BaseViewModel.swift

import Moya
import ReactiveSwift

class BaseViewModel<Element: Codable> {
    var value: Element? {
        didSet {
            valueDidSet()
        }
    }
    
    let moyaProvider: MoyaProvider<Service>?
    
    init(provider: MoyaProvider<Service> = MoyaProvider<Service>()) {
        moyaProvider = provider
    }
    
    public func fetchData(service: Service) -> SignalProducer<Element, MoyaError> {
        return SignalProducer<Element, MoyaError> { observer, disposable in
            self.moyaProvider?.request(service, completion: { result in
                switch result {
                case let .success(response):
                    if response.statusCode >= 200 && response.statusCode <= 300 {
                        let mapResult = SignalProducer { try response.map(Element.self) }
                        mapResult.startWithResult({ result in
                            switch result {
                            case let .success(element):
                                self.value = element
                                observer.send(value: element)
                                observer.sendCompleted()
                            case let .failure(mappingError):
                                observer.send(error: MoyaError.objectMapping(mappingError, response))
                            }
                        })
                    } else {
                        observer.send(error: MoyaError.statusCode(response))
                    }
                case let .failure(error):
                    observer.send(error: error)
                }
            })
        }
    }
    
    public func valueDidSet() {}
}