SwiftUI - AppStorage doesn't work with GeometryReader

Question

Here is a simple example. You can create new SwiftUI iOS project and copy it to ContentView file.

import SwiftUI

struct Settings {
    static let onOff = "onOff"
}

struct ContentView: View {
    @AppStorage(wrappedValue: false, Settings.onOff) var onOff
    
    var body: some View {
        NavigationView {
            GeometryReader { reader in // < Comment out this line
                List {
                    Section (header:
                                VStack {
                                    HStack {
                                        Spacer()
                                        VStack {
                                            Text("THIS SHOULD BE FULL-WIDTH")
                                            Text("It is thanks to GeometryReader")
                                        }
                                        Spacer()
                                    }
                                    .padding()
                                    .background(Color.yellow)
                                    
                                    HStack {
                                        Text("This should update from AppStorage: ")
                                        Spacer()
                                        Text(onOff == true ? "ON" : "OFF")
                                    }
                                    .padding()
                                }
                                .frame(width: reader.size.width) // < Comment out this line
                                .textCase(nil)
                                .font(.body)
                    ) {
                        Toggle(isOn: $onOff) {
                            Text("ON / OFF")
                        }
                    }
                    
                }
                .listStyle(GroupedListStyle())
            } // < Comment out this line
            
        }
    }
}

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

I have 3 elements:

1. Text with yellow background - I need it full-width and I use GeometryReader to do it.
2. Text. Last word should switch ON/OFF based on toggle value. This is just for testing purposes to check if AppStorage works correctly.
3. Toggle - switches onOff variable and saves it to AppStorage (UserDefaults).

AppStorage works perfectly only without GeometryReader. Please comment out 3 tagged lines to check it out.

Is it a bug? Or something is wrong with my AppStorage code? Or maybe GeometryReader part is wrong? If I could set yellow part full-width, I could drop GeometryReader completely.

Answer 1

One solution that works in my testing is to factor out the GeometryReader's content, including the @AppStorage:

struct ContentView: View {
    var body: some View {
        NavigationView {
            GeometryReader { proxy in
                _ContentView(width: proxy.size.width)
            }
        }
    }
}

struct _ContentView: View {
    var width: CGFloat
    @AppStorage(wrappedValue: false, Settings.onOff) var onOff

    var body: some View {
        List {
            Section(header: header) {
                Toggle(isOn: $onOff) {
                    Text("ON / OFF")
                }
            }

        }
        .listStyle(GroupedListStyle())
    }

    var header: some View {
        VStack {
            VStack {
                Text("THIS SHOULD BE FULL-WIDTH")
                Text("It is thanks to GeometryReader")
            }
            .padding()
            .frame(width: width)
            .background(Color.yellow)

            HStack {
                Text("This should update from AppStorage: ")
                Spacer()
                Text(onOff == true ? "ON" : "OFF")
            }
            .padding()
        }
        .textCase(nil)
        .font(.body)
    }
}

Answer 2

Yes, this is definitely a bug. Looks like AppStorage doesn't behave completely same as State for some reason, and change doesn't trigger updates inside GeometryReader. Example can be reduced to this:

struct ContentView: View {
    @AppStorage("onOff") var onOff = false

    var body: some View {
        VStack {
            Text("Text updating: " + (onOff == true ? "ON" : "OFF"))
            GeometryReader { reader in
                Toggle(isOn: $onOff) {
                    Text("Text not updating: " + (onOff == true ? "ON" : "OFF"))
                }
            }
        }
    }
}

This is not even fixed in the latest iOS 15 beta. You can try to hack with some duplicate state variable, but instead I suggest you switching to custom AppStorage, it works fine in this circumstances.