SwiftUI view modifier that performs action whenever accelerometer-based UIDeviceOrientation changes

SwiftUI_DeviceOrientationObservingViewModifier.swift

import CoreMotion
import SwiftUI

struct DeviceOrientationObservingViewModifier: ViewModifier {
  let onChange: (UIDeviceOrientation) -> Void

  func body(content: Content) -> some View {
    content.background(DeviceOrientationObservingView(onChange: onChange))
  }
}

extension View {
  func onDeviceOrientationChange(perform: @escaping (UIDeviceOrientation) -> Void) -> some View {
    modifier(DeviceOrientationObservingViewModifier(onChange: perform))
  }
}

private struct DeviceOrientationObservingView: View {
  let onChange: (UIDeviceOrientation) -> Void
  @StateObject var observer = DeviceOrientationObserver()

  var body: some View {
    EmptyView().onChange(of: observer.value) { orientation in
      onChange(orientation)
    }
  }
}

private final class DeviceOrientationObserver: ObservableObject {
  init() {
    manager.startAccelerometerUpdates(to: OperationQueue.main) { [weak self] data, error in
      guard error == nil, let acceleration = data?.acceleration else {
        self?.value = .unknown
        return
      }
      self?.value = acceleration.deviceOrientation
    }
  }

  @Published var value: UIDeviceOrientation = .unknown

  deinit {
    manager.stopAccelerometerUpdates()
  }

  let manager = CMMotionManager()
}

private extension CMAcceleration {
  var deviceOrientation: UIDeviceOrientation {
    if z <= -0.95 { return .faceUp }
    if z >= 0.95 { return .faceDown }

    switch atan2(y, -x) {
    case -2.25 ... -0.75: return .portrait
    case -0.75 ... 0.75: return .landscapeRight
    case 0.75 ... 2.25: return .portraitUpsideDown
    default: return .landscapeLeft
    }
  }
}