PreferenceKey SwiftUI

PreferenceKey.swift

import SwiftUI

struct ContentView: View {

    var body: some View {

        TabView {
            ListView()
                .tabItem {
                    Text("First")
            }

            Text("hey")
                .tabItem {
                    Text("Second")
            }

        }
        .overlayPreferenceValue(TitlePreferenceKey.self) { titles in
            VStack {
                Text(titles.last ?? "")
                Spacer()
            }
        }
    }
}

struct ListView: View {
    var body: some View {
        NavigationView {
            VStack(spacing: 36) {
                NavigationLink(destination: DetailView()) {
                    Text("Push detail view1")
                }
                NavigationLink(destination: DetailView()) {
                    Text("Push detail view2")
                }
                NavigationLink(destination: DetailView()) {
                    Text("Push detail view3")
                }
            }
            .testTitle("List View - Title")
        }
    }
}

struct DetailView: View {
    var body: some View {
        Text("Detail View")
            .testTitle("Detail View - Title")
    }
}

struct TitlePreferenceKey: PreferenceKey {
    static var defaultValue: [String] = []

    static func reduce(value: inout [String], nextValue: () -> [String]) {
        let added = nextValue()
        value.append(contentsOf: added)
    }
}

struct TitleModifier: ViewModifier {
    let title: String
    @State var currentTitle: String?

    func body(content: Content) -> some View {
        return content
            .transformPreference(TitlePreferenceKey.self) { value in
                if let title = self.currentTitle {
                    value.append(title)
                }
        }
        .onDisappear {
            self.currentTitle = nil
        }
        .onAppear {
            self.currentTitle = self.title
        }
        // This hack is helping SwiftUI to notice there was a change in the view tree.
        // There's something weird going on with NavigationView making changes not propagating back to ancestors properly.
        .background(EmptyView().tag(currentTitle))
    }
}

extension View {
    func testTitle(_ title: String) -> some View {
        return modifier(TitleModifier(title: title))
    }
}