SwiftUI relative frame

View+Relative.swift

struct SizePreferenceKey: PreferenceKey {
    static var defaultValue: CGSize = .zero

    static func reduce(value: inout CGSize, nextValue: () -> CGSize) {
        value = nextValue()
    }
}

struct RelativeSizeModifier: ViewModifier {
    let percentWidth: CGFloat

    @State private var contentSize: CGSize = .zero

    func body(content: Content) -> some View {
        GeometryReader { geometry1 in
            content
                .frame(width: geometry1.size.width * percentWidth)
                .background(
                    GeometryReader { geometry2 in
                        Color.green
                            .preference(key: SizePreferenceKey.self, value: geometry2.size)
                    }
                )
                .frame(maxWidth: .infinity)
        }
        .frame(height: contentSize.height)
        .onPreferenceChange(SizePreferenceKey.self) { self.contentSize = $0 }
    }
}

extension View {
    func relative(width: CGFloat) -> some View {
        self.modifier(RelativeSizeModifier(percentWidth: width))
    }
}