如何在flutter inappwebview中添加文件下载器

dependencies:
  flutter_inappwebview: ^5.3.2

import 'package:flutter/material.dart';
import 'package:flutter_inappwebview/flutter_inappwebview.dart';

class WebViewPage extends StatefulWidget {
  @override
  _WebViewPageState createState() => _WebViewPageState();
}

class _WebViewPageState extends State<WebViewPage> {
  InAppWebViewController? webViewController;
  String url = 'https://example.com';

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('WebView'),
      ),
      body: InAppWebView(
        initialUrlRequest: URLRequest(url: Uri.parse(url)),
        onWebViewCreated: (controller) {
          webViewController = controller;
        },
      ),
    );
  }
}

ElevatedButton(
  onPressed: () {
    downloadFile('https://example.com/file.pdf');
  },
  child: Text('Download File'),
),

void downloadFile(String fileUrl) async {
  if (webViewController != null) {
    await webViewController!.evaluateJavascript(
      source: "window.location.href = '$fileUrl';",
    );
  }
}