Let's say I have an interval that each second sends an heartbeat. At each beat i'd like to inspect something on my web page and react accordingly. I'd also like the option to unsubscribe from the inner Observables actions, but keep getting the heartbeat so when i subscribe back, everything will flow as before.
Creating a Subscription from Interval and piping it leaves no option to unsubscribe from the inner action, but only the whole subscription as whole.
Is there a way to return the inner Observable so i can unsubscribe from it while still retaining the heartbeat created from the Interval?
Edit: I've tried to create a class to describe what I'm talking about:
class Monitor {
sub: Subscription | null = null;
start() {
this.sub = this.monitor().subscribe();
}
monitor() {
const dom$ = someSelectorObserver(this.win.document, '#someSelector').pipe(
mergeMap(newElementOrBail => {
if (newElementOrBail) {
return handle(newElementOrBail);
} else {
return bail();
}
}),
tap({
error: error => this.log.error(error),
}),
);
return dom$;
}
handle(ele: HTMLElement) {
// do stuff
}
bail() {
this.sub.unsubscribe();
}
}
So basically my monitor starts with creating the subscription, as long as there's a new element to handle everything is fine, but when a bail signal appears I'd like to unsubscribe while still monitoring the DOM changes for a return of the previous elements.
So the outer subscription is basically the DOM observer and the inner is the mergeMap handle function. Does it make more sense?
You could just put some conditional on your inner observable:
private takeSignal = true
interval(3000).pipe(switchMap(() => takeSignal ? inner$ : NEVER))
Then just flip takeSignal as needed.
But it seems easier to just unsubscribe from the whole thing and resubscribe when needed. Why keep the interval going when you’re not using it?
You can split your logic in two (or more) streams.
Store heartbeat$ in a separate variable and subscribe to multiple times for different reasons.
In this way, you'd be able to split your logic into different streams and control subscriptions individually.
const heartbeat$ = interval(3000);
const inspectWeb = heartbeat$.pipe(
// do stuff
).subscribe()
inspectWeb.unsubscribe()
heartbeat$.pipe(
// do other stuff
).subscribe()
I'm new to RxJS and trying to wrap my brain around how I should be writing my code. I'm trying to write a function that extends an existing http which returns an observable array of data. I'd like to then loop over the array and make an http request on each object and return the new array with the modified data.
Here's what I have so far:
private mapEligibilitiesToBulk(bulkWarranties: Observable<any[]>): Observable<IDevice[]> {
const warranties: IDevice[] = [];
bulkWarranties.subscribe((bulk: any[]) => {
for (let warranty of bulk) {
// Check if another device already has the information
const foundIndex = warranties.findIndex((extended: IDevice) => {
try {
return warranty.device.stockKeepingId.equals(extended.part.partNumber);
} catch (err) {
return false;
}
});
// Fetch the information if not
if (foundIndex > -1) {
warranty.eligibilityOptions = warranties[foundIndex];
} else {
this.getDevices(warranty.device.deviceId.serialNumber).subscribe((devices: IDevice[]) => {
warranty = devices[0];
}); // http request that returns an observable of IDevice
}
warranties.push(warranty);
}
});
return observableOf(warranties);
}
Currently, my code returns an observable array immediately, however, its empty and doesn't react the way I'd like. Any advice or recommended reading would be greatly appreciated!
Without knowing a lot more about your data and what would make sense, it is impossible to give you the exact code you would need. However, I made some assumptions and put together this StackBlitz to show one possible way to approach this. The big assumption here is that the data is groupable and what you are actually trying to achieve is making only a single http call for each unique warranty.device.stockKeepingId.
I offer this code as a starting point for you, in the hopes it gets you a little closer to what you are trying to achieve. From the StackBlitz, here is the relevant method:
public mapEligibilitiesToBulk(bulk: Warranty[]): Observable<IDevice[]> {
return from(bulk).pipe(
tap(warranty => console.log('in tap - warranty is ', warranty)),
groupBy(warranty => warranty.device.stockKeepingId),
mergeMap(group$ => group$.pipe(reduce((acc, cur) => [...acc, cur], []))),
tap(group => console.log('in tap - group is ', group)),
concatMap(group => this.getDevices(group[0].device.deviceId.serialNumber)),
tap(device => console.log('in tap - got this device back from api: ', device)),
toArray()
)
}
A couple of things to note:
Be sure to open up the console to see the results.
