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$),
map((tuple) => tuple[1]),
take(1)
)
);
// do stuff with current value
}
This will emit a value "Right away" because of withLatestFrom, and then will resolve the promise.
live example
I've an Array of Filters as an Observable and I'd like to add/remove filters from it. Here is the code I have that is currently only adding a Filter the first time the function runs.
The second time nothing happens.
private _filters$ = new BehaviorSubject<Filter[]>([]);
addFilter(added: Filter) {
debugger
// adding to array of filters
this._filters$.pipe(
tap(d => { debugger; }),
first(),
map(filters => ([...filters, added]))
).subscribe(this._filters$);
}
So my question is: why does this happen ? Why does it run only once ? (By the way first() is not the reason).
I know I can make the code work like so:
private _filters$ = new BehaviorSubject<Filter[]>([]);
currentFilters;
init() {
this._filters$.subscribe(f => this.currentFilters = f);
}
addFilter(added: Filter) {
this._filters$.next([...this.currentFilters, added]);
}
Actually, it is because of first. When you run the function the first time it is creating the stream and subscribing to the BehaviorSubject. When it receives the first event it forwards it to BehaviorSubject and then it completes BehaviorSubject. The second time you run it BehaviorSubject is already shutdown so it immediately unsubscribes any new subscriptions to it.
Without knowing too much about your actual goal my suggestion is that instead of putting the BehaviorSubject at the bottom of the pipeline you instead put it at the top.
// You don't actually need the caching behavior yet so just use a `Subject`
private _filters$ = new Subject<Filter>()
// Hook this up to whatever is going to be using these filters
private _pipeline$ = this._filters.pipe(
// Use scan instead mapping back into self
scan((filters, newFilter) => ([...filters, newFilter]), []),
// Store the latest value for new subscribers
shareReplay(1)
);
// Now this method is just pushing into the `Subject` and the pipeline never has to be torn down
addFilter(added: Filter) {
debugger
this._filters$.next(added);
}
I'm looking for some guidance on the correct way to setup a WebSocket connection with RxJS 5. I am connecting to a WebSocket that uses JSON-RPC 2.0. I want to be able to execute a function which sends a request to the WS and returns an Observable of the associated response from the server.
I set up my initial WebSocketSubject like so:
const ws = Rx.Observable.webSocket("<URL>")
From this observable, I have been able to send requests using ws.next(myRequest), and I have been able to see responses coming back through the ws` observable.
I have struggled with creating functions that will filter the ws responses to the correct response and then complete. These seem to complete the source subject, stopping all future ws requests.
My intended output is something like:
function makeRequest(msg) {
// 1. send the message
// 2. return an Observable of the response from the message, and complete
}
I tried the following:
function makeRequest(msg) {
const id = msg.id;
ws.next(msg);
return ws
.filter(f => f.id === id)
.take(1);
}
When I do that however, only the first request will work. Subsequent requests won't work, I believe because I am completing with take(1)?
Any thoughts on the appropriate architecture for this type of situation?
There appears to be either a bug or a deliberate design decision to close the WebSocket on unsubscribe if there are no further subscribers. If you are interested here is the relevant source.
Essentially you need to guarantee that there is always a subscriber otherwise the WebSocket will be closed down. You can do this in two ways.
Route A is the more semantic way, essentially you create a published version of the Observable part of the Subject which you have more fine grained control over.
const ws = Rx.Observable.webSocket("<URL>");
const ws$ = ws.publish();
//When ready to start receiving messages
const totem = ws$.connect();
function makeRequest(msg) {
const { id } = msg;
ws.next(msg);
return ws$.first(f => f.id === id)
}
//When finished
totem.unsubscribe();
Route B is to create a token subscription that simply holds the socket, but depending on the actual life cycle of your application you would do well to attach to some sort of closing event just to make sure it always gets closed down. i.e.
const ws = Rx.Observable.webSocket("<URL>");
const totem = ws.subscribe();
//Later when closing:
totem.unsubscribe();
As you can see both approaches are fairly similar, since they both create a subscription. B's primary disadvantage is that you create an empty subscription which will get pumped all the events only to throw them away. They only advantage of B is that you can refer to the Subject for emission and subscription using the same variable whereas A you must be careful that you are using ws$ for subscription.
