StateObject alternative on iOS 13 / macOS Catalina

StateWrappedView.swift

import SwiftUI

protocol ViewModelContainable: View {
    
    associatedtype ViewModel : ObservableObject
    
    init(model: ViewModel)
}

// This struct is a direct MVVM alternative to @StateObject in iOS 14 and Mac OS Big Sur.
struct StateWrapped<U: ViewModelContainable, T> : View
    where U.ViewModel == T
{
    @State var model: T
    
    var body : some View {
        U(model: model)
    }
}

Thanks! Relevant discussion: https://twitter.com/dentelezhkin/status/1308774256050941954?s=21

Hi Denys!
I gave your gist a try in the following way:

struct FooView: ViewModelContainable {
    @ObservedObject var model: FooViewModel

    init(model: FooViewModel) {
        self.model = model
    }

    var body: some View {
        ZStack {
            model.color.edgesIgnoringSafeArea(.all)
            Button("Show sheet") {
                model.isShowingSheet = true
                DispatchQueue.main.asyncAfter(deadline: .now() + .seconds(1)) {
                    model.color = .yellow
                }
            }
        }
        .sheet(isPresented: $model.isShowingSheet) {
            StateWrapped<BarView, BarViewModel>(model: .init())
        }
    }
}

class FooViewModel: ObservableObject {
    @Published var color = Color.red
    @Published var isShowingSheet = false {
        didSet {
            if !isShowingSheet {
                color = .red
            }
        }
    }
}

struct BarView: ViewModelContainable {
    @ObservedObject var model: BarViewModel

    init(model: BarViewModel) {
        self.model = model
    }

    var body: some View {
        model.color.edgesIgnoringSafeArea(.all)
    }
}

class BarViewModel: ObservableObject {
    init() {
        print("New BarViewModel")
    }

    @Published var color = Color.green
}

Unfortunately, I still see "New BarViewModel" twice, which doesn't happen using @StateObject... Am I missing something?

@cicerocamargo Yep, that's unfortunate :( This trick seems to work only for root object in view hierarchy. SwiftUI does not understand that BarViewModel does not need to be recreated when using this approach.

The only workaround that comes in my mind is that you can create BarViewModel inside FooViewModel and push it down to BarView when it's needed:

.sheet(isPresented: $model.isShowingSheet) {
    BarView(model: model.barViewModel)
}

That's not pretty, and will lead to immediate creation of BarViewModel, instead of delaying it to actual sheet presentation, but I don't see another option here. Seems that we need StateObject to do better.

If you just want a propertyWrapper that simply replaces @StateObject: https://gist.github.com/Amzd/8f0d4d94fcbb6c9548e7cf0c1493eaff