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Is-there an opposite of the `race` operator in RxJS?
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I have 2 observable and I would like to have the value of the last one which emitted.
Example:
private obs1: Observable<number> = interval(1000);
private obs2: Observable<number> = interval(700);
public lastValue; // should take the value of the last observable which emitted.
expected behavior
I can't find the operator to use for this. One solution could be to subscribe to all observable, and then make lastValue a Subject and update its value with next, but I would like to avoid subscribing to all the observable.
Thanks
you can use merge to merge the two observables into one and the values are returned in the order they're emitted.. so when subscribing you'll always receive the latest value..
merge(obs1, obs2).subscribe(result => {console.log(result)})
if the observables complete you can combine the above with takeLast to get the last x values emitted.
You can also wrap the implementation and make your own BehaviourSubject
You need combineLatest:
import { combineLatest } from 'rxjs';
combineLatest(obs1, obs2)
.subscribe(([valueOfObs1, valueOfObs2]) => //...
Related
I use the following code in an angular app. I used the RxJS map call similar to how array map is used. After reading about RxJS switchmap operator, I'm not sure whether I should use map or switchmap. Should I use switchmap so the observable which is returned from the http call is closed so there is no memory leak?
getPeopleForTypeahead(term: string): Observable<IPersonForTypeahead[]> {
var peopleUrl = `https://localhost:5001/api/peoplesearch?name=${term}`;
return this.http.get<any>(peopleUrl)
.pipe(
map(pl => {
return this.peopleAsFlattened(pl.peopleList).reduce((p, c) => p.concat(c));
}),
catchError(this.handleError('getPeopleForTypeahead', []))
);
}
peopleAsFlattened = (pla: IPeopleList[]) => {
return pla.map(pl => pl.people.map(p => {
return {
id: p.id,
fullName: p.fullNames[0].firstName + " " + p.fullNames[0].lastName
};
}));
}
map and switchMap have completely different purposes:
map - transform the shape of an emission
switchMap - subscribe to an observable and emit its emissions into the stream
map
Use map when you want transform the shape of each emission. Ex: emit the user name property, instead of the entire user object:
userName$: Observable<string> = this.service.getUser(123).pipe(
map(user => user.name)
);
switchMap
Use switchMap when you want to map an emission to another observable and emit its emissions. Ex: You have an observable of some id and want to emit the resource after fetching it:
user$: Observable<User> = this.userId$.pipe(
switchMap(id => this.service.getUser(id)),
);
When user$ is subscribed to, the user returned from service.getUser(id) is emitted (not the userId string).
switchMap is not interchangeable with the map operator, nor vise versa. Although both of them has to do with mapping (as their names suggest), they have two separate use-cases.
In your particular case, the map operator is the way to go.
When to use switchMap?
You can only use switchMap(cb) when you check all these requirements:
Your callback function, cb, passed into switchMap returns an observable, observable$.
If your cb (callback function) does not return an observable, you should look into operators that don't handle higher-level observables, such as filter and map (what you actually needed); not operators that handle higher-level observables such as concatMap and well, switchMap.
You want to execute your cb sequentially before the next operation down the pipeline (after switchMap) executes.
Maybe you want to run logic inside of cb, and optionally get the return value of cb after executing, so that you can pass it down the pipeline for further processing, for example.
When you want to "discard" what will happen to cb's execution and re-execute cb every time the source observable (the thing that trickles down to switchMap(cb)) emits a new value/notification.
Applying what we hopefully learned, we know that your cb:
pl => {
return this.peopleAsFlattened(pl.peopleList).reduce((p, c) => p.concat(c));
}
returns a plain JavaScript array; not an observable. This takes using switchMap out of the question since it violates the first requirement I made up above.
Hopefully that makes sense. :)
We use switchMap when the source observable is a hot observable. In which case you prefer the behaviour that cancel the succeeding observable when source emits.
In your code, you source is a one-off http call which means it will not emit multiple times and the follow up action is not executing observable but to mutate an array. There is no need to use switchMap
how can i subscribe for example three observables, and to emit when one of them emit new value depend on the order they are, like forkJoin but emit but the order of them is important so if ob3 emit first, i want the value of obv1 to be null, also obv2
and only obv3 that was emitted will have value
for example
forkJoin(obs1,obs2,ob3)
.subscribe([ob1v,ob2v,ob3v]=>{
if (obv1v){ 'do somthing'}
if (obv2v){ 'do somthing'}
if (obv3v){ 'do somthing'}
})
thanks
Maybe combineLatest with an initial value would work for you. combineLatest will emit the latest value from each source observable whenever any of its sources emit. However, it doesn't emit for the first time until all sources have emitted at least one value.
