final Completer<WebViewController> _webViewController =
      Completer<WebViewController>();

  Widget getWebView() {
    var url = "https://$_userPool" +
        ".amazoncognito.com/oauth2/authorize?identity_provider=Google&redirect_uri=" +
        "myapp://&response_type=CODE&client_id=$clientId" +
        "&scope=email%20openid%20profile%20aws.cognito.signin.user.admin";
    return WebView(
      initialUrl: url,
      userAgent: 'Mozilla/5.0 (Linux; Android 6.0; Nexus 5 Build/MRA58N) ' +
          'AppleWebKit/537.36 (KHTML, like Gecko) Chrome/62.0.3202.94 Mobile Safari/537.36',
      javascriptMode: JavascriptMode.unrestricted,
      onWebViewCreated: (WebViewController webViewController) {
        _webViewController.complete(webViewController);
      },
      navigationDelegate: (NavigationRequest request) {
        if (request.url.startsWith("myapp://?code=")) {
          String code = request.url.substring("myapp://?code=".length);
          signUserInWithAuthCode(code);
          return NavigationDecision.prevent;
        }

        return NavigationDecision.navigate;
      },
      gestureNavigationEnabled: true,
    );
  }

  Future signUserInWithAuthCode(String authCode) async {
    String url = "https://$_userPool" +
        ".amazoncognito.com/oauth2/token?grant_type=authorization_code&client_id=" +
        "$clientId&code=" +
        authCode +
        "&redirect_uri=myapp://";
    final response = await http.post(Uri.parse(url),
        body: {},
        headers: {'Content-Type': 'application/x-www-form-urlencoded'});
    if (response.statusCode != 200) {
      throw Exception("Received bad status code from Cognito for auth code:" +
          response.statusCode.toString() +
          "; body: " +
          response.body);
    }
  }


now inside sign in with google button

GestureDetector(
      onTap: () {
        provider.getWebView;
       },