SwiftUI view modifier that constrains the width of a view to be no bigger than the readable width.

ReadableContentWidth.swift

import SwiftUI

/// View modifier that constrains the width of a view to be no bigger than the readable width.
struct ReadableContentWidth: ViewModifier {
  private let measureViewController = UIViewController()

  @State private var orientation: UIDeviceOrientation = UIDevice.current.orientation

  func body(content: Content) -> some View {
    content
      .frame(maxWidth: readableWidth(for: orientation))
      .onReceive(NotificationCenter.default.publisher(for: UIDevice.orientationDidChangeNotification)) { _ in
        orientation = UIDevice.current.orientation
      }
  }

  private func readableWidth(for orientation: UIDeviceOrientation) -> CGFloat {
    measureViewController.view.frame = UIScreen.main.bounds
    let readableContentSize = measureViewController.view.readableContentGuide.layoutFrame.size
    return readableContentSize.width
  }
}

extension View {
  func readableContentWidth() -> some View {
    modifier(ReadableContentWidth())
  }
}