i'm kinda new to rxjs and can't get my head around this problem:
I have two streams:
one with incoming objects
---a----b----c----d----->
one with the selected object from a list
-------------------c---->
From the incoming objects stream make a stream of the list of objects (with scan operator)
incoming: ----a--------b-------c----------d----------------\>
list: -------[a]----[a,b]----[a,b,c]----[a,b,c,d]---------\>
When a list object is selected (n), start a new stream
the first value of the new stream is the last value of the list sliced ( list.slice(n))
incoming: ----a--------b-------c----------d--------------------e-------->
list: -------[a]----[a,b]----[a,b,c]----[a,b,c,d]--------->
selected object: ---------------------------------c------->
new stream of list: ------[c,d]-----[c,d,e]--->
i can't get the last value of the list stream when the object is selected,,,
made a marble diagram for better understanding,
selectedObject$ = new BehaviorSubject(0);
incomingObjects$ = new Subject();
list$ = incomingObjects$.pipe(
scan((acc, val) => {
acc.push(val);
return acc;
}, [])
)
newList$ = selectedObject$.pipe(
withLastFrom(list$),
switchMap(([index,list])=> incomingObjects$.pipe(
scan((acc, val) => {
acc.push(val);
return acc;
}, list.slice(index))
))
)
A common pattern I use along with the scan operator is passing reducer functions instead of values to scan so that the current value can be used in the update operation. In this case you can link the two observables with a merge operator and map their values to functions that are appropriate - either adding to a list, or slicing the list after a selection.
// these are just timers for demonstration, any observable should be fine.
const incoming$ = timer(1000, 1000).pipe(map(x => String.fromCharCode(x + 65)), take(10));
const selected$ = timer(3000, 3000).pipe(map(x => String.fromCharCode(x * 2 + 66)), take(2));
merge(
incoming$.pipe(map(x => (s) => [...s, x])), // append to list
selected$.pipe(map(x => (s) => { // slice list starting from selection
const index = s.indexOf(x);
return (index !== -1) ? s.slice(index) : s;
}))
).pipe(
scan((list, reducer) => reducer(list), []) // run reducer
).subscribe(x => console.log(x)); // display list state as demonstration.
If I understand the problem right, you could follow the following approach.
The key point is to recognize that the list Observable (i.e. the Observable obtained with the use of scan) should be an hot Observable, i.e. an Observable that notifies independent on whether or not it is subscribed. The reason is that each new stream you want to create should have always the same source Observable as its upstream.
Then, as you already hint, the act of selecting a value should be modeled with a BehaviorSubject.
As soon as the select BehaviorSubject notifies a value selected, the previous stream has to complete and a new one has to be subscribed. This is the job of switchMap.
The rest is to slice the arrays of numbers in the right way.
This is the complete code of this approach
const selectedObject$ = new BehaviorSubject(1);
const incomingObjects$ = interval(1000).pipe(take(10));
const incomingObjectsHot$ = new ReplaySubject<number[]>(1);
incomingObjects$
.pipe(
scan((acc, val) => {
acc.push(val);
return acc;
}, [])
)
.subscribe(incomingObjectsHot$);
selectedObject$
.pipe(
switchMap((selected) =>
incomingObjectsHot$.pipe(
map((nums) => {
const selIndex = nums.indexOf(selected);
if (selIndex > 0) {
return nums.slice(selIndex);
}
})
)
),
filter(v => !!v)
)
.subscribe(console.log);
An example can be seen in this stackblitz.
I have a BehaviorSubject which emits JavaScript objects periodically. I want to construct another observable which will emit both previous and current values of the underlying observable in order to compare two objects and determine the delta.
The pairwise() or bufferCount(2, 1) operators are looking like a good fit, but they start emitting only after buffer is filled, but I require this observable to start emitting from the first event of the underlying observable.
subject.someBufferingOperator()
.subscribe([previousValue, currentValue] => {
/** Do something */
})
;
On first emission the previousValue could be just null.
Is there some built-in operators that I can use to achieve the desired result?
