showDialog(
      context: context,
      barrierDismissible: false,
      builder: (BuildContext context) => CustomDialog(),
    );


class CustomDialog extends StatelessWidget {

  @override
  Widget build(BuildContext context) {
    return Dialog(
      shape: RoundedRectangleBorder(
        borderRadius: BorderRadius.circular(16),
      ),
      elevation: 0.0,
      backgroundColor: Colors.transparent,
      child: Stack(
        children: <Widget>[
          Positioned(
            child: Align(
              alignment: Alignment.center,
              child: Material(
                color: Colors.white,
                child: Image.asset(
                  'assets/loader.gif',
                  height: 80,
                  width: 80,
                ),
                shape: CircleBorder(),
                clipBehavior: Clip.antiAlias,
              ),

            ),
          ),
        ],
      )
    );
  }

}