オーディオのフェードイン・フェードアウトを DOTween を使わずに実装する（ブログ公開用）

SoundManager.cs

using System;
using System.Collections;
using UnityEngine;
using UnityEngine.Audio;

// Singleton として持ち続ける
public class SoundManager : MonoBehaviour {
    static SoundManager instance;

    public AudioMixerGroup BGMMixerGroup;
    public AudioMixerGroup SEMixerGroup;

    GameObject soundObject;
    AudioMixer mixer;
    AudioSource bgmSource;
    FadeComponent fadeComponent;

    static string bgmVolumeParam = "BGMVolume";
    static string seVolumeParam = "SEVolume";
    static float defaultVolume = 0.8f; // 0 dB

    void Awake() {
        if (instance == null) {
            instance = this;
            mixer = BGMMixerGroup.audioMixer;
            createSoundObject();
            DontDestroyOnLoad(this.gameObject);
        } else {
            Destroy(this.gameObject);
        }
    }

    void createSoundObject() {
        soundObject = new GameObject("SoundObject");
        GameObject.DontDestroyOnLoad(soundObject);
        bgmSource = soundObject.AddComponent<AudioSource>();
    }

    public static void PlayBGM(string path) {
        instance.playBGM(path);
    }

    void playBGM(string path) {
        if (fadeComponent != null) fadeComponent.Stop();
        SetBGMMixerVolume(defaultVolume);

        var clip = Resources.Load<AudioClip>(path);
        bgmSource.clip = clip;
        bgmSource.outputAudioMixerGroup = BGMMixerGroup;
        bgmSource.loop = true;
        bgmSource.Play();
        Resources.UnloadUnusedAssets();
    }

    public static void SetBGMMixerVolume(float volume = 0.8f) {
        instance.mixer.SetFloat(bgmVolumeParam, volumeToDecibel(volume));
    }

    public static void SetSEMixerVolume(float volume = 0.8f) {
        instance.mixer.SetFloat(seVolumeParam, volumeToDecibel(volume));
    }

    static float volumeToDecibel(float volume) {
        return volume * 100 - 80; // -80dB ~ +20dB
    }

    public static void FadeInBGM(float fadeDuration = 2.0f) {
        instance.fadeStart(FadeComponent.ModeSet.FadeIn, defaultVolume, fadeDuration);
    }

    public static void FadeOutBGM(float fadeDuration = 2.0f) {
        instance.fadeStart(FadeComponent.ModeSet.FadeOut, defaultVolume, fadeDuration);
    }

    void fadeStart(int fadeMode, float maxVolume = 0.8f, float fadeDuration = 2.0f) {
        fadeComponent = soundObject.AddComponent<FadeComponent>();
        fadeComponent.Mode = fadeMode;
        fadeComponent.MaxVolume = maxVolume;
        fadeComponent.FadeDuration = fadeDuration;
        fadeComponent.Start();
    }

    public class FadeComponent : MonoBehaviour {
        public float MaxVolume;
        public float FadeDuration;

        public int Mode;
        public struct ModeSet {
            public static int FadeIn = 1, FadeOut = 2;
        }

        float startVolume;
        float endVolume;
        float elapsedTime; // 経過時間

        public void Start() {
            StartCoroutine(fadeCoroutine());
        }

        public void Stop() {
            StopCoroutine(fadeCoroutine());
            Destroy(this);
        }

        IEnumerator fadeCoroutine() {
            if (Mode == ModeSet.FadeIn) {
                startVolume = 0;
                endVolume = MaxVolume;
            } else if (Mode == ModeSet.FadeOut) {
                startVolume = MaxVolume;
                endVolume = 0;
            }

            while (elapsedTime <= FadeDuration) {
                elapsedTime += Time.deltaTime;
                float volume = Mathf.Lerp(startVolume, endVolume, elapsedTime / FadeDuration);
                SoundManager.SetBGMMixerVolume(volume);
                yield return null;
            }

            Destroy(this);
        }
    }
}