I changed the first parameter to an array rather than an observable, assuming you need a complete array to start with. Let me know if you want this to extend an existing observable, that is quite simple to achieve.
I put in some tap()s so you can see what the code does at two of the important points.
In the StackBlitz currently the getDevices() returns the same thing for every call, I did this for simplicity in mocking, not because I believe it would function that way. :)
I have an Angular 2 service:
import {Storage} from './storage';
import {Injectable} from 'angular2/core';
import {Subject} from 'rxjs/Subject';
#Injectable()
export class SessionStorage extends Storage {
private _isLoggedInSource = new Subject<boolean>();
isLoggedIn = this._isLoggedInSource.asObservable();
constructor() {
super('session');
}
setIsLoggedIn(value: boolean) {
this.setItem('_isLoggedIn', value, () => {
this._isLoggedInSource.next(value);
});
}
}
Everything works great. But I have another component which doesn't need to subscribe, it just needs to get the current value of isLoggedIn at a certain point in time. How can I do this?
A Subject or Observable doesn't have a current value. When a value is emitted, it is passed to subscribers and the Observable is done with it.
If you want to have a current value, use BehaviorSubject which is designed for exactly that purpose. BehaviorSubject keeps the last emitted value and emits it immediately to new subscribers.
It also has a method getValue() to get the current value.
The only way you should be getting values "out of" an Observable/Subject is with subscribe!
If you're using getValue() you're doing something imperative in declarative paradigm. It's there as an escape hatch, but 99.9% of the time you should NOT use getValue(). There are a few interesting things that getValue() will do: It will throw an error if the subject has been unsubscribed, it will prevent you from getting a value if the subject is dead because it's errored, etc. But, again, it's there as an escape hatch for rare circumstances.
There are several ways of getting the latest value from a Subject or Observable in a "Rx-y" way:
Using BehaviorSubject: But actually subscribing to it. When you first subscribe to BehaviorSubject it will synchronously send the previous value it received or was initialized with.
Using a ReplaySubject(N): This will cache N values and replay them to new subscribers.
A.withLatestFrom(B): Use this operator to get the most recent value from observable B when observable A emits. Will give you both values in an array [a, b].
A.combineLatest(B): Use this operator to get the most recent values from A and B every time either A or B emits. Will give you both values in an array.
shareReplay(): Makes an Observable multicast through a ReplaySubject, but allows you to retry the observable on error. (Basically it gives you that promise-y caching behavior).
publishReplay(), publishBehavior(initialValue), multicast(subject: BehaviorSubject | ReplaySubject), etc: Other operators that leverage BehaviorSubject and ReplaySubject. Different flavors of the same thing, they basically multicast the source observable by funneling all notifications through a subject. You need to call connect() to subscribe to the source with the subject.
I had similar situation where late subscribers subscribe to the Subject after its value arrived.
I found ReplaySubject which is similar to BehaviorSubject works like a charm in this case.
And here is a link to better explanation: http://reactivex.io/rxjs/manual/overview.html#replaysubject
const observable = of('response')
function hasValue(value: any) {
return value !== null && value !== undefined;
}
function getValue<T>(observable: Observable<T>): Promise<T> {
return observable
.pipe(
filter(hasValue),
first()
)
.toPromise();
}
const result = await getValue(observable)
// Do the logic with the result
// .................
// .................
// .................
You can check the full article on how to implement it from here.
https://www.imkrish.com/blog/development/simple-way-get-value-from-observable
I encountered the same problem in child components where initially it would have to have the current value of the Subject, then subscribe to the Subject to listen to changes. I just maintain the current value in the Service so it is available for components to access, e.g. :
import {Storage} from './storage';
import {Injectable} from 'angular2/core';
import {Subject} from 'rxjs/Subject';
#Injectable()
export class SessionStorage extends Storage {
isLoggedIn: boolean;
private _isLoggedInSource = new Subject<boolean>();
isLoggedIn = this._isLoggedInSource.asObservable();
constructor() {
super('session');
this.currIsLoggedIn = false;
}
setIsLoggedIn(value: boolean) {
this.setItem('_isLoggedIn', value, () => {
this._isLoggedInSource.next(value);
});
this.isLoggedIn = value;
}
}
A component that needs the current value could just then access it from the service, i.e,:
sessionStorage.isLoggedIn
Not sure if this is the right practice :)
A similar looking answer was downvoted. But I think I can justify what I'm suggesting here for limited cases.
