Set frame aspect ratio - SwiftUI

View+frameRatio.swift

import SwiftUI

// MARK: - ReadSize

struct SizePreferenceKey: PreferenceKey {
  static var defaultValue: CGSize = .zero
  static func reduce(value: inout CGSize, nextValue: () -> CGSize) {}
}

extension View {
  func readSize(onChange: @escaping (CGSize) -> Void) -> some View {
    background(
      GeometryReader { geometryProxy in
        Color.clear
          .preference(key: SizePreferenceKey.self, value: geometryProxy.size)
      }
    )
    .onPreferenceChange(SizePreferenceKey.self, perform: onChange)
  }
}

// MARK: - FrameRatio

extension View {
    func frameRatio(_ aspectRatio: CGFloat) -> some View {
        self.modifier(FrameRatioModifier(aspectRatio: aspectRatio))
    }
}

struct FrameRatioModifier: ViewModifier {
    var aspectRatio: CGFloat
    @State private var viewSize: CGSize = .zero

    func body(content: Content) -> some View {
        content
            .frame(maxWidth: .infinity)
            .frame(height: viewSize.width / aspectRatio)
            .readSize { viewSize = $0 }
    }
}

// MARK: - Preview

struct FrameRatio_Previews: PreviewProvider {
    static var previews: some View {
        Color.red
            .frameRatio( 16 / 9)
            .previewLayout(.fixed(width: 500, height: 500))
    }
}