RiftMayhem/Assets/Scripts/Audio/MusicFader.cs

using UnityEngine;
using System.Collections;
using System;

public class MusicFader : MonoBehaviour
{
    [Header("Events:")]
    [SerializeField] GameEventListener_AudioClip OnNewSceneLoaded_AudioRequest_AudioEvent;
    [Header("Component:")]
    public AudioSource audioSource;
    [Header("Settings:")]
    public float fadeDuration = 1f;

    private Coroutine fadeCoroutine;
    private AudioClip currentClip;

    float startingMaxVolume;

    private void Awake()
    {
        currentClip = null;
        OnNewSceneLoaded_AudioRequest_AudioEvent.Response.AddListener(FadeOutAndIntoNewMusic);
    }

    private void Start()
    {
        if (audioSource == null)
        {
            audioSource = GetComponent<AudioSource>();
        }
        audioSource.loop = true;
        startingMaxVolume = audioSource.volume;
    }

    public void FadeIn(Action callback = null)
    {
        StopFade();
        fadeCoroutine = StartCoroutine(FadeCoroutine(0f, startingMaxVolume, callback));
    }

    public void FadeOut(Action callback = null)
    {
        StopFade();
        fadeCoroutine = StartCoroutine(FadeCoroutine(audioSource.volume, 0f, callback));
    }

    private void StopFade()
    {
        if (fadeCoroutine != null)
        {
            StopCoroutine(fadeCoroutine);
        }
    }

    private IEnumerator FadeCoroutine(float startVolume, float targetVolume, Action callback = null)
    {
        float elapsedTime = 0f;
        audioSource.volume = startVolume;

        while (elapsedTime < fadeDuration)
        {
            elapsedTime += Time.deltaTime;
            float t = elapsedTime / fadeDuration;
            audioSource.volume = Mathf.Lerp(startVolume, targetVolume, t);
            yield return null;
        }

        audioSource.volume = targetVolume;

        if (targetVolume == 0f)
        {
            audioSource.Pause();
        }
        else
        {
            audioSource.Play();
        }

        if (callback != null)
            callback.Invoke();
    }

    public void FadeOutAndIntoNewMusic(AudioClip newMusic)
    {
        if (newMusic == currentClip)
        {
            // If it's the same music, just ensure it's playing and fade in if necessary
            if (!audioSource.isPlaying)
            {
                audioSource.Play();
                FadeIn();
            }
        }
        else
        {
            if (audioSource.clip == null)
            {
                StartNewMusic(newMusic);
                return;
            }
            FadeOut(() => StartNewMusic(newMusic));
        }
    }

    private void StartNewMusic(AudioClip newMusic)
    {
        audioSource.clip = newMusic;
        currentClip = newMusic;
        FadeIn();
    }
}