Map.swift

extension ObservableType {    
    public func map<U>(_ transform: @escaping (E) throws -> U) -> Observable<U> {
        return Observable<U> { observer in
            return self.subscribe(observer: Observer { (event) in
                switch event {
                case .next(let element):
                    do {
                        try observer.on(event: .next(transform(element)))
                    } catch {
                        observer.on(event: .error(error))
                    }
                case .error(let e):
                    observer.on(event: .error(e))
                case .completed:
                    observer.on(event: .completed)
                }
            })
        }
    }
}