PassViewUsingPreference.swift

import SwiftUI

struct ContentView: View {
    var body: some View {
        VStack {
            ChildView()
            ChildView()
        }
        .padding()
        .overlayPreferenceValue(ViewPreferenceKey.self) { $0 }
    }
}

struct ChildView: View {
    @State private var showInParent = false

    var body: some View {
        Toggle("Show in parent", isOn: $showInParent)
            .preferenceView {
                if showInParent {
                    Text("👋").font(.system(size: 64))
                }
            }
    }
}

struct ViewPreferenceKey: PreferenceKey {
    static var defaultValue: AnyView = AnyView(EmptyView())

    static func reduce(value: inout Value, nextValue: () -> Value) {
        value = AnyView(VStack {
            value
            nextValue()
        })
    }
}

extension View {
    func preferenceView(@ViewBuilder view: () -> some View) -> some View {
        self.preference(key: ViewPreferenceKey.self, value: AnyView(view()))
    }
}