Any `Reader` views doesn't update content.

[wlsdms0122]

Problem

• Apple Developer Forum | Posted issue
• Stackoverflow | Relative issue

It seems to be a SwiftUI bug. All Reader type views (such as GeometryReader or ScrollViewReader) don't update their content even if state changes.

This is the code that reproduces the behavior.

class MainViewController: ComposableController {
    final class Environment: ObservableObject {
        @Published
        var show: Bool = false
    }
    
    // MARK: - Property
    
    // MARK: - Initializer
    init() {
        let env = Environment()
        super.init(env)
        
        run {
            VStack {
                GeometryReader { _ in
                    // Not updated
                    Text(env.show ? "A" : "B")
                }
                
                // Updated
                Text(env.show ? "AA" : "BB")
                Button("toggle") {
                    env.show.toggle()
                }
            }
        }
    }
    ...
}

Pure SwiftUI version

class ViewModel: ObservableObject {
    @Published
    var show: Bool = false
}

struct Parent: View {
    let viewModel = ViewModel()
    
    var body: some View {
        Child(viewModel) {
            VStack {
                GeometryReader { _ in
                    // Not updated
                    Text(viewModel.show ? "A" : "B")
                }
                
                // Updated
                Text(viewModel.show ? "AA" : "BB")
                Button("toggle") {
                    viewModel.show.toggle()
                }
            }
        }
    }
}

struct Child<Content: View>: View {
    var body: some View {
        content()
    }
    
    @StateObject
    var viewModel: ViewModel
    let content: () -> Content
    
    init(_ viewModel: ViewModel, content: @escaping () -> Content) {
        self._viewModel = .init(wrappedValue: viewModel)
        self.content = content
    }
}

There seems to be a problem with the state capture system for rendering. When the content closure captures a StateObject, the content closure of the reader view isn't capturing the state correctly.

Workaround

Make as sub view that include reader view.

AS-IS

VStack {
    GeometryReader { _ in
        Text(env.show ? "A" : "B")
    }
    
    Text(env.show ? "AA" : "BB")
    Button("toggle") {
        env.show.toggle()
    }
}

TO-BE

VStack {
    SubView(show: .init(
        get: { env.show },
        set: { env.show = $0 }
    ))
    
    Text(env.show ? "AA" : "BB")
    Button("toggle") {
        env.show.toggle()
    }
}

/// Or make as `struct`.
@ViewBuilder
func SubView(show: Binding<Bool>) -> some View {
    GeometryReader { _ in
        Text(show.wrappedValue ? "A" : "B")
    }
}