WidthReaderView: Read the width using a GeometryReader background, pass it out using a PreferenceKey, to be able to compute the height of a view based on the available width

WidthReaderView.swift

import SwiftUI

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

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

struct WidthReaderView: View {
    @Binding var width: CGFloat

    var body: some View {
        GeometryReader { geometry in
            Color.clear.preference(
                key: SizePreferenceKey.self,
                value: geometry.size
            )
        }
        .onPreferenceChange(SizePreferenceKey.self) { size in
            self.width = size.width
        }
    }
}

struct WidthReaderExampleView: View {
    @State var width : CGFloat = 0

    var body: some View {
        Text("Hello \(width)")
            .frame(height: width / 2)
            .background(.yellow)
            .background(WidthReaderView(width: $width))
    }
}

#Preview {
    WidthReaderExampleView()
}