top_bar_contents.dart

FlatButton(
  color: Colors.blueGrey,
  hoverColor: Colors.blueGrey[700],
  highlightColor: Colors.blueGrey[800],
  onPressed: _isProcessing
      ? null
      : () async {
          setState(() {
            _isProcessing = true;
          });
          await signOut().then((result) {
            print(result);
            Navigator.of(context).pushReplacement(
              MaterialPageRoute(
                fullscreenDialog: true,
                builder: (context) => HomePage(),
              ),
            );
          }).catchError((error) {
            print('Sign Out Error: $error');
          });
          setState(() {
            _isProcessing = false;
          });
        },
  shape: RoundedRectangleBorder(
    borderRadius: BorderRadius.circular(15),
  ),
  child: Padding(
    padding: EdgeInsets.only(
      top: 8.0,
      bottom: 8.0,
    ),
    child: _isProcessing
        ? CircularProgressIndicator()
        : Text(
            'Sign out',
            style: TextStyle(
              fontSize: 14,
              color: Colors.white,
            ),
          ),
  ),
)