观察类变量以了解RxSwift中viewModel的任何更改

Question

I have a ViewModel where an service class is defined, This service class is initialized in this ViewModel's initializer. Now I need to update my View when there is any change in its value in ViewModel(getting it from service). Although I have found some examples of using Variable but I could not find it with Class injection. ViewController:

    var presenter:Variable<CardViewModelProtocol>!
        func setPresenter() {
//Following line has error
            self.presenter = Variable<CardViewModelProtocol>(CardViewModelProtocol(service:CardService()))
            self.presenter.value.attachView(view: self)
        }

the line where presenter is initialized is giving error. Error:

'CardPresenterProtocol' cannot be constructed because it has no accessible initializers

Following is how the ViewModel initialized:

var cardService:CardServiceContract!
var materialNum = Variable<String>("")

init(service:CardServiceContract){
    self.cardService = service
}

Edit1: In short I want to convert following line to RxSwift supporting Variables and observe a variable(class object) in CardViewModel:

self.presenter = CardViewModel(service: CardService())

Edit2:

    protocol CardViewModelProtocol : BaseViewModelProtocol {

    func loadCardInfo(Serial serial:String)

    var materialNum: Variable<String> {get set}
}
protocol BaseViewModelProtocol {
    func attachView(view: BaseViewProtocol)
}
protocol BaseViewProtocol{
    func setViewModel()
}

Answer 1

This is an example of usage - https://github.com/RxSwiftCommunity/RxSegue/blob/master/Example/RxSegue/ProfileViewController.swift

But keep in mind that RxSwift 4.0.0 "Deprecates Variable in favor of BehaviorRelay."

If you are not obliged to follow MVVM, you can try unidirectional dataflow approach instead - https://github.com/maxvol/RaspSwift

In case to have a continuous subscription to server updates, you can use .switchLatest() operator like that -

let stream = PublishSubject<Observable<Response>>()
stream.switchLatest().subscribe(consumer)
stream.onNext(serverRequestReturningResponseObservable) // repeat whenever needed