Actually, it was as easy as pairing pairwise() with startWith() operators:
subject
.startWith(null) // emitting first empty value to fill-in the buffer
.pairwise()
.subscribe([previousValue, currentValue] => {
if (null === previousValue) {
console.log('Probably first emission...');
}
})
;
Here's a simple operator:
function withPreviousItem<T>(): OperatorFunction<
T,
{
previous?: T;
current: T;
}
> {
return pipe(
startWith(undefined),
pairwise(),
map(([previous, current]) => ({
previous,
current: current!
}))
);
}
The nice thing about this is that the result has meaningful property names and correct types:
previous is T | undefined
current is T (not T | null)
Stackblitz example
Here's the snippet for rxjs 6+
subject
.pipe(
startWith(undefined),
pairwise()
)
.subscribe(([previousValue, currentValue]) => {
/** Do something */
});
The value in startWith() should be undefined because there is no value. Typically null is defined as "we have a value and this value is empty".
scan (RX equivalent of a reduce) is an option here:
subject
.scan((accumulator, currentValue) => {
const previousValue = ...accumulator.slice(-1);
return [previousValue, currentValue];
}, [null]) // emitting first empty value to fill-in the buffer
.subscribe(([previousValue, currentValue]) => {
// ...
});
This can be extended to a more general case when you want to look at more than two items.
I'm making use of the withLatestFrom operator in RxJS in the normal way:
var combined = source1.withLatestFrom(source2, source3);
...to actively collect the most recent emission from source2 and source3 and to emit all three value only when source1 emits.
But I cannot guarantee that source2 or source3 will have produced values before source1 produces a value. Instead I need to wait until all three sources produce at least one value each before letting withLatestFrom do its thing.
The contract needs to be: if source1 emits then combined will always eventually emit when the other sources finally produce. If source1 emits multiple times while waiting for the other sources we can use the latest value and discard the previous values. Edit: as a marble diagram:
--1------------2---- (source)
----a-----b--------- (other1)
------x-----y------- (other2)
------1ax------2by--
--1------------2---- (source)
------a---b--------- (other1)
--x---------y------- (other2)
------1ax------2by--
------1--------2---- (source)
----a-----b--------- (other1)
--x---------y------- (other2)
------1ax------2by--
I can make a custom operator for this, but I want to make sure I'm not missing an obvious way to do this using the vanilla operators. It feels almost like I want combineLatest for the initial emit and then to switch to withLatestFrom from then on but I haven't been able to figure out how to do that.
Edit: Full code example from final solution:
var Dispatcher = new Rx.Subject();
var source1 = Dispatcher.filter(x => x === 'foo');
var source2 = Dispatcher.filter(x => x === 'bar');
var source3 = Dispatcher.filter(x => x === 'baz');
var combined = source1.publish(function(s1) {
return source2.publish(function(s2) {
return source3.publish(function(s3) {
var cL = s1.combineLatest(s2, s3).take(1).do(() => console.log('cL'));
var wLF = s1.skip(1).withLatestFrom(s2, s3).do(() => console.log('wLF'));
return Rx.Observable.merge(cL, wLF);
});
});
});
var sub1 = combined.subscribe(x => console.log('x', x));
// These can arrive in any order
// and we can get multiple values from any one.
Dispatcher.onNext('foo');
Dispatcher.onNext('bar');
Dispatcher.onNext('foo');
Dispatcher.onNext('baz');
// combineLatest triggers once we have all values.
// cL
// x ["foo", "bar", "baz"]
// withLatestFrom takes over from there.
Dispatcher.onNext('foo');
Dispatcher.onNext('bar');
Dispatcher.onNext('foo');
// wLF
// x ["foo", "bar", "baz"]
// wLF
// x ["foo", "bar", "baz"]
I think the answer is more or less as you described, let the first value be a combineLatest, then switch to withLatestFrom. My JS is hazy, but I think it would look something like this:
var selector = function(x,y,z) {};
var combined = Rx.Observable.concat(
source1.combineLatest(source2, source3, selector).take(1),
source1.withLatestFrom(source2, source3, selector)
);
You should probably use publish to avoid multiple subscriptions, so that would look like this:
var combined = source1.publish(function(s1)
{
return source2.publish(function(s2)
{
return source3.publish(function(s3)
{
return Rx.Observable.concat(
s1.combineLatest(s2, s3, selector).take(1),
s1.withLatestFrom(s2, s3, selector)
);
});
});
});
or using arrow functions...
