Rx + more generic observable

Rx-generic-observable.swift

protocol EventProtocol {
    var isStopEvent: Bool { get }
}

enum InfiniteEvent<Element>: EventProtocol {
    case element(Element)   // many
    var isStopEvent: Bool { return false }
}

enum SingleEvent<Element, Error>: EventProtocol {
    case element(Element)
    case error(Error)
    var isStopEvent: Bool { return true }
}

enum Event<Element, Error>: EventProtocol {
    case element(Element)
    case error(Error)
    case completed
    var isStopEvent: Bool {
        switch self {
        case .element: return false
        default: return true
        }
    }
}

// ... and tons of other varieties of events! (e.g. non-stopping errors like Bacon.js)

class _Observable<Ev: EventProtocol, Element, Error> {}

typealias InfiniteObservable<Element, Error> = _Observable<InfiniteEvent<Element>, Element, Error>
typealias SingleObservable<Element, Error> = _Observable<SingleEvent<Element, Error>, Element, Error>
typealias Observable<Element, Error> = _Observable<Event<Element, Error>, Element, Error>

// For InfiniteObservable (UI binding is only allowed from here).
extension _Observable where Ev == InfiniteEvent<Element> { }

// For SingleObservable.
extension _Observable where Ev == SingleEvent<Element, Error> { }

// For Observable.
extension _Observable where Ev == Event<Element, Error> { }

First discussed in ReactiveX/RxSwift#1470

ReactiveSwift proposal: Introduce Generalized Event by inamiy · Pull Request #722 · ReactiveCocoa/ReactiveSwift