/// <summary>
 /// 微信许可令牌
 /// </summary>
 public class AccessToken
 {
  /// <summary>
  /// 保存已获取到的许可令牌，键为公众号，值为公众号最后一次获取到的令牌
  /// </summary>
  private static ConcurrentDictionary<string, Tuple<AccessToken, DateTime>> accessTokens = new ConcurrentDictionary<string, Tuple<AccessToken, DateTime>>();

  /// <summary>
  /// 获取access token的地址
  /// </summary>
  private const string urlForGettingAccessToken = "https://api.weixin.qq.com/cgi-bin/token?grant_type=client_credential&appid={0}&secret={1}";
  /// <summary>
  /// 获取access token的http方法
  /// </summary>
  private const string httpMethodForGettingAccessToken = WebRequestMethods.Http.Get;
  /// <summary>
  /// 保存access token的最长时间（单位：秒），超过时间之后，需要重新获取
  /// </summary>
  private const int accessTokenSavingSeconds = 7000;

  /// <summary>
  /// access token
  /// </summary>
  public string access_token { get; set; }
  /// <summary>
  /// 有效时间，单位：秒
  /// </summary>
  public int expires_in { get; set; }

  /// <summary>
  /// 构造函数
  /// </summary>
  /// <param name="_access_token">access token</param>
  /// <param name="_expires_in">有效时间</param>
  internal AccessToken(string _access_token, int _expires_in)
  {
   access_token = _access_token;
   expires_in = _expires_in;
  }

  /// <summary>
  /// 返回AccessToken字符串
  /// </summary>
  /// <returns></returns>
  public override string ToString()
  {
   return string.Format("AccessToken：{0}\r\n有效时间：{1}秒", access_token, expires_in);
  }

  /// <summary>
  /// 从JSON字符串解析AccessToken
  /// </summary>
  /// <param name="json">JSON字符串</param>
  /// <returns>返回AccessToken</returns>
  internal static AccessToken ParseFromJson(string json)
  {
   var at = JsonConvert.DeserializeAnonymousType(json, new { access_token = "", expires_in = 1 });
   return new AccessToken(at.access_token, at.expires_in);
  }

  /// <summary>
  /// 尝试从JSON字符串解析AccessToken
  /// </summary>
  /// <param name="json">JSON字符串</param>
  /// <param name="msg">如果解析成功，返回AccessToken；否则，返回null。</param>
  /// <returns>返回是否解析成功</returns>
  internal static bool TryParseFromJson(string json, out AccessToken token)
  {
   bool success = false;
   token = null;
   try
   {
    token = ParseFromJson(json);
    success = true;
   }
   catch { }
   return success;
  }

  /// <summary>
  /// 得到access token
  /// </summary>
  /// <param name="userName">公众号</param>
  /// <returns>返回access token</returns>
  public static AccessToken Get(string userName)
  {
   Tuple<AccessToken, DateTime> lastToken = accessTokens.ContainsKey(userName) ? accessTokens[userName] : null;
   AccessToken token = lastToken == null ? null : lastToken.Item1;
   DateTime refreshTime = lastToken == null ? DateTime.MinValue : lastToken.Item2;
   double ms = (DateTime.Now - refreshTime).TotalSeconds;
   if (ms > accessTokenSavingSeconds || token == null)
   {
    //尝试从微信服务器获取2次
    ErrorMessage msg;
    AccessToken newToken = GetFromWeixinServer(userName, out msg);
    if (newToken == null)
     newToken = GetFromWeixinServer(userName, out msg);
    if (newToken != null)
    {
     lastToken = new Tuple<AccessToken, DateTime>(newToken, DateTime.Now);
     accessTokens[userName] = lastToken;
     token = newToken;
    }
   }
   return token;
  }

  /// <summary>
  /// 从微信服务器获取access token
  /// </summary>
  /// <param name="userName">公众号</param>
  /// <param name="msg">从服务器返回的错误信息。</param>
  /// <returns>返回许可令牌；如果获取失败，返回null。</returns>
  private static AccessToken GetFromWeixinServer(string userName, out ErrorMessage msg)
  {
   AccessToken token = null;
   msg = new ErrorMessage(ErrorMessage.ExceptionCode, "");
   string url = string.Format(urlForGettingAccessToken, WxPayConfig.APPID, WxPayConfig.APPSECRET);
   string result;
   if(!HttpHelper.Request(url, out result, httpMethodForGettingAccessToken, string.Empty))
   {
    msg.errmsg = "从微信服务器获取响应失败。";
    return token;
   }
   if (ErrorMessage.IsErrorMessage(result))
    msg = ErrorMessage.Parse(result);
   else
   {
    try
    {
     token = AccessToken.ParseFromJson(result);
    }
    catch (Exception e)
    {
     msg = new ErrorMessage(e);
    }
   }
   return token;
  }
 }