While it's true that an observable doesn't have a current value, very often it will have an immediately available value. For example with redux / flux / akita stores you may request data from a central store, based on a number of observables and that value will generally be immediately available.
If this is the case then when you subscribe, the value will come back immediately.
So let's say you had a call to a service, and on completion you want to get the latest value of something from your store, that potentially might not emit:
You might try to do this (and you should as much as possible keep things 'inside pipes'):
serviceCallResponse$.pipe(withLatestFrom(store$.select(x => x.customer)))
.subscribe(([ serviceCallResponse, customer] => {
// we have serviceCallResponse and customer
});
The problem with this is that it will block until the secondary observable emits a value, which potentially could be never.
I found myself recently needing to evaluate an observable only if a value was immediately available, and more importantly I needed to be able to detect if it wasn't. I ended up doing this:
serviceCallResponse$.pipe()
.subscribe(serviceCallResponse => {
// immediately try to subscribe to get the 'available' value
// note: immediately unsubscribe afterward to 'cancel' if needed
let customer = undefined;
// whatever the secondary observable is
const secondary$ = store$.select(x => x.customer);
// subscribe to it, and assign to closure scope
sub = secondary$.pipe(take(1)).subscribe(_customer => customer = _customer);
sub.unsubscribe();
// if there's a delay or customer isn't available the value won't have been set before we get here
if (customer === undefined)
{
// handle, or ignore as needed
return throwError('Customer was not immediately available');
}
});
Note that for all of the above I'm using subscribe to get the value (as #Ben discusses). Not using a .value property, even if I had a BehaviorSubject.
Although it may sound overkill, this is just another "possible" solution to keep Observable type and reduce boilerplate...
You could always create an extension getter to get the current value of an Observable.
To do this you would need to extend the Observable<T> interface in a global.d.ts typings declaration file. Then implement the extension getter in a observable.extension.ts file and finally include both typings and extension file to your application.
You can refer to this StackOverflow Answer to know how to include the extensions into your Angular application.
// global.d.ts
declare module 'rxjs' {
interface Observable<T> {
/**
* _Extension Method_ - Returns current value of an Observable.
* Value is retrieved using _first()_ operator to avoid the need to unsubscribe.
*/
value: Observable<T>;
}
}
// observable.extension.ts
Object.defineProperty(Observable.prototype, 'value', {
get <T>(this: Observable<T>): Observable<T> {
return this.pipe(
filter(value => value !== null && value !== undefined),
first());
},
});
// using the extension getter example
this.myObservable$.value
.subscribe(value => {
// whatever code you need...
});
There are two ways you can achieve this.
BehaviorSubject has a method getValue() which you can get the value in a specific point of time.
You can subscribe directly with the BehaviorSubject and you may pass the subscribed value to a class member, field or property.
I wouldn't recommend both approaches.
In the first approach, it's a convenient method you can get the value anytime, you may refer to this as the current snapshot at that point of time. Problem with this is you can introduce race conditions in your code, you may invoke this method in many different places and in different timing which is hard to debug.
The second approach is what most developers employ when they want a raw value upon subscription, you can track the subscription and when do you exactly unsubscribe to avoid further memory leak, you may use this if you're really desperate to bind it to a variable and there's no other ways to interface it.
I would recommend, looking again at your use cases, where do you use it? For example you want to determine if the user is logged in or not when you call any API, you can combine it other observables:
const data$ = apiRequestCall$().pipe(
// Latest snapshot from BehaviorSubject.
withLatestFrom(isLoggedIn),
// Allow call only if logged in.
filter(([request, loggedIn]) => loggedIn)
// Do something else..
);
With this, you may use it directly to the UI by piping data$ | async in case of angular.
A subscription can be created, then after taking the first emitted item, destroyed. In the example below, pipe() is a function that uses an Observable as its input and returns another Observable as its output, while not modifying the first observable.
Sample created with Angular 8.1.0 packages "rxjs": "6.5.3", "rxjs-observable": "0.0.7"
ngOnInit() {
...
