On the way of understanding Observable, Observer and subscribe I just came to this example and unable to understand this example and I'm confuse
var observable = Rx.Observable.create(
function(observer) {
observer.onNext('Simon');
observer.onNext('Jen');
observer.onNext('Sergi');
observer.onCompleted(); // We are done }
);
whats happening here? what we are creating here stream of observer? but observer is the one who receives data from stream when subscribing to the stream with subscribe method and have 3 methods onNext, complete and error.
Also give me example. where we create a stream, a observer(standalone) and how this standalone observer subscribe to a observable.
The documentation is your entry door to understanding observables and their mechanics. Now the truth is that it can be pretty dense some times. I would recommend you have a look here :
Creating and Subscribing to Simple Observable Sequences
learning-observable-by-building-observable by Ben Lesh (development lead on RxJS)
The introduction to Reactive Programming you've been missing
Hot and Cold observables : are there 'hot' and 'cold' operators?
The short answer to your questions is that :
Rx.Observable.create returns an object (an observable) which implements an interface (described in the documentation) which includes a subscribe function. When you execute that subscribe function as in .subscribe(observer), the factory function you passed as parameter of Rx.Observable.create is executed with that observer parameter. In your case, Simon etc. values will be pushed to the observer (which implements the observer interface which includes onNext, onCompleted etc.).
Generally the overload form .subscribe(function yournamehere(){...}) is used. Hence an observer is made internally from that function, with its onNext being the function you passed, and its onError and onCompleted being default values.
For the second part of your question, cf. documentation https://github.com/Reactive-Extensions/RxJS/blob/master/doc/api/core/observer.md
Related
I am trying to understand the observables RxJs and I am using angular framework! I can't understand what is actually happening in 'subscriber function' ,it has a parameter named 'observer', and this parameter has a method in the function body, and its name is next()!can a function parameter own a method? based on which rule?!
and the next question is : what is happening in the 'Observable' class? I think the subscriber function returns or to be more precise, creates and passes a value to the Observable instance! and when we call the subscribe method on 'customIntervalObservable' , it passes that data or value to the subscribe method?
am I right?
const customIntervalObservable = new Observable(function subscriber(
observer
) {
let count = 0;
setInterval(() => {
count++;
observer.next(+count);
if (count > 3) {
observer.error(new Error("count is greater than 3"));
}
}, 1000);
});
this.firstObjSubs = customIntervalObservable.subscribe(
(data: number) => {
console.log(data);
},
(error) => {
console.log(error), alert(error.message);
}
);
}
ngOnDestroy() {
//this.firstObjSubs.unsubscribe();
this.firstObjSubs.unsubscribe();
}
}
Here's a bit more:
You can think of an Observer as something that watches the Observable and reacts to notifications.
What notifications?
Next: When another item is emitted into the Observable
Error: When an error occurs
Complete: When there are no more items to emit
Observer is an object with three functions: one for each type of notification. You can define an Observer in code as shown below. But this is uncommon.
And then pass that Observer object into the subscribe as shown below:
More often, you'll pass either the next callback as shown in the first example below. Or an object with one, two, or three of the callback functions defined as shown in the second example.
The subscribe method tells the Observable stream to start emitting its values. It does not itself emit any values. Think of it as a streaming service, like Disney+ or hulu. You have to first subscribe to the service before you can stream movies.
Does this help?
can a function parameter own a method?
Javascript functions parameters are not typed, so you can pass any value you want - including an object.
what is happening in the 'Observable' class?
I'll give this a try:
An observable is basically just a wrapper around a - subscribe() - function that essentially describes the logic of the observable behaviour.
An observable has the ability to notify subjects - or subscribers, or observers - of changes occuring in its state during its lifecycle. To do so, a contract exists that states that the observable should call a subject next() method. This method should describe the logic for how the subject wants to react to such changes.
When a subject is interested in being notified by an observable of its state changes, it executes the observable subscribe() method, passing itself to it as an argument. This effectively provides the observable the ability to call the subject next() method whenever its logic dictates to do so.
To basically illustrate this in code:
// Function describing the observable logic.
function subscribe(observer){
// Observable logic, including calling observer.next() as many times as the logic dictates to notify the observer of state changes.
}
// Observable wrapper.
let observable = new Observable(subscribe)
// Subject interested in being notified of the observable state changes.
let observer = {
next(value){
// Logic for how to react to notifications from the observable.
