Flutter evaluateJavascript Not working for me

You can use MutationObserver to Observe changes only within the specific container or element where the button is expected to appear. This reduces the number of observed nodes and improves performance

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

class MyHomePage extends StatefulWidget {
  const MyHomePage({super.key});

  @override
  State<MyHomePage> createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  InAppWebViewController? _webViewController;

  @override
  void initState() {
    super.initState();
  }

  @override
  void dispose() {
    // Clean up resources (e.g., disconnect observers)
    if (_webViewController != null) {
      _webViewController!.clearCache(); 
    }
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: InAppWebView(
        initialUrlRequest: URLRequest(url: Uri.parse('your_web_page_url')),
        onWebViewCreated: (controller) {
          _webViewController = controller;
        },
        onPageFinished: (controller, url) async {
          await controller.evaluateJavascript(source: """
            const targetNode = document.querySelector('#your-target-container'); // Replace with the actual container ID
            const config = { childList: true, subtree: true };

            const observer = new MutationObserver(mutations => {
              mutations.forEach(mutation => {
                if (mutation.type === 'childList') {
                  const button = document.querySelector("a[href^='/GenericUpload/GetFile']");
                  if (button) {
                    button.addEventListener('click', () => {
                      window.flutter_inappwebview.callHandler('MobileAppDownload', button.getAttribute('href'));
                    });
                    observer.disconnect(); // Stop observing after the button is found
                  }
                }
              });
            });

            if (targetNode) {
              observer.observe(targetNode, config);
            }
          """);
        },
      ),
    );
  }
}