ReadFrame.swift

// Works
struct TestView1: View {
    var body: some View {
        SmartView {
            VStack {
                Text("random text")
                Text("More random text")
            }
        }
    }
}

// Doesn't work
struct TestView2: View {
    var body: some View {
        SmartView {
            VStack {
                Text("random text")
                if true {
                    Text("More random text")
                }
            }
        }
    }
}


struct SmartView<Content: View>: View {
    let content: () -> Content

    @State private var contentHeight: Length = 0

    var body: some View {
        HStack(spacing: 0) {
            Color.purple
                .frame(width: 10, height: contentHeight)
            content()
                .background(FrameReader())
                .onPreferenceChange(FramePreferenceKey.self) { self.contentHeight = $0.size.height }
            Color.purple
                .frame(width: 10, height: contentHeight)
        }
    }
}

struct FramePreferenceKey: PreferenceKey {
    static var defaultValue: CGRect = .zero

    static func reduce(value: inout CGRect, nextValue: () -> CGRect) {
        value = nextValue()
        print("FramePreferenceKey: \(value)")
    }
}

struct FrameReader: View {
    var coordinateSpace: CoordinateSpace = .global

    var body: some View {
        GeometryReader { geometry in
            Rectangle()
                .fill(Color.clear)
                .preference(key: FramePreferenceKey.self, value: geometry.frame(in: self.coordinateSpace))
        }
    }
}