var combined = source1.publish(s1 => source2.publish(s2 => source3.publish(s3 =>
Rx.Observable.concat(
s1.combineLatest(s2, s3, selector).take(1),
s1.withLatestFrom(s2, s3, selector)
)
)));
EDIT:
I see the problem with concat, the withLatestFrom isn't getting the values. I think the following would work:
var combined = source1.publish(s1 => source2.publish(s2 => source3.publish(s3 =>
Rx.Observable.merge(
s1.combineLatest(s2, s3, selector).take(1),
s1.skip(1).withLatestFrom(s2, s3, selector)
)
)));
...so take one value using combineLatest, then get the rest using withLatestFrom.
I wasn't quite satisfied with the accepted answer, so I ended up finding another solution. Many ways to skin a cat!
My use-case involves just two streams - a "requests" stream and a "tokens" stream. I want requests to fire as soon as they are received, using the whatever the latest token is. If there is no token yet, then it should wait until the first token appears, and then fire off all the pending requests.
I wasn't quite satisfied with the accepted answer, so I ended up finding another solution. Essentially I split the request stream into two parts - before and after first token arrives. I buffer the first part, and then re-release everything in one go once I know that the token stream is non-empty.
const first = token$.first()
Rx.Observable.merge(
request$.buffer(first).mergeAll(),
request$.skipUntil(first)
)
.withLatestFrom(token$)
See it live here: https://rxviz.com/v/VOK2GEoX
For RxJs 7:
const first = token$.first()
merge(
request$.pipe(
buffer(first),
mergeAll()
),
request$.pipe(
skipUntil(first)
)
).pipe(
withLatestFrom(token$)
)
I had similar requirements but for just 2 observables.
I ended up using switchMap+first:
observable1
.switchMap(() => observable2.first(), (a, b) => [a, b])
.subscribe(([a, b]) => {...}));
So it:
waits until both observables emit some value
pulls the value from second observable only if the first one has changed (unlike combineLatest)
doesn't hang subscribed on second observable (because of .first())
In my case, second observable is a ReplaySubject. I'm not sure if it will work with other observable types.
I think that:
flatMap would probably work too
it might be possible to extend this approach to handle more than 2 observables
I was surprised that withLatestFrom will not wait on second observable.
In my mind, the most elegant way to achieve the different behavior of an existing RxJS operator is to wrap it into a custom operator. So that from the outside it looks just like any regular operator and doesn't require you to restructure your code each time you need this behavior.
Here is how you can create your own operator which behaves just like withLatestFrom, except that at the very beginning it will emit as soon as the first value of the target observable is emitted (unlike standard withLatestFrom, which will ignore the first emission of the source if the target hasn't yet emitted once). Let's call it delayedWithLatestFrom.
Note that it's written in TypeScript, but you can easily transform it to plain JS. Also, it's a simple version that supports only one target observable and no selector function - you can extend it as needed from here.
export function delayedWithLatestFrom<T, N>(
target$: Observable<N>
): OperatorFunction<T, [T, N]> {
// special value to avoid accidental match with values that could originate from target$
const uniqueSymbol = Symbol('withLatestFromIgnore');
return pipe(
// emit as soon target observable emits the first value
combineLatestWith<T, [N]>(target$.pipe(first())),
// skip the first emission because it's handled above, and then continue like a normal `withLatestFrom` operator
withLatestFrom(target$.pipe(skip(1), startWith(uniqueSymbol))),
map(([[rest, combineLatestValue], withLatestValue]) => {
// take combineLatestValue for the first time, and then always take withLatestValue
const appendedValue =
withLatestValue === uniqueSymbol ? combineLatestValue : withLatestValue;
return [rest, appendedValue];
})
);
}
// SAMPLE USAGE
source$.pipe(
delayedWithLatestFrom(target$)
).subscribe(console.log);
So if you compare it with the original marble diagram for withLatestFrom, it will differ only in one fact: while withLatestFrom ignores the first emissions and produces b1 as the first value, the delayedWithlatestFrom operator will emit one more value a1 at the beginning, as soon as the second observable emits 1.
