Demonstrates setting a preference based on an action

swiftui-prefs.swift

import SwiftUI

struct ContentView: View {
    var body: some View {
        WrapperView {
            VStack {
                PrefView()
                PrefView()
            }
        }
    }
}

struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ContentView()
    }
}

struct BooleanKey: PreferenceKey {
    static var defaultValue: Bool = false

    static func reduce(value: inout Bool, nextValue: () -> Bool) {
        let next = nextValue()
        guard value == false else { return }
        value = next
    }
}

struct PrefView: View {

    @State var pressed: Bool = false

    var body: some View {
        Button(action: { self.pressed.toggle() }) { Text(pressed ? "unpress me" : "press me") }
            .preference(key: BooleanKey.self, value: pressed)
    }
}

struct WrapperView<Content>: View where Content: View {

    var content: () -> Content
    @State var pressed: Bool = false

    init(@ViewBuilder content: @escaping () -> Content) {
        self.content = content
    }

    var body: some View {
        VStack {
            Text(pressed ? "pressed" : "unpressed")
            content()
        }
        .backgroundPreferenceValue(BooleanKey.self) { value -> AnyView in
            DispatchQueue.main.async { self.pressed = value }
            return AnyView(Color.clear)
        }
    }
}