public void RecordSubMail(IAsyncMail mail)
 {
     if (!subMails.Contains(mail))
     {
         subMails.Add(mail);
     }
 }
 public void OnSubMailComplete(IAsyncMail mail)
 {
     if (subMails.Contains(mail))
     {
         subMails.Remove(mail);
     }
     if (subMails.Count == 0)
     {
         if (onComplete != null)
         {
             onComplete.Invoke(this);
         }
     }
 }