// If loading with previously saved value
if (this.controlValue) {
// Take says once you have 1, then close the subscription
this.selectList.pipe(take(1)).subscribe(x => {
let opt = x.find(y => y.value === this.controlValue);
this.updateValue(opt);
});
}
}
You could store the last emitted value separately from the Observable. Then read it when needed.
let lastValue: number;
const subscription = new Service().start();
subscription
.subscribe((data) => {
lastValue = data;
}
);
The best way to do this is using Behaviur Subject, here is an example:
var sub = new rxjs.BehaviorSubject([0, 1])
sub.next([2, 3])
setTimeout(() => {sub.next([4, 5])}, 1500)
sub.subscribe(a => console.log(a)) //2, 3 (current value) -> wait 2 sec -> 4, 5
Another approach, If you want / can to use async await (has to be inside of an async functions) you can do this with modern Rxjs:
async myFunction () {
const currentValue = await firstValueFrom(
of(0).pipe(
withLatestFrom(this.yourObservable$),
map((tuple) => tuple[1]),
take(1)
)
);
// do stuff with current value
}
This will emit a value "Right away" because of withLatestFrom, and then will resolve the promise.
I have an SPA that is loading some global/shared data (let's call this APP_LOAD_OK) and page-specific data (DASHBOARD_LOAD_OK) from the server. I want to show a loading animation until both APP_LOAD_OK and DASHBOARD_LOAD_OK are dispatched.
Now I have a problem with expressing this in RxJS. What I need is to trigger an action after each DASHBOARD_LOAD_OK, as long as there had been at least one APP_LOAD_OK. Something like this:
action$
.ofType(DASHBOARD_LOAD_OK)
.waitUntil(action$.ofType(APP_LOAD_OK).first())
.mapTo(...)
Does anybody know, how I can express it in valid RxJS?
You can use withLatestFrom since it will wait until both sources emit at least once before emitting. If you use the DASHBOARD_LOAD_OK as the primary source:
action$.ofType(DASHBOARD_LOAD_OK)
.withLatestFrom(action$.ofType(APP_LOAD_OK) /*Optionally*/.take(1))
.mapTo(/*...*/);
This allows you to keep emitting in the case that DASHBOARD_LOAD_OK fires more than once.
I wanted to avoid implementing a new operator, because I thought my RxJS knowledge was not good enough for that, but it turned out to be easier than I thought. I am keeping this open in case somebody has a nicer solution. Below you can find the code.
Observable.prototype.waitUntil = function(trigger) {
const source = this;
let buffer = [];
let completed = false;
return Observable.create(observer => {
trigger.subscribe(
undefined,
undefined,
() => {
buffer.forEach(data => observer.next(data));
buffer = undefined;
completed = true;
});
source.subscribe(
data => {
if (completed) {
observer.next(data);
} else {
buffer.push(data);
}
},
observer.error.bind(observer),
observer.complete.bind(observer)
);
});
};
If you want to receive every DASHBOARD_LOAD_OK after the first APP_LOAD_OK You can simply use skipUntil:
action$ .ofType(DASHBOARD_LOAD_OK)
.skipUntil(action$.ofType(APP_LOAD_OK).Take(1))
.mapTo(...)
This would only start emitting DASHBOARD_LOAD_OK actions after the first APP_LOAD_OK, all actions before are ignored.
I want to listen to an event stream, then conditionally stop listening based on the received event.
In short, I'd like to use:
var subscription = stream.listen((event) {
if (f(event)) {
doStuff();
subscription.cancel();
} else {
doOtherStuff();
}
});
This obviously doesn't work; subscription doesn't exist until the listener has been created, resulting in an error.
How do I do this?
Just declare the subscription before subscribing to the stream:
var subscription;
subscription = stream.listen((event) {
if (f(event)) {
doStuff();
subscription.cancel();
} else {
doOtherStuff();
}
});
Divide and conquer.
First, let's consider the if (f(event)) part. That takes only the first item that matches the predicate and performs an operation. There are two ways of doing this:
stream.where((event) => f(event)).take(1).listen((event) => doStuff());
That's the generic way of doing things, and it means we use the Stream interface all through. Another way of doing it, which might avoid a few addition and comparison operations, requires switching to the Future interface:
stream.firstWhere((event) => f(event)).then((event) => doStuff());
That only handles the first part of the condition. What about the second part? We want to grab everything until the condition holds true, so we'll use takeWhile:
stream.takeWhile((event) => !f(event)).listen((event) => doOtherStuff());
And Bob's your uncle.
If the code is essentially the same for both listeners, you can separate it out into another function, of course.