Add start method to Observers

[benlesh]

The TC39 spec has a start method to prevent this scenario from being a problem:

const subscription = range(0, 10000000).subscribe({
  next(value) { 
    if (value > 3) {
      // try to unsubscribe here, I dare you.
      subscription.unsubscribe();
    }
  }
});

The solution was to add an optional start method to Observer:

const subscription = range(0, 10000000).subscribe({
  start(subs) { this.subscription = subs; },
  next(value) { 
    if (value > 3) {
      // this.subscription is literally the same reference you'd get back
      // from the subscribe call itself.
      this.subscription.unsubscribe();
    }
  }
});

[benlesh]

And no, I won't be adding this to the 3-callbacks version upfront, it's sort of to cover an edge case, not sure it needs to have some ergonomic considerations.

[felixfbecker]

Why is the scenario a problem?

[benlesh]

@felixfbecker if a consumer wishes to cancel a synchronous firehose, they cannot do so easily with the basic observable impelmentation. They could "Rx" around it:

const stop = new Subject();
// a hypothetical synchronous firehose of data.
const firehose = range(0, Number.POSITIVE_INFINITY);

firehose.takeUntil(stop);
  .subscribe({
    next(value) {
      doSomeSideEffect(value);
      if (checkSomeExternalState()) stop.next();
      console.log(value);
    }
  });

... but that's a lot of extra hoops to jump through.

[benlesh]

Regardless, I think we're going to move away from the start method if we use abortSignal: #3122

[felixfbecker]

Ah, got it.

[martinsik]

It feel like this could be easily solved by using publish():

const source = range(0, 10000000).publish();

const subscription = source.subscribe({
  next(value) { 
    if (value > 3) {
      // try to unsubscribe here, I dare you.
      subscription.unsubscribe();
    }
  }
});

source.connect();

[benlesh]

@martinsik this is meant to round out the type on its own and it's coming from the standards proposals in flight.

[benlesh]

And, of note, I was wrong.. even with abort signal, we'd need start.

[lock]

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