators';
// simulate a element that appear at t0, then at t30s, then at t10s
const obs1$ = of('aa ');
const obs2$ = of('bb ').pipe(delay(3000));
const obs3$ = of('cc ').pipe(delay(10000));
const example2 = obs1$.pipe(concat(obs2$.pipe(concat(obs3$))), timeout(4000), catchError(a => of('timeout'))); // here in the catchError, execute your function
const subscribe = example2.subscribe(val => console.log(val + ' ' + new Date().toLocaleTimeString()));
I have the following code (httpObservable completes on first emission) which polls service with fixed "dead time":
return serviceObservable.expand(() => Observable.timer(period).concatMap(() => serviceObservable));
How can I make the timer "resettable" by using Subject which emits every time the timer should be resetted?
return serviceObservable
.expand(() => Observable
.timer(period)
.race(subject.take(1))
.concatMap(() => serviceObservable));