Manual testing code for `@StateObject` backport

StateObjectBackportTest.swift

import Combine
import SwiftUI
import SwiftUIBackports

@main
struct BackportTestApp: App {
    var body: some Scene {
        WindowGroup {
            ContentView()
        }
    }
}

final class Model: ObservableObject {
    let id: String

    @Published var i = 0

    private var cancellables: Set<AnyCancellable> = []

    init(id: String) {
        self.id = id

        print("Model", id, #function)
        Timer.publish(every: 1, on: .main, in: .common)
            .autoconnect()
            .sink { [weak self] _ in self?.i += 1 }
            .store(in: &cancellables)
    }

    deinit {
        print("Model", id, #function)
    }
}

struct ContentView: View {
    @State var i = 0
    var body: some View {
        VStack(spacing: 16) {
            if #available(iOS 14.0, *) {
                StateObjectView(model: Model(id: "StateObject")).border(Color.accentColor)
            } else {
                // Fallback on earlier versions
            }

            ObservableObjectView(model: Model(id: "ObservableObject")).border(Color.accentColor)

            StateObjectBackportView(model: Model(id: "Backport.StateObject")).border(Color.accentColor)

            Button {
                i += 1
            } label: {
                Text("i \(i.description)")
            }
        }
    }
}

@available(iOS 14.0, *)
struct StateObjectView: View {
    @StateObject var model: Model

    var body: some View {
        VStack {
            Text("\(model.id) \(model.i.description)")

            BindingView(i: $model.i)
        }
    }
}

struct ObservableObjectView: View {
    @ObservedObject var model: Model

    var body: some View {
        VStack {
            Text("\(model.id) \(model.i.description)")

            BindingView(i: $model.i)
        }
    }
}

struct StateObjectBackportView: View {
    @Backport.StateObject var model: Model

    var body: some View {
        VStack {
            Text("\(model.id) \(model.i.description)")

            BindingView(i: $model.i)
        }
    }
}

struct BindingView: View {
    @Binding var i: Int

    var body: some View {
        Text("Binding \(i.description)")
    }
}