Observable.swift

public class Observable<Element>: ObservableType {
    public typealias E = Element
    private let _subscribeHandler: (Observer<Element>) -> Disposable
    
    public init(_ subscribtionClosure: @escaping (Observer<Element>) -> Disposable) {
        _subscribeHandler = subscribtionClosure
    }
    
    public func subscribe<O : ObserverType>(observer: O) -> Disposable where O.E == E {
        let composite = CompositeDisposable()
        let subscription = _subscribeHandler(Observer { (event) in
            observer.on(event: event)
            switch event {
            case .error, .completed:
                composite.dispose()
            default:
                break
            }
        })
        composite.add(disposable: subscription)
        
        return composite
    }
}