ObservableContentView.swift

import SwiftUI

struct ContentView: View {
    @State var model = ViewModel.shared
    var body: some View {
        VStack {
            Slider(value:
                    $model.width, in: 1...200)
            Rectangle()
                .frame(maxWidth: model.width, maxHeight: .infinity)
                .foregroundColor(.red)
        }
    }
}

#Preview { ContentView() }