Proposal: add a switchScan operator

[lestathc]

It will be similar as the mergeScan but instead of merging all events of inner observables, if the outer source emitted a new event, it switch to the recently accumulated inner observable.

[kwonoj]

while it is not concluded yet, we're discussing of plan for mergeScan itself (#2737) and for now, we recommend to create operator as user-level first instead of direct addition into core lib. (https://github.com/ReactiveX/rxjs/blob/master/doc/operator-creation.md#diy-custom-operators-for-end-users).

[kylecordes]

I have stumbled across a situation where a switchScan() would be perfect; implementing it myself does not sound like all that much fun.

[bradleyayers]

@kylecordes is your scenario solved by just passing concurrency=1?

[kylecordes]

@bradleyayers I'm not sure - I ended up solving the problem I was having a completely different way, with a group operation plus a switch map.

[benlesh]

This just came up at Google... I'm going to reopen it.

Here's a minimal implementation in JS: http://jsbin.com/kiyoyir/3/edit?js,console

[fljot]

Here's a minimal implementation in JS: http://jsbin.com/kiyoyir/3/edit?js,console

@benlesh I think you forgot of disposal of outer subscription in that demo.

[cartant]

Closing this in favour of #4442 - which has an associated PR and has been discussed in a core-team meeting.