ReflectiveUI.swift

import SwiftUI
import CoreMotion
import PlaygroundSupport

struct ContentView: View {
    /// モーションマネージャー
    private let motionManager: CMMotionManager = .init()

    /// 角度
    @State var angle: Angle = .init()

    /// Body
    var body: some View {
        ZStack {
            Circle()
                .fill(AngularGradient(gradient: .init(colors: [.gray, .black, .gray]), center: .center, angle: angle))
        }
        .frame(width: 300, height: 300)
        .onAppear {
            if motionManager.isDeviceMotionAvailable {
                motionManager.startDeviceMotionUpdates(to: .current!) { motion, _ in
                    guard let motion else { return }
                    angle = get(angle: motion.attitude)
                }
            }
        }
    }

    /// スクリーンの傾き角度を検出
    /// - Parameter attitude: 姿勢
    /// - Returns: 傾き角度
    private func get(angle attitude: CMAttitude) -> Angle {
        var pitch = attitude.pitch
        var roll = attitude.roll
        // pitchとrollの値をデバイスの向きに基づいて変換する
        switch UIDevice.current.orientation {
        case .portraitUpsideDown:
            pitch = -pitch
            roll = -roll
        case .landscapeLeft:
            let tmp = pitch
            pitch = -roll
            roll = tmp
        case .landscapeRight:
            let tmp = pitch
            pitch = roll
            roll = -tmp
        default:
            break
        }
        return .radians(atan2(pitch, roll))
    }
}

PlaygroundPage.current.needsIndefiniteExecution = true
PlaygroundPage.current.setLiveView(ContentView())