a) Standard withLatestFrom:
b) Custom delayedWithLatestFrom:
Use combineLatest and filter to remove tuples before first full set is found then set a variable to stop filtering. The variable can be within the scope of a wrapping defer to do things properly (support resubscription). Here it is in java (but the same operators exist in RxJs):
Observable.defer(
boolean emittedOne = false;
return Observable.combineLatest(s1, s2, s3, selector)
.filter(x -> {
if (emittedOne)
return true;
else {
if (hasAll(x)) {
emittedOne = true;
return true;
} else
return false;
}
});
)
I wanted a version where tokens are fetched regularly - and where I want to retry the main data post on (network) failure. I found shareReplay to be the key. The first mergeWith creates a "muted" stream, which causes the first token to be fetched immediately, not when the first action arrives. In the unlikely event that the first token will still not be available in time, the logic also has a startWith with an invalid value. This causes the retry logic to pause and try again. (Some/map is just a Maybe-monad):
Some(fetchToken$.pipe(shareReplay({refCount: false, bufferSize: 1})))
.map(fetchToken$ =>
actions$.pipe(
// This line is just for starting the loadToken loop immediately, not waiting until first write arrives.
mergeWith(fetchToken$.pipe(map(() => true), catchError(() => of(false)), tap(x => loggers.info(`New token received, success: ${x}`)), mergeMap(() => of()))),
concatMap(action =>
of(action).pipe(
withLatestFrom(fetchToken$.pipe(startWith(""))),
mergeMap(([x, token]) => (!token ? throwError(() => "Token not ready") : of([x, token] as const))),
mergeMap(([{sessionId, visitId, events, eventIds}, token]) => writer(sessionId, visitId, events, token).pipe(map(() => <ISessionEventIdPair>{sessionId, eventIds}))),
retryWhen(errors =>
errors.pipe(
tap(err => loggers.warn(`Error writing data to WG; ${err?.message || err}`)),
mergeMap((_error: any, attemptIdx) => (attemptIdx >= retryPolicy.retryCount ? throwError(() => Error("It's enough now, already")) : of(attemptIdx))), // error?.response?.status (int, response code) error.code === "ENOTFOUND" / isAxiosError: true / response === undefined
delayWhen(attempt => timer(attempt < 2 ? retryPolicy.shortRetry : retryPolicy.longRetry, scheduler))
)
)
)
),
)
)
Thanks to everyone on this question-page for good inputs.
Based on the answer from #cjol
Here's a RxJs 7 implementation of a waitFor operator that will buffer the source stream until all input observables have emitted values, then emit all buffered events on the source stream. Any subsequent events on the source stream are emitted immediately.
// Copied from the definition of withLatestFrom() operator.
export function waitFor<T, O extends unknown[]>(
inputs: [...ObservableInputTuple<O>]
): OperatorFunction<T, [T, ...O]>;
/**
* Buffers the source until every observable in "from" have emitted a value. Then
* emit all buffered source values with the latest values of the "from" array.
* Any source events are emitted immediately after that.
* #param from Array of observables to wait for.
* #returns Observable that emits an array that concatenates the source and the observables to wait.
*/
export function waitFor(
from: Observable<unknown>[]
): (source$: Observable<unknown>) => Observable<unknown> {
const combined$ = combineLatest(from);
// This served as a conditional that switched on and off the streams that
// wait for the the other observables, or emits the source right away because
// the other observables have emitted.
const firstCombined$ = combined$.pipe(first());
return function (source$: Observable<unknown>): Observable<unknown> {
return merge(
// This stream will buffer the source until the other observables have all emitted.
source$.pipe(
takeUntil(firstCombined$), // without this it continues to buffer new values forever
buffer(firstCombined$),
mergeAll()
),
// This stream emits the source straight away and will take over when the other
// observables have emitted.
source$.pipe(skipUntil(firstCombined$))
).pipe(
withLatestFrom(combined$),
// Flatten it to behave like withLatestFrom() operator.
map(([source, combined]) => [source, ...combined])
);
};
}
All of the above solutions are not really on the point, therefore I made my own. Hope it helps someone out.
import {
combineLatest,
take,
map,
ObservableInputTuple,
OperatorFunction,
pipe,
switchMap
} from 'rxjs';
/**
* ### Description
* Works similar to {#link withLatestFrom} with the main difference that it awaits the observables.