}
}
// Effectively execute the observable logic.
observable.subscribe(observer)
For simplicty's sake, I omitted a number of more minor concepts - such as the subject complete() or error() methods, as well as the unsubscribe() function returned by the observable subscribe() function.
Is the takeUntil will manage the subscription of the mergeMap when destroyed$ is emitted in the following code? In other words, is the mergeMap leak a subscription?
combineLatest([
this.selectedCustomerId$.pipe(
-->mergeMap<--(customerId => invoicesService.getInvoices(customerId)
),
this.configuration$
]).pipe(
takeUntil(this.destroyed$)
).subscribe([invoices, configuration] => this.displayInvoices(invoices, configuration));
Thanks!
Yes, there will be no memory leakage.
To understand that you need to understand how pipe works -
Without calling subscribe - there is no subscriptions, doesn't matter what operators you have put it, what pipe does, is it creates a new observable, which, while subscribing to, subscribe to the source observable, and while unsubscribing, it unsubscribe to the source observable.
Even though you have two pipes here - the "un-subscription" will bubble up. i.e. when destroyed$ emits the combine latest subscription will end, than, each of the combineLatest parameters subscription will end, than response of the pipe in which you used the mergeMap will end, which means the return value of the mergeMap will end too.
I hope it wasn't too complicated of an explanation, the short version is that when using pipe everything is connected so when you unsubscribe it unsubscribes all sources.
I have learned of two different ways of making an Observable. First one was with a Subject, like so:
// file A
const message$ = new Subject();
// file B
message$.subscribe( (message) => console.log(message) );
// file C
message$.next("Hello there!");
This method of creating an Observable allows me to have a way to exchange data from file B to file C.
The second way is via the Observable class, like so:
// file A
const click$ = new Observable( function(observer) {
//Alternatively, I can use Observable.create()
document.addEventListener('click', (e) => observer.next(e));
});
// file B
click$.subscribe( (cl) => console.log(cl) );
The main difference that I can gather between the Subject way and the Observable way is that I am not sure how to have some sort of communication between some file C, to the subscribers of the Observable. Basically, click$ does not have a .next() method, and the observer methods are in the function that we pass to the observable.
Other than this difference in behavior, is there another difference between observables made with Subject, and those made with Observable
A Subject is both Observable and Observer at the same time. That makes it so tempting to use, because you get a reference to an Observer that you can pass around in your code and emit items from wherever you want. However, that increases the error-proneness of your code by a lot, as you switch from a declarative definition of your Observable to a imperative one.
Generally speaking, you should use Observable creation functions (of, from, create) wherever possible. I'd say most cases can be solved without Subjects. There is a steep learning-curve though, as you must get to know most of the Observable creation functions in order to follow that pattern.
Subject might come more natural to developers who are used to code imperatively (that is: with a script language like JS), as it kind of resembles a simple wrapper object for a callback function. And one might ask, why is there a Subject anyway if its not desirable.
According to this article, Subjects are to be used in one case only:
To generate a hot observable imperatively and statefully, without any direct external source.
In short, that means: Use Subject when you don't have any external source (like an Observable, Promise or Event) and need to multicast the state of a class from inside a function. You shouldn't expose that Subject to others, though!
I suggest reading this article to you, it will clear up things.
Subject implements both the Observable and the Observer interface.
Implementing the Observable interface means, among other things, that you can subscribe to a Subject.
Implementing the Observer interface means, among other things, that with a Subject you can call the methods next error and complete.
You create Observables using Subject in the case you want to control programmatically the flow of events of that Observable, which is the case you mentioned of managing communication between C and B.
The relation between Subject and Observable goes like this:
class Observable {}
class Subject extends Observable {}
Subject implements Subscriber interface. So you can use Subject as a Subscriber (Observable cannot be used like this):
const subj = new Subject()
observable.subscribe(subj)
Subject can hide it's Subscriber interface by calling asObservable method - turning Subject into plain Observable;
I generally use Subject when my code is going to generate events and I use Observable when I already have some source of events
For example using Subject as a queue:
const queue = new Subject();
queue.pipe(
concatMap(doStuff)
).subscribe()
queue.next('do this')
queue.next('do that')
or using Subject as a signaller to stop other Observable
const stop = new Subject();
someObservable.pipe(
map(x => x + 1)
takeUntil(stop)
)
// somewhere in my code
stop.next()
Other than this difference in behavior, is there another difference between observables made with Subject, and those made with Observable
I wouldn't say there are differences - It is more like Subject is a complement to plain Observable - allowing us to do more things when needed.
