Rewrite executing observable

[fpillet]

Rewrote the executing observable, leading to simpler code.

Also Integrated @stefanomondomino's idea of using a shareReplay(1) to guarantee that late subscribers get the current execution state.

[fpillet]

This PR supersedes #67

[stefanomondino]

Brilliant! I struggled for a while for a better way to express that executing state, this is really clever.

[fpillet]

... and of course after I pushed the first commits I found a better way to do it. Always looking to achieve the zen of Rx :)

[stefanomondino]

Great! I'll try it directly in my codebase to check if it solves my problem (Action sometimes doesn't get re-enabled when a .just() observable is directly returned) and let you know. Hopefully I'll be able to update again to RxSwift 3.1 (right now I'm stuck at 3.0 because of this)

[fpillet]

Let me know if you have more issues. If you want to chat directly I'm on the RxSwift Slack (come join https://rxswift.slack.com, we can discuss Action in the Community channel)

[stefanomondino]

Will surely do! I have something I'd like to propose. Meanwhile, I can confirm my problem is resolved with your fix.

[fpillet]

Perfect, we'll merge it ASAP (just waiting for someone else to review, that's how the process goes)

@ishkawa ?

[ishkawa]

Putting these operations here looks nice 👍

I think it might be better to replace ignoreElements().map { _ in true } with flatMap { _ in Observable<Bool>.empty() if it is possible, because the former returns unrelated value true but the latter doesn't.

Additionally, naming $0.ignoreElements().map { _ in true }.catchError { _ in Observable.empty() } may improve readability.

executing = executionObservables
    .flatMap { execution -> Observable<Bool> in
        let execution = execution
            .flatMap { _ in Observable<Bool>.empty() }
            .catchError { _ in Observable.empty() }

        return Observable.concat([
            Observable.just(true),
            execution,
            Observable.just(false)])
    }
    .startWith(false)
    .shareReplay(1)

[fpillet]

Excellent ideas, let me push these ones and we're good to go.

[ishkawa]

@fpillet Your PR looks good 👍 I left 2 trivial comments.