Atom.fromObservable

[blacktaxi]

Hi all,

In this PR, I'm proposing a new utility API to convert observables to read-only atoms. This is a part of a bigger track to improve reactiveList and dynamic trees in general (will elaborate more in next PR(s)).

For the context of this PR, the motivation is to provide a way to temporarily convert an existing observable into a read-only atom, mainly to leverage the .get() API and 'distinct value' semantics in conjunction with .view(...) to gain performance benefits.

The initial proposal for the API is as follows:

function fromObservable<T>(src: Observable<T>): Observable<ReadOnlyAtom<T>>

One of the intended use cases for this is applying observable operators to an atom, and then being able to convert back from an observable.

Things to discuss:

• I'm using Observable as the return value to make it easier to use within existing Focal environment (e.g. as a child of an F element). It's a bit clunky as this observable can only emit a single item. Promise, on the other hand, wouldn't work here as we need explicit lifecycle control via subscribe/unsubscribe. We could also do something like Promise<{ atom: ...; dispose(): void }>. It would be almost the same thing, and while trivially converted to an observable to embed into an F element, it would be less ergonomic.
• Should the returned observable immediately complete, or should it be left hanging indefinitely? Probably more of an implementation detail but maybe I'm wrong.
• Naming and location. fromObservable vs toAtom? Put it in the Atom namespace or as an exported free function?

Let me know what you think.