Accomplishing shareValue/shareBehavior

Question

I have an Observable that is based on some events and at some point does some expensive computation. I would like to render the results from that Observable in multiple different places. If I naively subscribe to this Observable in two places I will end up doing the expensive computation twice. Here is a code snippet to drive my point home:

var s = new rx.Subject();
var o = s.map(x => { console.log('expensive computation'); return x });
o.subscribe(x => console.log('1: ' + x));
o.subscribe(x => console.log('2: ' + x));
s.next(42);

The output is:

expensive computation
1: 42
expensive computation
2: 42

I would like to perform the expensive computation in the map only once. share accomplishes this, but it makes it so late-arriving subscribers do not get the current value to render. In previous RxJS versions, shareValue allowed late-arriving subscribers to get the current value. However, it appears that this was renamed to shareBehavior in RxJS 5 and then removed altogether:

https://github.com/ReactiveX/rxjs/pull/588

https://github.com/ReactiveX/rxjs/pull/712

There is a long discussion in this issue where it was decided that they would 'Remove shareBehavior and shareReplay to prevent user confusion.' I don't understand what the potential for confusion was (so maybe that means I am one of the users saved by this decision?).

publishBehavior also looks promising but I don't fully understand publish and it seems like it adds more complexity than I need or want.

Anyway, I would like to know if there is a recommended way to accomplish this in RxJS 5. The migration doc doesn't provide any recommendations.

Answer 1

After some more research I've found that the behavior I've described can be implemented with

.publishBehavior(startValue).refCount().

This discovery is based on the fact that share() is an alias for publish().refCount(). I still don't fully understand publish() but this seems to have the desired effect in practice.

Similar is cache(1) (which is an alias for publishReplay(1).refCount()). It has a similar effect as publishBehavior(defaultValue).refCount() except that it does not start with a default value. So if no items have been emitted, new subscribers will not immediately receive a value.