In our Single Page Application we've developed a centralized store class that uses an RxJS behavior subject to handle the state of our application and all its mutation. Several components in our application are subscribing to our store's behavior subject in order to receive any update to current application state. This state is then bound to UI so that whenever state changes, UI reflect those changes. Whenever a component wants to change a part of the state, we call a function exposed by our store that does the required work and updates the state calling next on the behavior subject. So far nothing special. (We're using Aurelia as a framework which performs 2 way binding)
The issue we are facing is that as soon as a component changes it's local state variable it receives from the store, other components gets updated even if next() wasn't called on the subejct itself.
We also tried to subscribe on an observable version of the subject since observable are supposed to send a different copy of the data to all subscriber but looks like it's not the case.
Looks like all subject subscriber are receiving a reference of the object stored in the behavior subject.
import { BehaviorSubject, of } from 'rxjs';
const initialState = {
data: {
id: 1,
description: 'initial'
}
}
const subject = new BehaviorSubject(initialState);
const observable = subject.asObservable();
let stateFromSubject; //Result after subscription to subject
let stateFromObservable; //Result after subscription to observable
subject.subscribe((val) => {
console.log(`**Received ${val.data.id} from subject`);
stateFromSubject = val;
});
observable.subscribe((val) => {
console.log(`**Received ${val.data.id} from observable`);
stateFromObservable = val;
});
stateFromSubject.data.id = 2;
// Both stateFromObservable and subject.getValue() now have a id of 2.
// next() wasn't called on the subject but its state got changed anyway
stateFromObservable.data.id = 3;
// Since observable aren't bi-directional I thought this would be a possible solution but same applies and all variable now shows 3
I've made a stackblitz with the code above.
https://stackblitz.com/edit/rxjs-bhkd5n
The only workaround we have so far is to clone the sate in some of our subscriber where we support edition through binding like follow:
observable.subscribe((val) => {
stateFromObservable = JSON.parse(JSON.stringify(val));
});
But this feels more like a hack than a real solution. There must be a better way...
Yes, all subscribers receive the same instance of the object in the behavior subject, that is how behavior subjects work. If you are going to mutate the objects you need to clone them.
I use this function to clone my objects I am going to bind to Angular forms
const clone = obj =>
Array.isArray(obj)
? obj.map(item => clone(item))
: obj instanceof Date
? new Date(obj.getTime())
: obj && typeof obj === 'object'
? Object.getOwnPropertyNames(obj).reduce((o, prop) => {
o[prop] = clone(obj[prop]);
return o;
}, {})
: obj;
So if you have an observable data$ you can create an observable clone$ where subscribers to that observable get a clone that can be mutated without affecting other components.
clone$ = data$.pipe(map(data => clone(data)));
So components that are just displaying data can subscribe to data$ for efficiency and ones that will mutate the data can subscribe to clone$.
Have a read on my library for Angular https://github.com/adriandavidbrand/ngx-rxcache and my article on it https://medium.com/#adrianbrand/angular-state-management-with-rxcache-468a865fc3fb it goes into the need to clone objects so we don't mutate data we bind to forms.
It sounds like the goals of your store are the same as my Angular state management library. It might give you some ideas.
I am not familar with Aurelia or if it has pipes but that clone function is available in the store with exposing my data with a clone$ observable and in the templates with a clone pipe that can be used like
data$ | clone as data
The important part is knowing when to clone and not to clone. You only need to clone if the data is going to be mutated. It would be really inefficient to clone an array of data that is only going to be displayed in a grid.
The only workaround we have so far is to clone the state in some of our subscriber where we support edition through binding like follow:
I don't think I can answer that without rewriting your store.
const initialState = {
data: {
id: 1,
description: 'initial'
}
}
That state object has deeply structured data. Everytime you need to mutate the state the object needs to be reconstructed.
Alternatively,
const initialState = {
1: {id: 1, description: 'initial'},
2: {id: 2, description: 'initial'},
3: {id: 3, description: 'initial'},
_index: [1, 2, 3]
};
That is about as deep of a state object that I would create. Use a key/value pair to map between IDs and the object values. You can now write selectors easily.
function getById(id: number): Observable<any> {
return subject.pipe(
map(state => state[id]),
distinctUntilChanged()
);
}
function getIds(): Observable<number[]> {
return subject.pipe(
map(state => state._index),
distinctUntilChanged()
);
}
When you want change a data object. You have to reconstruct the state and also set the data.
function append(data: Object) {
const state = subject.value;
subject.next({...state, [data.id]: Object.freeze(data), _index: [...state._index, data.id]});
}
function remove(id: number) {
const state = {...subject.value};
delete state[id];
subject.next({...state, _index: state._index.filter(x => x !== id)});
}
Once you have that done. You should freeze downstream consumers of your state object.
const subject = new BehaviorSubject(initialState);
function getStore(): Observable<any> {
return subject.pipe(
map(obj => Object.freeze(obj))
);
}
function getById(id: number): Observable<any> {
return getStore().pipe(
map(state => state[id]),
distinctUntilChanged()
);
}
function getIds(): Observable<number[]> {
return getStore().pipe(
map(state => state._index),
distinctUntilChanged()
);
}
Later when you do something like this:
stateFromSubject.data.id = 2;
You'll get a run-time error.
FYI: The above is written in TypeScript
The big logical issue with your example is that the object forwarded by the subject is actually a single object reference. RxJS doesn't do anything out of the box to create clones for you, and that is fine otherwise it would result in unnecessary operations by default if they aren't needed.
So while you can clone the value received by the subscribers, you're still not save for access of BehaviorSubject.getValue(), which would return the original reference. Besides that having same refs for parts of your state is actually beneficial in lots of ways as e.g arrays can be re-used for multiple displaying components vs having to rebuild them from scratch.
