info_tren/lib/utils/webview_invoke.dart

import 'dart:convert';
import 'dart:io' show Platform;

import 'package:flutter/foundation.dart';
import 'package:webview_flutter/webview_flutter.dart';

/// Evaluates a JavaScript function on the given WebView.
/// 
/// The JavaScript function must return a String.
/// 
/// On Android, the `String` resulted from the evaluation 
/// is JSON parsed. On iOS, the `String` is returned as is.
/// 
/// Other platforms are not supported. The returned value
/// in this case will be `null`.
Future<String> wInvoke({
  @required WebViewController webViewController, 
  @required String jsFunctionContent,
  bool isFunctionAlready = false
}) async {
  final actualJS = isFunctionAlready ? 
  jsFunctionContent :
  """
    (() => {
      $jsFunctionContent
    })()
  """;

  final res = await webViewController.evaluateJavascript(actualJS);

  if (Platform.isAndroid) return JsonDecoder().convert(res) as String;
  else if (Platform.isIOS) return res;
  else return null;
}