title: Wait until Refrotfit return success or failure
body: <p>I have login button that when user click on it , I call Api-Service ( I use retrofit ) to check is he/she signed up or not , something like this </p>
  
  <pre><code>    private void loginUserService() {
      ServiceHelper.getInstance().loginUser(usernameEdtTxt.getText().toString(), passwordEdtTxt.getText().toString(), new Callback&lt;String&gt;() {
          @Override
          public void success(String s, Response response) {
              if (!TextUtils.isEmpty(s)) {
                  isLoginSuccessfull = true;
              }
          }
  
          @Override
          public void failure(RetrofitError retrofitError) {
              isLoginSuccessfull = false;
          }
      });
  
  }
  </code></pre>
  
  <p>How can i wait until this process finished and return proper value ? ( while this call is async )</p>
  
  <p>My last try : I put this method in AsynTask and get return value from OnPostExecute but it's seems not works fine !</p>