/// <summary>
/// call in AssetProvider
/// </summary>
/// <param name="item"></param>
internal void Add(AssetProvider item)
{
Debug.Assert(providers.Contains(item) == false);
providers.Add(item);
}
/// <summary>
/// call in AssetProvider
/// </summary>
/// <param name="item"></param>
internal void Remove(AssetProvider item)
{
providers.Remove(item);
}