在 SwiftUI 组件之间共享 ViewModel

Question

I'd like to implement a navigator/router for an architecture implemented with SwiftUI and Combine. In a few words the View will share viewModel with Router . When the View triggers a change on the viewModel the Router should navigate to a new sheet.

This is a version of my code where I'm directly passing the viewModel from View to Router . Is there anything wrong? My biggest doubt is that since I'm using @ObservedObject on both the Router and the View , two different instances of the viewModel are created.

VIEW MODEL

class BootViewModel:ObservableObject{
    @Published var presentSignIn = false
}

VIEW

struct BootView: View {

    @ObservedObject var viewModel:BootViewModel
    var navigator:BootNavigator<BootView>? = nil

    init(viewModel:BootViewModel) {
        self.viewModel = viewModel
        self.navigator = BootNavigator(view: self, viewModel: viewModel)
        self.navigator.setSubscriptions()
    }

    var body: some View {
        VStack{
            Text("Hello")
            Button("Button"){
                self.viewModel.presentSignIn.toggle()
            }
        }
    }
}

NAVIGATOR

class BootNavigator<T:View>{
    var view:T? = nil
    @ObservedObject var viewModel:BootViewModel

    init(view:T, viewModel:BootViewModel) {
        self.view = view
        self.viewModel = viewModel
    }

    func setSubscriptions(){
        subscribe(onSigninPressed: $viewModel.presentSignIn)
    }


    func subscribe(onSigninPressed : Binding<Bool>){
        _ = view.sheet(isPresented: $viewModel.presentSignIn){
            SignInView()
        }
    }
}

• Why the SignInView is never presented?
• Without taking into account the fact that using a router with swiftUI is not needed in general(I'm mostly doing an exercise)... is there anything wrong with this implementation?

Answer 1

This

view.sheet(isPresented: $viewModel.presentSignIn){
    SignInView()

MUST be somewhere in body (directly or via computed property or func) but inside body's ViewBuilder

Answer 2

Some notes I have to point out here:

ValueType

There is a difference between an UIView and a SwiftUI View . All SwiftUI View s are value type! So they get copied when you pass them around. Be aware of that.

Single instance

If you want a single instance like a regular navigator for your entire app, you can use singleton pattern. But there is a better approach in SwiftUI universe called @Environment objects. You can take advantage of that.

Trigger a view refresh

To refresh the view (including presenting something), you must code inside the var body . But it can be directly written on indirectly through a function or etc.