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AudioFade.cs

using UnityEngine;
using System.Collections;
using System;

[RequireComponent( typeof(AudioSource))]
public class AudioFade : MonoBehaviour
{
  
	int volume = 0;
	
#region Unity_delegate
	void OnEnable ()
	{
		StopAllCoroutines();
		if( gameObject.activeSelf )
			StartCoroutine (Fadein ());
	}
	
	void OnDisable ()
	{
		StopAllCoroutines();
		if( gameObject.activeSelf )
			StartCoroutine (Fadeout ());
	}
#endregion
	
	IEnumerator Fadein ()
	{
		audio.enabled = true;
		for (; volume<100; volume++) {
			audio.volume = (float)volume / 100;
			yield return null;
		}
		audio.volume = 1;
	}
	
	IEnumerator Fadeout ()
	{
		for (; volume>0; volume--) {
			audio.volume =  (float)(volume) / 100;
			yield return null;
		}
		audio.volume = 0;
		audio.enabled = false;
	}
}