用作PartialObserver的BehaviorSubject会中断BehaviorSubject上的订阅

Question

I was looking through the interfaces of observables and saw you can pass anything that implements PartialObserver to the subscribe function. So I did that with BehaviorSubject.

Like this (A)

source$
      .pipe(
        tap(() => console.log('X')),
      )
      .subscribe(this._titlesX$);

So I did, but found something very strange. If I pass the behavior subject to the subscribe function the values get emitted, but to see that, you have to subscribe before using the BehaviorSubject as PartialObserver.

In the docs for BehaviorSubject you can find this:

A variant of Subject that requires an initial value and emits its current value whenever it is subscribed to.

So when using BehaviorSubject as partialObserver this behavior breaks.(No pun intended)

But when I use this method: Like this (B)

source$
     .pipe(
        tap(() => console.log('X')),
      )
      .subscribe(res => this._titlesX$(res));

Everything works as expected.

Why does method A not work? Is this a bug or am I just using it wrong?

Here is a link to the stackblitz all setup. Link to stackblitz project

titleX: represents method A titleY: represents method B

Subscribing with an BehaviorSubject should be possible. And the time when you subscribed should not matter. Otherwise it shouldn't be allowed as a parameter for the subscribe method.

Answer 1

the reason is because when you do

source$.subscribe(this._titlesX$)

you've fully casted the source to the BehaviorSubject, as BehaviorSubject is a full observer, including the complete and error handlers. of completes after one emission, so it also completes your BehaviorSubject, and completed subjects do not emit. but just calling next in the subscribe of source$ clearly doesn't pass through the complete / error handlers.

if your source was more like:

const source$ = interval(1000).pipe(map(v => [v.toString()]));

where it does not complete, you'll see expected behavior.

blitz: https://stackblitz.com/edit/angular-gfmdgf?file=src/app/app.component.ts