Merging progressively sources in a subject to implement the channel

At Potions we are implementing an actor model based on RxJS.

We therefore need to be able to “hookup” an actor to the main channel.

An actor holds a behaviorSubject _state$, an in$ stream, an out$ stream and an

action function

(in$, state$) ⇒ (out$)

It is very similar to the redux-observable epic : https://redux-observable.js.org/docs/basics/Epics.html

Except that we consider that we can have multiple “stores”… Our “actors”.

As explained in this video https://www.youtube.com/watch?v=Vg60lf92EkM&list=PLNYkxOF6rcIDjlCx1PcphPpmf43aKOAdF&index=19

the actor model shift the mental model toward state rather then UI framework.

Hooking up a new actor means that the in$ stream becomes the channel$ stream

and the out$ stream pours its data in the channel$ stream.

The first part seams easy

javascript
in$ = channel$.asObservable()

However the second part is trickier.

We could think to just subscribe the channel$ to the out$ like that

javascript
// This is wrong
actorA.out$.subscribe(channel$)
actorB.out$.subscribe(channel$)

But the problem with this code is that we don’t want the channel$ subject to complete as soon as an actor complete !

javascript
a = new Subject()
b = new Observable(observer => observer.complete())
c = new Observable(observer => observer.next(42))

a.subscribe(console.log)
b.subscribe(a)
c.subscribe(a)
// Doesn't log 42 !!

javascript
a = new Subject()
b = new Observable(observer => observer.complete())
c = new Observable(observer => observer.next(42))

merge(b,c).subscribe(a)
// Works but requires to know b and c prior to suubscribing

Here is our little trick to avoid completing the channel when a source completes :

javascript
a = new Subject()
b = new Observable(observer => observer.complete())
c = new Observable(observer => observer.next(42))
d = new Observable(observer => observer.next(50))

const neverComplete = mergeWith(NEVER)

const mergeIn = a => b => {
	b.pipe(neverComplete).subscribe(a)
}

[b, c, d].map(mergeIn(a))
// Works !!