[Idea] Make it possible for BehaviorSubject to emit an initial "summary" value to late subscribers

[vkryachko]

Description:

Note: this PR is intended to open a discussion on whether this change is considered broadly useful

BehaviorSubject currently allows emitting the initial or latest value when it's subscribed to. However there is a use case that it non-trivial to model with it.

Imagine a BehaviorSubject is used to stream log events and consumers can subscribe and render them. However the logs subscribers will receive depends on when they subscribe, late subscribers won't see log events that happened before they subscribed.

This change proposes to make it possible for late subscribers to receive a "summary" value upon initial subscription, e.g.

interface LogEvent {
  readonly log: string;
}

const events = new BehaviorSubject<LogEvent>(
    {log: "start"},
    (prev, next) => {log: `${prev.log}\n${next.log}`.slice(-2000)});

With this code, late subscribers will receive up to 2000 characters of previously emitted logs and then keep receiving new log lines as long as they are subscribed.
While this example it contrived, the proposed change is general enough to allow for more interesting folds to generate a meaningful "summary" value.

Thoughts?

BREAKING CHANGE: NA

[voliva]

If you don't need to "summarise" and you only need to keep the last 2000 logs, you can just use new ReplaySubject(2000) instead of new BehaviourSubject(). The consumer then can take those 2000 logs when it subscribes and change them / summarise them as they need.

But if for whatever reason you want to pre-summarise it and store that as a separate value, then I think you can just break it in a separate observable:

const events = new Subject<LogEvent>(); // BehaviourSubject not needed

const eventSummary = connectable(events.pipe(
  scan((prev, next) => ({log: `${prev.log}\n${next.log}`.slice(-2000)}), {log: "start"}),
), {
  connector: () => new ReplaySubject(1)
});
eventSummary.connect()

In a way, if I subscribe to events I expect not to receive the last log event, that's why I think Subject is usually more suitable.
And if I need the summary I can subscribe to eventSummary, or even combine them both so I can receive the summary followed by the new ones.