read response from redirection url using webView in flutter

I was trying to add a payment gateway using WebView flutter library.On successful completion of payment, the gateway redirects to a return url or webhook with transaction response. Now instead of redirection to some outside url, I want redirection to my app and want to read the status of transaction from response that is sent.
All I want to know whether transaction is success or fail.

”’

WebView(
  navigationDelegate: (action) {
    return NavigationDecision.navigate;
  },
  onPageStarted: (url) => _onPageStart(url),
  onPageFinished: (url) => _onPageFinish(url),
   gestureNavigationEnabled: true,
  debuggingEnabled: true,
  javascriptMode: JavascriptMode.unrestricted,
  initialUrl: url,
  onWebViewCreated: (WebViewController webcontroller) {
    _controller = webcontroller;
  },
),

void _onPageStart(url) {
  Future<String> future = _controller
      .runJavascriptReturningResult("window.document.body.outerHTML");
  future.then((data) {
    print("ONpage start $data");
  });
}


Future<void> _onPageFinish(url) async {
    Future<String> future = _controller
        .runJavascriptReturningResult("window.document.body.outerHTML");
    future.then((data) {
      print(data);
    });
  }

”’