ObservableFilter.swift

protocol ResultProtocol {
    associatedtype WrappedType
    associatedtype ErrorType
    var value: WrappedType? { get }
    var error: ErrorType? { get }
}

extension Result: ResultProtocol {
    typealias WrappedType = Value
    typealias ErrorType = Error
}

extension Observable where Element: ResultProtocol {
    func filtered() -> Observable<Element.WrappedType> {
        return self.filter { $0.value != nil }.map { $0.value! }
    }

    func filtered(handler: @escaping (Element.ErrorType) -> Void) -> Observable<Element.WrappedType> {
        return self
            .do(onNext: { result in
                guard let error = result.error else { return }
                handler(error)
            })
            .filter { $0.value != nil }
            .map { $0.value! }
    }
}