I was going through this blog and reading about Observables and couldn't figure out the difference between the Observable and a Subject.
In stream programming there are two main interfaces: Observable and Observer.
Observable is for the consumer, it can be transformed and subscribed:
observable.map(x => ...).filter(x => ...).subscribe(x => ...)
Observer is the interface which is used to feed an observable source:
observer.next(newItem)
We can create new Observable with an Observer:
var observable = Observable.create(observer => {
observer.next('first');
observer.next('second');
...
});
observable.map(x => ...).filter(x => ...).subscribe(x => ...)
Or, we can use a Subject which implements both the Observable and the Observer interfaces:
var source = new Subject();
source.map(x => ...).filter(x => ...).subscribe(x => ...)
source.next('first')
source.next('second')
Observables are unicast by design and Subjects are multicast by design.
If you look at the below example, each subscription receives the different values as observables developed as unicast by design.
import {Observable} from 'rxjs';
let obs = Observable.create(observer=>{
observer.next(Math.random());
})
obs.subscribe(res=>{
console.log('subscription a :', res); //subscription a :0.2859800202682865
});
obs.subscribe(res=>{
console.log('subscription b :', res); //subscription b :0.694302021731573
});
This could be weird if you are expecting the same values on both the subscription.
We can overcome this issue using Subjects. Subjects is similar to event-emitter and it does not invoke for each subscription. Consider the below example.
import {Subject} from 'rxjs';
let obs = new Subject();
obs.subscribe(res=>{
console.log('subscription a :', res); // subscription a : 0.91767565496093
});
obs.subscribe(res=>{
console.log('subscription b :', res);// subscription b : 0.91767565496093
});
obs.next(Math.random());
Both of the subscriptions got the same output value!
Observables
They are cold: Code gets executed when they have at least a single observer.
Creates copy of data: Observable creates copy of data for each observer.
Uni-directional: Observer can not assign value to observable(origin/master).
The code will run for each observer . If its a HTTP call, it gets called for each observer.
if its a service we want to share among all the components, it wont have latest result all new subscribers will still subscribe to same observable and get value from scratch
Unicast means can emit values from the observable not from any other component.
Subject
They are hot: code gets executed and value gets broadcast even if there is no observer.
Shares data: Same data get shared between all observers.
bi-directional: Observer can assign value to observable(origin/master).
If are using using subject then you miss all the values that are broadcast before creation of observer. So here comes Replay Subject
multicast, can cast values to multiple subscribers and can act as both subscribers and emmitter
I found the accepted answer slightly confusing!
An Observer isn't the interface for feeding an Observable source, it's the interface for observing an Observable source... which makes more sense from the name, right?
So, the reason that:
var observable = Observable.create(observer => {
observer.next('first');
observer.next('second');
...
});
works - creating an observable which emits 'first' then 'second' - is that the argument to Observable.create(...) is a subscribe function, it basically defines which Observer events will happen on a direct Observer of that Observable.
If you want to go into it a little bit further again, it's important to understand that the subscribe function isn't directly called on the Observer object when you subscribe, instead it's mediated by a Subscription object which can enforce correct observable rules, e.g. that an Observable will never emit a new value after observer.complete() has been called, even if your subscribe function looks as if it would.
REF: http://reactivex.io/rxjs/manual/overview.html#creating-observables
A Subject is both an Observable and an Observer and once again it looks just like the Observer interface is the way to 'feed' events to the Subject. But it's easier to understand the naming if you realise that a Subject is a bit like an Observable with the equivalent of the subscribe function (i.e. where you define what events will happen to things observing it) sitting there right on the object, even after it has been created. So, you call Observer methods on the Subject to define what Observer events will happen on things observing it! 😊 (And again, there are intermediate objects involved, to make sure that you can only do legal sequences of things.)
REF: http://reactivex.io/rxjs/manual/overview.html#subject
See rxjs document (more information and examples there):
http://reactivex.io/rxjs/manual/overview.html#subject
What is a Subject? An RxJS Subject is a special type of Observable that allows values to be multicasted to many Observers. While plain Observables are unicast (each subscribed Observer owns an independent execution of the Observable), Subjects are multicast.
A Subject is like an Observable, but can multicast to many Observers. Subjects are like EventEmitters: they maintain a registry of many listeners.
and code, Subject extending Observable: https://github.com/ReactiveX/rxjs/blob/master/src/internal/Subject.ts#L22
/**
* #class Subject<T>
*/
export class Subject<T> extends Observable<T> implements SubscriptionLike {
//...
