javascript - Rx.Subject loses events

Question

Can anybody explain what the differents between these 3 variants?

http://jsfiddle.net/8vx2g3fr/2/

1. First works as excpect, all events are processed.
2. But second loses last event (3)
3. Third loses second event (2)

Could you please help me to understand what the issue is and how to make the third variant process all events?

1

let bs = new Rx.Subject();
bs
    .subscribe(v=>{
        console.log("in", v);
        if (v % 2 == 0) {
            setTimeout(()=>{
                console.log(" out", v, "->" , v + 1);
                bs.next(v+1);
            }, 0);
        }
    });

bs.next(0);
bs.next(2);

Output:

in 0
in 2
 out 0 -> 1
in 1
 out 2 -> 3
in 3

2

let bs2 = new Rx.Subject();
bs2
    .subscribe(v=>{
        console.log("in", v);
        if (v % 2 == 0) {            
            Rx.Observable.interval(0).take(1)
                .map(()=>{console.log(" out", v, "->" , v + 1);return v+1;})
                .subscribe(bs2);
        }
    });

bs2.next(0);
bs2.next(2);

Output:

in 0
in 2
 out 0 -> 1
in 1
 out 2 -> 3

3

let bs3 = new Rx.Subject();
bs3
    .switchMap(v=>{
        console.log("in", v);
        if (v % 2 == 0) {            
            return Rx.Observable.interval(0).take(1)
                .map(()=>{console.log(" out", v, "->" , v + 1);return v+1;});
        }

    return Rx.Observable.empty();     
    }).subscribe(bs3);

bs3.next(0);
bs3.next(2);

Output:

in 0
in 2
 out 2 -> 3
in 3

Answer 1

This all is in fact expected behavior.

The confusing thing is what happens when you reuse Subjectand an operator such as take() multiple times.

Operator take(1) takes just a single value and send complete notification. This notification is received by the Subject because of .subscribe(bs2). Now comes the most important part.
When a Subject receives a complete or error notification it marks itself as stopped. This means it will never remit any items or notifications which is correct and expected behavior in Rx. Notifications complete or error have to be the last emissions.

So the Subject is completed by the first take(1) which is triggered by value 0 (the bs2.next(0) call).

Then when value 2 triggers the second run of the Observable.interval(0).take(1) it's received by the Subject but it's automatically ignored because the Subject is already marked as stopped.

The process in you third demo is exactly the same.

You can see it in the source code in Subject.ts:

• https://github.com/ReactiveX/rxjs/blob/master/src/Subject.ts#L86

• https://github.com/ReactiveX/rxjs/blob/master/src/Subject.ts#L56