SpringJam2026/Assets/Scripts/Management/BGM.cs

using System.Collections;
using UnityEngine;

namespace Management
{
    /// <summary>
    /// Singleton pattern object that holds an audio source, designed as single source of
    /// truth and driver for the BGM music.
    /// </summary>
    [RequireComponent(typeof(AudioSource))]
    public class BGM : MonoBehaviour
    {
        // hold a static reference to enforce singleton pattern
        private static BGM _instance;
        
        // the audiosource that will play the music
        private AudioSource _audioSource;
        
        // get the original volume set in the editor so when we fade in/out we know
        // what volume to return to.
        private float _originalVolume;
        
        // Fades out the currently playing music if playing, then fades in
        // the provided clip over the amount of time specified.
        // if t == 0 swap is instant.
        public void FadeIn(AudioClip clip, float t = 1f)
        {
            if (clip == null)
            {
                Debug.LogWarning("BGM: Selected clip is null");
                return;
            }
            
            StopAllCoroutines();
            if (t <= 0)
            {
                _audioSource.Stop();
                _audioSource.clip = clip;
                _audioSource.Play();
                _audioSource.volume = _originalVolume;
            }
            else
            {
                StartCoroutine(FadeInRoutine(clip, t));   
            }
        }

        private IEnumerator FadeInRoutine(AudioClip clip, float t)
        {
            var timer = 0f;
            var startVolume = _audioSource.volume;
            var fadeInTarget = _audioSource.isPlaying ? t / 2f : t; 
            
            if (_audioSource.isPlaying)
            {
                while (timer < t / 2f)
                {
                    timer += Time.deltaTime;
                    _audioSource.volume = Mathf.Lerp(startVolume, 0f, timer / (t / 2f));
                    yield return null;
                }

                _audioSource.volume = 0f;
                timer = 0f;
            }

            _audioSource.Stop();
            _audioSource.clip = clip;
            _audioSource.Play();
            
            while (timer < fadeInTarget)
            {
                timer += Time.deltaTime;
                _audioSource.volume = Mathf.Lerp(0f, _originalVolume, timer / fadeInTarget);
                yield return null;
            }
            _audioSource.volume = _originalVolume;
        }
        
        private void Awake()
        {
            // enforce singleton
            if (_instance != null && _instance != this)
            {
                Destroy(gameObject);
                return;
            }
            
            _instance = this;
            _audioSource = GetComponent<AudioSource>();
            _originalVolume = _audioSource.volume;
            DontDestroyOnLoad(gameObject);
        }
    }
}