What you want to do instead is to leverage a single-source-of-truth pattern, similar to Redux, where instead of making sure that subscribers get clones, you're treating your state as immutable object. That means every modification results in a new state. That further means you should restrict modifications to actions, (actions + reducers in Redux) which construct a new state of the current plus the necessary changes and return the new copy.
Now all of that might sound like a lot of work but you should take a look at the official Aurelia Store Plugin, which is sharing pretty much the same concept as you have plus making sure that best ideas of Redux are brought over to the world of Aurelia.
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$.next(added);
}
I have the following requirement.
I have An Angular service with an BehaviorSubject.
A http request is done and when this is done the BehaviorSubject.next method is invoked with the value.
This value can change during the lifecycle of the single page.
Different subscribers are registered to it and get invoked whenever this changes.
The problem is that while the http request is pending the BehaviorSubject already contains a default value and subscribers are already immediately getting this value.
What I would want is that subscribers have to wait till the http request is done (deferred) and get the value when the http request is done and sets the value.
So what I need is some kind of deferred Behavior subject mechanism.
How would i implement this using rxjs?
Another requirement is that if I subscribe to the behaviorsubject in a method we want the subcriber to get the first non default value and that the subscription ends. We don't want local subscriptions in functions to be re-executed.
Use a filter on your behavior subject so your subscribers won't get the first default emitted value:
mySubject$: BehaviorSubject<any> = new BehaviorSubject<any>(null);
httpResponse$: Observable<any> = this.mySubject$.pipe(
filter(response => response)
map(response => {
const modifyResponse = response;
// modify response
return modifyResponse;
}),
take(1)
);
this.httpResponse$.subscribe(response => console.log(response));
this.myHttpCall().subscribe(response => this.mySubject$.next(response));
You can of course wrap the httpResponse$ observable in a method if you need to.
I think the fact that you want to defer the emitted default value, straight away brings into question why you want to use a BehaviorSubject. Let's remember: the primary reason to use a BehaviorSubject (instead of a Subject, or a plain Observable), is to emit a value immediately to any subscriber.
If you need an Observable type where you need control of the producer (via .next([value])) and/or you want multicasting of subscription out of the box, then Subject is appropriate.
If an additional requirement on top of this is that subscribers need a value immediately then you need to consider BehaviorSubject.
If you didn't say you need to update the value from other non-http events/sources, then I would've suggested using a shareReplay(1) pattern. Nevertheless...
private cookieData$: Subject<RelevantDataType> = new
Subject<RelevantDataType>(null);
// Function for triggering http request to update
// internal Subject.
// Consumers of the Subject can potentially invoke this
// themselves if they receive 'null' or no value on subscribe to subject
public loadCookieData(): Observable<RelevantDataType> {
this.http.get('http://someurl.com/api/endpoint')
.map(mapDataToRelevantDataType());
}
// Function for dealing with updating the service's
// internal cookieData$ Subject from external
// consumer which need to update this value
// via non-http events
public setCookieData(data: any): void {
const newCookieValue = this.mapToRelevantDataType(data); // <-- If necessary
this.cookieData$.next(newCookieValue); // <-- updates val for all subscribers
}
get cookieData(): Observable<RelevantDataType> {
return this.cookieData$.asObservable();
}
The solution is based on OPs comments etc.
- deals with subscribing to subject type.
- deals with external subscribers not being able to 'next' a new value directly
- deals with external producers being able to set a new value on the Subject type
- deals with not giving a default value whilst http request is pending
So Im struggling a bit with my understanding of RxJs and observables.
Can it be explained why 'fromEvent' always reacts to a new event no matter how much time has passed without a new value...yet pushing new values into an existing array that is being observed using 'from' doesn't work in the same way if they are both observables at this point and should react to async events why do we need to use 'Subject' for arrays?
I have seen we need to use a 'Subject' for an array ...but why? I would like to understand the reason/mechanism
This is a little bit like asking why and Array.forEach doesn't also iterate over items I add to an array via .push while addEventListener() continues listening for events even far in the future.
The behaviors of the two are different because the underlying data structures are different.
#fromArray
For an Array the implementation is essentially:
function fromArray(array) {
return new Observable(observer => {
try {
// Iterate through each item in the array and emit it to the Observer
array.forEach(item => observer.next(item));
// Array iteration is synchronous, which when we get here we are done iterating
observer.complete();
} catch (e) { observer.error(e) }
})
}
Where the function passed to the observable gets run each time a subscriber subscribes to the Observable. Because Arrays don't have a mechanism to detect changes to them there is no way to listen for additional updates to a native array (note: I am ignoring monkey-patching or creating some sort of substitute array data type that does support such things for simplicity).
#fromEvent
The fromEvent on the other hand would look more like:
function fromEvent(node, eventName, selector) {
// Convert the passed in event or just use the identity
let transform = selector || x => x;
// Construct an Observable using the constructor
return new Observable(observer => {
// Build a compatible handler, we also use this for the unsubscribe logic
const nextHandler = (value) => {
try {
observer.next(transform(value));
} catch (e) { observer.error(e); }
}
// Start listening for events
node.addEventListener(eventName, nextHandler);
// Return a way to tear down the subscription when we are done
return () => node.removeEventListener(eventName, nextHandler);
})
// Shares the underlying Subscription across multiple subscribers
// so we don't create new event handlers for each.
.share();
}
Here we are simply wrapping the native event handler (obviously the real implementation is more robust than this). But because the underlying source is actually an event handler which does have a mechanism to report new event (by definition really), we continue to get events in perpetuity (or until we unsubscribe).