Unity上からiOS端末のシステムボリュームを取得する

AVAudioSessionBridge.cs

using System.Runtime.InteropServices;

namespace iOSNative
{
    /// <summary>
    /// `AVAudioSession`のBridge
    /// </summary>
    /// <remarks>
    /// - https://developer.apple.com/documentation/avfaudio/avaudiosession
    /// </remarks>
    public static class AVAudioSessionBridge
    {
        /// <summary>
        /// システムボリューム
        /// </summary>
        /// <remarks>
        /// NOTE:
        ///  - 値は[0f~1f]の範囲
        ///  - iOS実機以外は常に1fを返す
        /// </remarks>
        public static float OutputVolume
        {
            get
            {
#if !UNITY_EDITOR && UNITY_IOS
                return OutputVolumeNative();
#else
                return 1f;
#endif
            }
        }

        #region P/Invoke

        [DllImport("__Internal", EntryPoint = "outputVolume")]
        static extern float OutputVolumeNative();

        #endregion P/Invoke
    }
}

AVAudioSessionBridge.mm

#import <Foundation/Foundation.h>
#import <AVFoundation/AVFoundation.h>

#ifdef __cplusplus
extern "C" {
#endif

// システムボリュームの取得
// NOTE: 値は[0f~1f]の範囲
// ref: https://developer.apple.com/documentation/avfaudio/avaudiosession/1616533-outputvolume?language=objc
float outputVolume() {
    return [[AVAudioSession sharedInstance] outputVolume];
}

#ifdef __cplusplus
}
#endif