* When all observables can emit at least one value, then takes the latest state of all observables and proceeds execution of the pipe.
* Will execute this pipe only once and will only retrigger pipe execution if source observable emits a new value.
*
* ### Example
* ```ts
* import { BehaviorSubject } from 'rxjs';
* import { awaitLatestFrom } from './await-latest-from.ts';
*
* const myNumber$ = new BehaviorSubject<number>(1);
* const myString$ = new BehaviorSubject<string>("Some text.");
* const myBoolean$ = new BehaviorSubject<boolean>(true);
*
* myNumber$.pipe(
* awaitLatestFrom([myString$, myBoolean$])
* ).subscribe(([myNumber, myString, myBoolean]) => {});
* ```
* ### Additional
* #param observables - the observables of which the latest value will be taken when all of them have a value.
* #returns a tuple which contains the source value as well as the values of the observables which are passed as input.
*/
export function awaitLatestFrom<T, O extends unknown[]>(
observables: [...ObservableInputTuple<O>]
): OperatorFunction<T, [T, ...O]> {
return pipe(
switchMap((sourceValue) =>
combineLatest(observables).pipe(
take(1),
map((values) => [sourceValue, ...values] as unknown as [T, ...O])
)
)
);
}
Actually withLatestFrom already
waits for every source
emits only when source1 emits
remembers only the last source1-message while the other sources are yet to start
// when source 1 emits the others have emitted already
var source1 = Rx.Observable.interval(500).take(7)
var source2 = Rx.Observable.interval(100, 300).take(10)
var source3 = Rx.Observable.interval(200).take(10)
var selector = (a,b,c) => [a,b,c]
source1
.withLatestFrom(source2, source3, selector)
.subscribe()
vs
// source1 emits first, withLatestFrom discards 1 value from source1
var source1 = Rx.Observable.interval(500).take(7)
var source2 = Rx.Observable.interval(1000, 300).take(10)
var source3 = Rx.Observable.interval(2000).take(10)
var selector = (a,b,c) => [a,b,c]
source1
.withLatestFrom(source2, source3, selector)
.subscribe()
Suppose we have a function getIds() which takes an array of some ids
like this:
getIds([4, 1, 32]);
This function will delay HTTP call for 100ms. But during 100ms if this
same function is called again:
getIds([1, 8, 5]);
It will reset the 100ms timer and keep merging the passed ids. It will
send HTTP request only if it's not called by anyone for more than 100ms.
I am new to RxJS and here's my attempt to solve this problem but I have
a feeling that there could be better solution for this problem.
https://jsfiddle.net/iFadey/v3v3L0yd/2/
function getIds(ids) {
let observable = getIds._observable,
subject = getIds._subject;
if (!observable) {
subject = getIds._subject = new Rx.ReplaySubject();
observable = getIds._observable = subject
.distinct()
.reduce((arr, id) => {
arr.push(id);
return arr;
}, [])
// Some HTTP GET request will go here
// whose results may get flatMapped here
.publish()
.refCount()
;
}
ids.forEach((id) => {
console.log(id);
subject.next(id);
});
clearTimeout(getIds._timer);
getIds._timer = setTimeout(() => {
getIds._observable = null;
getIds._subject = null;
subject.complete();
}, 100);
return observable;
}
getIds([1, 2, 3])
.subscribe((ids) => {
console.log(ids);
});
getIds([3, 4, 5])
.subscribe((ids) => {
console.log(ids);
});
edit:
I am looking for an operator which behaves like debounce but without dropping previous values. Instead it must queue them.
I am not certain to have captured exactly which of the following you are looking for, so I will simply describe both. There are two "time based patterns" that are most often suited for this type of problem in my experience:
debounce
rxmarbles url: http://rxmarbles.com/#debounce ; github doc
As it says in its documentation, it
Emits an item from the source Observable after a particular timespan
has passed without the Observable omitting any other items.
throttle
rxmarbles url: none yet ; github doc
Returns an Observable that emits only the first item emitted by the
source Observable during sequential time windows of a specified
duration.
Basically, if you would like to wait until the inputs have quieted for a certain period of time before taking action, you want to debounce. If you do not want to wait at all, but do not wish to make more than 1 query within a specific amount of time, you will want to throttle.
Hope it makes sense.