GrowthBookSDK initializeSDK(Map<String, String> userAttr) {
  GrowthBookSDK growthBookSDK = GBSDKBuilderApp(
          apiKey: "this is api key",
          hostURL: 'https://cdn.growthbook.io/api/features/this is api key',
          attributes: userAttr,
          growthBookTrackingCallBack: (experiment, experimentResult) {
            /// Track feature.
          })
      .initialize()
    ..afterFetch = () {
      print('success fetch');
    };
  print(growthBookSDK.feature("param").value);
  return growthBookSDK;
}