implement RxJS-like BehaviorSubject

[TylorS]

Overview

Create a subject which behaves similarly to RxJS' BehaviorSubject

Motivation

Easier transitioning from RxJS to Most

[TylorS]

@briancavalier @Frikki

Any suggestions on better naming?

[axefrog]

If I were naming this I'd first try to describe its behaviour.

• It always retains the latest value
• It has an initial value to be used before anything has been emitted
• It emits (replays?) the currently-retained value to new subscribers

How to say this in a class name? Ideas:

• RetainedValueSubject
• RetentionSubject
• ValueRetentionSubject
• LatestValueSubject
• RecentValueSubject
• ValueCaptureSubject
• SingleCaptureSubject
• CaptureLatestSubject
• ReplayOnceSubject
• SingularReplaySubject

[TylorS]

I'm actually beginning to wonder if it would be a better to simply provide a single function with options instead. No real reason to continue to emulate RxJS.
e.g.

const {sink, stream} = subject() // regular subject 
const {sink, stream} = subject({replay: true}) // replaySubject
const {sink, stream} = subject({replay: true, initialValue: 1}) // behaviorSubject

[wishfoundry]

I would vote for something like

 const {sink, stream}  cacheLatest(size: number)
sink.add(initial)

supporting an initial value in the constructor seems superfluous at best, especially for buffers > 1.

[TylorS]

I actually went ahead with a similar API to the above. If you check the README now, it has the API I decided upon. It does include the initial values.