@Test
 public void interceptorDoesAddUserAgent() throws Exception {
   Request request = new Request.Builder().url("https://test.desk.com").build();
   assertFalse(doesHaveUserAgent(request));
   OkHttpClient.Builder builder = new OkHttpClient.Builder();
   builder.interceptors().add(userAgentInterceptor);
   Call call = builder.build().newCall(request);
   Response response = call.execute();
   assertTrue(doesHaveUserAgent(response.request()));
   assertEquals(TEST_USER_AGENT, response.request().header(HEADER_USER_AGENT));
 }
  @Override
  public T call() throws Exception {
    Request request = null;
    Response response = null;
    Exception exception = null;
    Call call = null;

    try {
      MNSLog.logD("[call] - ");

      if (context.getCancellationHandler().isCancelled()) {
        throw new InterruptedIOException("This task is cancelled!");
      }

      Request.Builder requestBuilder = new Request.Builder();

      // build request url
      String url = message.buildCanonicalURL();
      MNSUtils.signRequest(message);
      requestBuilder = requestBuilder.url(url);

      // set request headers
      for (String key : message.getHeaders().keySet()) {
        requestBuilder = requestBuilder.addHeader(key, message.getHeaders().get(key));
      }

      String contentType = message.getHeaders().get(MNSHeaders.CONTENT_TYPE);
      String content = message.getContent();

      // set request body
      if (message.getContent() != null) {
        MNSUtils.assertTrue(contentType != null, "Content type can't be null when send data!");
        requestBuilder =
            requestBuilder.method(
                message.getMethod().toString(),
                new ProgressTouchableRequestBody(
                    message.getContent().getBytes(), contentType, context.getProgressCallback()));
      } else {
        switch (message.getMethod()) {
          case PUT:
            requestBuilder =
                requestBuilder.method(
                    message.getMethod().toString(), RequestBody.create(null, new byte[0]));
            break;
          case GET:
            requestBuilder = requestBuilder.get();
            break;
          case HEAD:
            requestBuilder = requestBuilder.head();
            break;
          case DELETE:
            requestBuilder = requestBuilder.delete();
            break;
          default:
            break;
        }
      }

      request = requestBuilder.build();

      if (MNSLog.isEnableLog()) {
        MNSLog.logD("request url: " + request.url());
        Map<String, List<String>> headerMap = request.headers().toMultimap();
        for (String key : headerMap.keySet()) {
          MNSLog.logD("requestHeader " + key + ": " + headerMap.get(key).get(0));
        }
      }

      call = client.newCall(request);
      context.getCancellationHandler().setCall(call);

      // send request
      response = call.execute();

      if (MNSLog.isEnableLog()) {
        MNSLog.logD("response code: " + response.code() + " for url: " + request.url());
        Map<String, List<String>> headerMap = response.headers().toMultimap();
        for (String key : headerMap.keySet()) {
          MNSLog.logD("responseHeader " + key + ": " + headerMap.get(key).get(0));
        }
      }
    } catch (Exception e) {
      MNSLog.logE("Encounter local execpiton: " + e.toString());
      if (MNSLog.isEnableLog()) {
        e.printStackTrace();
      }
      exception = new ClientException(e.getMessage(), e);
    }

    if (exception == null && (response.code() == 203 || response.code() >= 300)) {
      try {
        exception = ResponseParsers.parseResponseErrorXML(response);
      } catch (IOException e) {
        exception = new ClientException(e.getMessage(), e);
      }
    } else if (exception == null) {
      try {
        T result = responseParser.parse(response);
        if (context.getCompletedCallback() != null) {
          context.getCompletedCallback().onSuccess(context.getRequest(), result);
        }
        return result;
      } catch (IOException e) {
        exception = new ClientException(e.getMessage(), e);
      }
    }

    // reconstruct exception caused by manually cancelling
    if ((call != null && call.isCanceled()) || context.getCancellationHandler().isCancelled()) {
      exception = new ClientException("Task is cancelled!", exception.getCause(), true);
    }

    if (exception instanceof ClientException) {
      if (context.getCompletedCallback() != null) {
        context
            .getCompletedCallback()
            .onFailure(context.getRequest(), (ClientException) exception, null);
      }
    } else {
      if (context.getCompletedCallback() != null) {
        context
            .getCompletedCallback()
            .onFailure(context.getRequest(), null, (ServiceException) exception);
      }
    }
    throw exception;
  }