Inject an Error on a given Signal

Signal+InjectError.swift

import ReactiveSwift
import Result


extension Signal {
    func injectError(_ error: Error, on signal: Signal<(), NoError>) -> Signal<Value, Error> {
        return Signal { observer, lifetime in
            lifetime += signal.observe { event in
                if event.value != nil {
                    observer.send(error: error)
                }
            }
            lifetime += self.observe(observer)
        }
    }
}

extension SignalProducer {
    func injectError(_ error: Error, on signal: Signal<(), NoError>) -> SignalProducer<Value, Error> {
        return lift { $0.injectError(error, on: signal) }
    }
}