If you were really so inclined you could refine Route A using the Subject creation function:
const safeWS = Rx.Subject.create(ws, ws$);
The above would allow you to use the same variable, but you would still be responsible for shutting down ws$ and transitively, the WebSocket, when you are done with it.
I have a class that when instantiated makes some web service calls, pseudo code below:
Rx.Observable.fromPromise(jQuery.getJSON('https://api.github.com/users'))
.flatMap(function () {
return Rx.Observable.fromPromise(jQuery.getJSON('https://api.github.com/users'));
});
The same class is listening for an onclick event.
When this even is triggered, if the original web service calls are complete: do something
If they are not complete, wait for them to complete, before doing something.
I was wondering how to achieve this with the rxjs approach? rather than setting variables and using if statements.
I would refer to this as an Asynchronous Gate.
These are actually pretty easy to do with Rx.
You will need to cache the web service calls observable sequences.
Then in other calls that are predicated on these being complete, you simply flatMap off their results.
As these are from Promises I believe the result is retained for late subscribers, but if not then you just need to replay(1) the sequences.
So in psudeo code
var startUpData = Rx.Observable.fromPromise(jQuery.getJSON('https://api.github.com/users'))
.flatMap(function () {
return Rx.Observable.fromPromise(jQuery.getJSON('https://api.github.com/users'));
});
var events = Rx.Observable....//Your event wired up here.
//When an event
events
.flatMap(function(evt){
//Wait until the startUpData yeilds, but pass on the evt data instead.
return startUpData.map(function(){ return evt;})
//do something here knowing that your event has fired, but the web services have also completed.
.subscribe();
You can see Matt Barrett explain an Async gate in this video at about 51minutes in to this video - https://youtu.be/Tp5mRlHwZ7M?t=51m30s
You may also want to consider the switch operator incase you don't want overlapping events
I believe withLatestFrom or combineLatest will do what you're asking.
Depending on if you wish to only allow the button to be clicked once with the data provided from the service you could use withLatestFrom. If you wish to allow the button to continued to be clicked using the data previously provided by the service you can use combineLatest
const futureEvent$ = Rx.Observable.timer(3000);
const btnClick$ = Rx.Observable
.fromEvent(document.querySelector('button'), 'click');
const futureAndBtnClick$ = futureEvent$.combineLatest(btnClick$);
futureAndBtnClick$.subscribe(x => console.log('click + future stuff happened'));
jsbin example
In my application i am using chain of of web request call for fetching data from the net. Ie from the result of one request i will send other request and so on. But when i am disposing the web request, only the parent request is disposing. The two other request are still running. How i can cancel all these request in Rx
For your subscription to terminate everything, you either cannot break the monad or you need to make sure that you work into the IDisposable model.
To keep the monad (ie. stick with IObservables):
var subscription = initialRequest.GetObservableResponse()
.SelectMany(initialResponse =>
{
// Feel free to use ForkJoin or Zip (intead of Merge) to
// end up with a single value
return secondRequest.GetObservableResponse()
.Merge(thirdRequest.GetObservableResponse());
})
.Subscribe(subsequentResponses => { });
To make use of the IDisposable model:
var subscription = initialRequest.GetObservableResponse()
.SelectMany(initialResponse =>
{
return Observable.CreateWithDisposable(observer =>
{
var secondSubscription = new SerialDisposable();
var thirdSubscription = new SerialDisposable();
secondSubscription.Disposable = secondRequest.GetObservableResponse()
.Subscribe(secondResponse =>
{
// Be careful of race conditions here!
observer.OnNext(value);
observer.OnComplete();
});
thirdSubscription.Disposable = thirdRequest.GetObservableResponse()
.Subscribe(thirdResponse =>
{
// Be careful of race conditions here!
});
return new CompositeDisposable(secondSubscription, thirdSubscription);
});
})
.Subscribe(subsequentResponses => { });
One approah is by using TakeUntil extnsion method as described here. In your case, the event that takes this method as parameter could be some event thrown by the parent request.
If you could show us some code we can face the problem more specifically.
regards,