if系modifierがStateを壊す例

import SwiftUI

public extension View {
    func extend<Content: View>(@ViewBuilder transform: (Self) -> Content) -> some View {
        transform(self)
    }

    func `if`<T: View, F: View>(_ condition: Bool, @ViewBuilder _ then: (Self) -> T, @ViewBuilder `else`: (Self) -> F) -> some View {
        condition ? ViewBuilder.buildEither(first: then(self)) : ViewBuilder.buildEither(second: `else`(self))
    }
}

struct ContentView: View {
    @State var toggle = false

    var body: some View {
        VStack {
            TextView()

            Button {
                toggle.toggle()
            } label: {
                Text("toggle if")
            }
        }
        .if(toggle) { content in
            content.overlay {
                Text("true condition")
            }
        } else: { content in
            content
        }
//        .extend { content in
//            if toggle {
//                content.overlay {
//                    Text("true condition")
//                }
//            } else {
//                content
//            }
//        }
    }
}

final class ViewModel: ObservableObject {
    @Published var count = 0
}

struct TextView: View {
    @StateObject var viewModel: ViewModel

    init() {
        _viewModel = .init(wrappedValue: ViewModel())
    }

    var body: some View {
        let _ = Self._printChanges()
        Button {
            viewModel.count += 1
        } label: {
            Text("count: \(viewModel.count)")
        }
    }
}