perf(client): share replay of computed observables

[arnautov-anton]

Overview

Adds shareReplay to computed observables which when previously subscribed to by multiple components would be evaluated separately, i.e., each source data emission would evaluate defined pipeline for each subscription individually.

"Futureproofs" RxUtils.getCurrentValue method through usage of combineLatest function which will always take the last emitted value even from "pure" observables unlike take operator which "resolves" with first emitted value.

Fixes noop sorting (\w) test where "sort" mutates source data - when we move to TS5.2 we should replace that with Array.toSorted.

Implementation notes

I mentioned using share operator which uses refCount and Subject under the hood meaning that if amount of subscriptions on such observable reaches 0 (refCount === 0) - it resets the Subject and re-evaluates the pipeline and assignes new value on next subscription. Using it in our SDK proved to be difficult and was behaving unexpectedly (broke tests and certain observables would return "undefined" instead of initial values) though on paper it makes perfect sense. I opted to use shareReplay which by default (with only buffer size as argument) works like a ReplaySubject but continues running defined pipeline (never unsubscribes from the source observable) even after the refCount reached zero. With the overloaded variant shareReplay({ bufferSize: 1, refCount: true }) it behaves just as expected and does not break our SDK - meaning the values (pipelines) are being computed only once per multiple subscriptions and only if it has at least one subscription.

Sources:

• difference between share and shareReplay operators - though I found a discrepancy in the post - they mention that the shareReplay with refCount does not re-evaluate source observable but through my testing I found that to be false (yet it does something differently under the hood)
• shareReplay operator
• share operator

[szuperaz]

@arnautov-anton I think you will be our new RxJS expert, I've never used the refCount thing before.