InitialRendering.swift

class ParentViewModel: ObservableObject {
    @Published var counter1: Int = 0
    @Published var counter2: Int = 0
}

/// When using an `ObservableObject`, the first mutation of any `@Published` property
/// triggers a full re-computation of the parent view and all of its child views.
/// This initial update causes both `CounterView` instances to lose their previous
/// identity and re-render. After this first invalidation pass, SwiftUI appears to
/// stabilize the view identities, and subsequent updates only re-render the
/// `CounterView` whose `Binding` actually changed.
struct ParentWithViewModel: View {
    @StateObject var viewModel = ParentViewModel()
    
    var body: some View {
        VStack(spacing: 24.0) {
            CounterView(name: "counter 1", count: $viewModel.counter1)
                .border(Color.red)
            CounterView(name: "counter 2", count: $viewModel.counter2)
                .border(Color.blue)
        }
    }
}

/// The classic "demystifying" example without `ObservableObject` works as expected
/// only the relevant child view is re-rendered even in first mutations.
struct ParentWithState: View {
    @State private var counter1: Int = 0
    @State private var counter2: Int = 0
    
    var body: some View {
        VStack(spacing: 24.0) {
            CounterView(name: "counter 1", count: $counter1)
                .border(Color.red)
            CounterView(name: "counter 2", count: $counter2)
                .border(Color.blue)
        }
    }
}

struct CounterView: View {
    let name: String
    @Binding var count: Int

    var body: some View {
        if #available(iOS 15, *) {
            let _ = print("🤖", #function, "name:", name, Self._printChanges())
        }
        VStack {
            Text("name: \(name)")
            Text("count: \(count)" )
            HStack {
                Button("Increase") {
                    print("🤖", #function, "counter: \(name) increasing")
                    count += 1
                }
                
                Button("Decrease") {
                    print("🤖", #function, "counter: \(name) decreasing")
                    count -= 1
                }
            }
        }
        
    }
}