We can use startWith to overcome this by providing an initial value for each source.
combineLatest([
obs1.pipe(startWith(null)),
obs2.pipe(startWith(null)),
obs3.pipe(startWith(null)),
])
.pipe(
skip(1) // prevent emitting initial [null, null, null] value
)
.subscribe(([v1, v2, v3]) => {
// do something here
});
You can see the output in this StackBlitz.
It seems that you want to do different thing for every observable. Maybe you shouldn't gorup them? If you want to group them and do different side effect for every one of them you can do something similar to BizzyBob anwer but instead of having if statements in subscribe use tap() operator for every stream. Something like this:
combineLatest([
obs1.pipe(tap(() => 'do somthing'),
obs2.pipe(tap(() => 'do somthing')),
obs3.pipe(tap(() => 'do somthing')),
])
.subscribe(([v1, v2, v3]) => {
});
Good practise is not to use subscribe method but instead set this stream to some property in component and than use async pipe in the template.
I would like to combine two observables in such a way that
I mirror at most 1 value from the source observable (same moment it arrives),
Then ignore its subsequent values until the notifier observable emits;
Then, I allow to mirror at most 1 more value from the source;
After which I again ignore elements until the notifier observable emits
etc.
Source:
a----a----a----a----a----a Source
------------b---------b--- Notifier
a--------------a---------a Desired result
The observables are infinite.
Can you please suggest a recipe to achieve this?
I believe this is a simple use case of the throttle() operator.
// TypeScript
let source$: Observable<any>;
let notifier$: Observable<any>;
let result$: Observable<any>;
result$ = source$.pipe(
throttle(_=>notifier$)
);
Reference Link: https://www.learnrxjs.io/learn-rxjs/operators/filtering/throttle
I was originally searching how to return several actions in a ngrx effect, and found I need to return an array of actions.
Then I noticed that returning a simple array in the switchMap works as fine as returning an observable created from this array.
For example:
timer(1000).pipe(switchMap(val => from([val, val + 1)])).subscribe(val => console.log(val));
timer(1000).pipe(switchMap(val => [val, val + 1])).subscribe(val => console.log(val));
I expect the first to work and think it is the correct syntax.
I don't expect the second to work but it actually does and I would like to understand why.
Thanks,
Because switchMap, among other flattening-operators (mergeMap, exhaustMap, ...), takes an ObservableLike as the return type of its projection function.
An ObservableLike can be Observable, Promise or Array. If you provide an array, it is converted into a stream of its items - basically the same as if you had used from.
I would like to understand why take operator is used in code given below.
private _places = new BehaviorSubject<Place[]>(
// places for initialization
);
get places() {
return this._places.asObservable();
}
addPlace(title: string, description: string, price: number)
{
generatedId: string;
newPlace: Place;
// code to initialize newPlace
return this.http.post<{name: string}>(
'https://ionic-angular-ef2f8.firebaseio.com/offered-places.json',
{...newPlace, id: null})
.pipe(
switchMap(response => {
generatedId = response.name;
return this.places;
}),
take(1),
tap(places => {
newPlace.id = generatedId;
this._places.next(places.concat(newPlace));
})
);
}
post request returns an Observable, and we get a value from it in switchMap operator (note that we do not take a value before calling switchMap). In switchMap we replace the observable with a new observable got from _places, a BehaviourSubject object. After switchMap we use take operator.
Why don't we skip take operator, and use tap straight off? Do we take a value from an observable, because the observable is generated from a subject? Who can explain the use case of take operator in details?
UPDATE
I suspect that the reason I should use take operator after switchMap is that switchMap returns an observable received from an object of type BehaviorSubject which holds emitted values. One can subscribe to such BehaviorSubject object and take the latest emitted value - that's exactly what I did.
httpClient.post() emits one next notification and one complete notification.
However, they're using switchMap to merge another Observable to the chain (this.places). switchMap() won't complete until its source and the inner Observable complete so they're using take(1) to complete the chain after the first emission from this.places which is a BehaviorSubject that doesn't complete until you deliberately call complete() on it.
This is a very similar use-case to using takeUntil() to complete chains. takeUntil() always has to be the last operator in a chain because completing a source Observable to switchMap(), concatMap(), ... doesn't necessarily complete the whole chain. See this for more details https://medium.com/angular-in-depth/rxjs-avoiding-takeuntil-leaks-fb5182d047ef