Weird behaviour of share operator on UISegmentedControl observable

[jcavar]

Short description of the issue:

Using share operator on UISegmentedControl selectedSegmentIndex observable emits duplicated events on subscribe.

Expected outcome:

side effect
1 next(-1)
2 next(-1)

What actually happens:

side effect
1 next(-1)
2 next(-1)
side effect
2 next(-1)

Self contained code example that reproduces the issue:

var bag: DisposeBag? = DisposeBag()

var observable = Observable.just(UISegmentedControl())
    .do(onNext: { _ in
        print("side effect")
    })
    .flatMapLatest { segmented in
        return segmented.rx.selectedSegmentIndex
    }
    .share(replay: 1, scope: .forever)

observable.subscribe { event in
    print("1 \(event)")
}.disposed(by: bag!)

bag = DisposeBag()

observable.subscribe { event in
    print("2 \(event)")
}.disposed(by: bag!)

RxSwift/RxCocoa/RxBlocking/RxTest version/commit

4.3.1

Platform/Environment

• [x] iOS
• [ ] macOS
• [ ] tvOS
• [ ] watchOS
• [ ] playgrounds

How easy is to reproduce? (chances of successful reproduce after running the self contained code)

• [x] easy, 100% repro
• [ ] sometimes, 10%-100%
• [ ] hard, 2% - 10%
• [ ] extremely hard, %0 - 2%

Xcode version:

10.2.1

:warning: Fields below are optional for general issues or in case those questions aren't related to your issue, but filling them out will increase the chances of getting your issue resolved. :warning:

Installation method:

• [x] CocoaPods
• [ ] Carthage
• [ ] Git submodules

I have multiple versions of Xcode installed: (so we can know if this is a potential cause of your issue)

• [ ] yes (which ones)
• [x] no

Level of RxSwift knowledge: (this is so we can understand your level of knowledge and formulate the response in an appropriate manner)

• [ ] just starting
• [x] I have a small code base
• [ ] I have a significant code base

[freak4pc]

Hi,

There isn't anything unexpected with your code as far as I can tell. I also don't recommend using .forever for most things. (any reason for this?)

In any sense, this doesn't look like an issue with the project itself but more of a "how do I do XXX" question. Please ask these kinds of questions in our slack channel http://slack.rxswift.org/ or some other more appropriate communication channel.

We are asking this because:

• we can't scale development of this project if people are reporting false issues and we are spending lot of time answering those directly instead of working on this project.
• all who are interested in development of this project and watching it are being spammed by irrelevant information.
• those who want to know what are currently unresolved serious issues are having a harder time figuring it out because list is spammed by non issues, so the real issues are masked.

Thank you for understanding.

[jcavar]

How is it expected that the same event is triggered 2 times? Isn't expected behaviour of share operator to replay only latest value(with buffer 1)?

I am using .forever because I want to replay latest value even if all subscribers unsubscribe and then subscribe again.

[jcavar]

Also, I asked this question on Slack channel and other member agrees that this is weird behaviour and that it is worth opening an issue.

[freak4pc]

Hey there - I've answered some of your questions on Slack.

In general .forever isn't guaranteed to do what you want - it can potentially replay stale values, please read the documentation. In general using .forever as if it was a caching mechanism, it's not something you should rely on and as far as I know it's not meant for this.

Note that there are also some minor differences between rebuilding a new DisposeBag vs unsubscribing existing disposables / disposing them.