}
Imagine if you have a stream of data coming into your application like in a websocket connection. You want a way to handle it. There is a few solution:
1. normal ajax request:
This solution is not viable because it is
not applicable to process push data. It is more of a pull then a
push.
2. Promise:
Also not good because you have to trigger them and
they can only retrieve once. Also more of a pull then a push.
So in order to retrieve this data, in the old time, we do a long-polling. Which is where we set an interval function to retrieve that stream of data every 1 minute for an example. Though it works, it actually burdening resources like CPU and memory.
But now with option no 3,
3. Observable: You can subscribe and let the stream of data to come
in non-stop until the function complete has been called.
Cool right ? But then there is another problem. What if you want to observe incoming data only once somewhere in your application. But you want to use that data simultaneously around your application when the data arrived. That is when and where you use Subject.
You place subject.subscribe() at places you want to use throughout your application. When the data arrived, places where there is subject.subscribe() will process them simultaneously. But the observer must subscribe with the subject as its argument like this.
observer.subscribe(subject).
Example application is when you want to build a notification alert.
You cannot have multiple subscription of the same observable because chances are, each subscribers will received different input data. But with subject, all that subscribe() through subject will be retrieving the same data.
Another analogy is through magazine subscription. Each subscribers will received the magazine with their name on it. So, different subscription = different receiver name.(Normal Observable)
But when you share with your friends, all of your friend would receive the same magazine with only your name on it.(Normal Observable with Subject)
This guy explain it very well with code example. You can check it out at https://javascript.tutorialhorizon.com/2017/03/23/rxjs-subject-vs-observable/
Hopefully this answer helps.
Briefly,
subject: you can send to it and receive from it.
Observable: you can receive from it only.
In another words,
In subject you can subscribe to it and you can use it to broadcast to other subscribers any time and anywhere in code.
whilst,
in observable you can subscribe to it only (you can't use it to broadcast data after it have been initialized).
The only place you can broadcast data from observable is inside its constructor.
Observable can inform only one observer, while Subject can inform multiple observers.
From another perspective, it is good to note that the subscription to an Observable re-execute the Observable function. This can lead performance issue if the data source is a service for instance.
If you want several subscribers to get the same value, you may need a Subject.
For this, make sure that your subscription is set before the Subject subscribed to the data source. Otherwise, your process would be stuck.
More details here: https://javascript.tutorialhorizon.com/2017/03/23/rxjs-subject-vs-observable/
Observable:
Only the Observable knows how and when the events are triggered on the observable. i.e the next() method has to be called only inside the instantiated constructor. Also, on subscribing each time, a separate observer is created and calls next() method using particular observer inside constructor only, in the following example subscriber itself is the observer and it is subscribed when the instantiated constructor gets executed.
Ex:
import { Observable } from 'rxjs';
const observable = new Observable(subscriber => {
subscriber.next(1);
subscriber.next(2);
setTimeout(() => {
subscriber.next(3);
}, 1000);
});
Subject:
Here next() method can be used by subject anywhere outside the constructor. Also, when next() method is called before subscribing, the particular event will be missed. Hence next() method has to be called only after subscribing.
Ex:
import { Subject } from 'rxjs';
const subject = new Subject<number>();
subject.next(1); // this is missed
subject.subscribe({
next: (v) => console.log(`observerA: ${v}`)
});
subject.subscribe({
next: (v) => console.log(`observerB: ${v}`)
});
subject.next(2);
I'm creating a class for my app that keeps an internal BehaviorSubject. Is there a way I can expose a corresponding 'BehaviorObservable', which pipes through the values of the subject, but only has the 'read-only' stuff of the Observable interface? The .next(), .error() and .complete() methods should only be available internally.
It's not just a matter (I believe) of using Observable.create() to pipe the subject's values through. Users of my API should .subscribe() to the exposed observable, and then immediately get a callback for the stored current value.
I may be able to hack something together, but I'm sure I'm just missing something that RxJS can already do.
Are you by any chance looking for the .asObservable() method - which existed in Rxjs v4. Don't know if that is still the case in Rxjs v5 though.
According to this, the method should have been included in release 5.0.0-beta.2. Also I quote here their work-around if the functionality is not there :
You can get the same functionality by creating an observable with the private subject's subscribe function:
> const subj = new rx.Subject();
> const exposed = new rx.Observable(fn => subj.subscribe(fn));
For more details about subjects' semantics, you can have a look here.