I'm so confused: why @State var inside a View do not change when you call a method on this view from some outer View?

Using Swift
10

I don't like having the parent view passing in a binding to a @State var it owns, just so to kick start something to happen inside the child view. It seems wrong to me. So here is what I end up:

import SwiftUI

struct BarView: View {
    
    struct Foo: View {
        class Notifier: ObservableObject {
            @Published var counter = 0
        }
        @ObservedObject private var notifier = Notifier()
        @State private var animationFlag = false
        static private let animationDuration = 0.5
        
        
        // trigger some action when this is called
        func doFoo() {
            notifier.counter = 1
        }
        
        var body: some View {
            VStack {
                ZStack {
                   Text(verbatim: "🤔").font(.system(size: 150))
                        .scaleEffect(animationFlag ? 1.2 : 1)
                        .animation(Animation.linear(duration: Self.animationDuration))
                }
                .frame(width: 200, height: 200)
                Text(verbatim: "self.flag = \(self.animationFlag)")
            }
            .onReceive(notifier.objectWillChange) { _ in
                self.animationFlag = true
                DispatchQueue.main.asyncAfter(deadline: .now() + Self.animationDuration) {
                    self.animationFlag = false
                }
            }
        }
    }
    
    let foo = Foo()    // seems okay to do this way?
    
    var body: some View {
        VStack {
            foo
            Spacer()
            Button("Action!...") {
                self.foo.doFoo()
            }
        }
    }
}

struct ConfuseMe_Previews: PreviewProvider {
    static var previews: some View {
        BarView()
    }
}

So it's back to how I wanted originally. But instead use an @ObservedObject to indirectly cause the internal @State var change.

I wonder, the way I originally try to change the @State var, which is wrong, why SwiftUI does not catch it and print some error message...