Call Javascript function in WebView in Android

Question

I am trying to create Facebook Video Downloader using Webview where one has to click on video to download it. I tried the below code. But I don't know what's wrong with this code, when I click on any video, the downloading doesn't starts. Can anybody help?

webView = (WebView) findViewById(R.id.webview);
webView.getSettings().setJavaScriptEnabled(true);
webView.addJavascriptInterface(this, "FBDownloader");
webView.setWebViewClient(new WebViewClient() {
    @Override
    public void onPageStarted(WebView view, String url, Bitmap favicon) {
        super.onPageStarted(view, url, favicon);
    }

    @Override
    public void onPageFinished(WebView view, String url) {
        MainActivity.this.webView.loadUrl("javascript:(function() { var el = document.querySelectorAll('div[data-sigil]');for(var i=0;i<el.length; i++){var sigil = el[i].dataset.sigil;if(sigil.indexOf('inlineVideo') > -1){delete el[i].dataset.sigil;var jsonData = JSON.parse(el[i].dataset.store);el[i].setAttribute('onClick', 'FBDownloader.processVideo(\"'+jsonData['src']+'\",\"'+jsonData['videoID']+'\");');}}})()");
        super.onPageFinished(view, url);
    }

    @Override
    public void onLoadResource(WebView view, String url) {
        MainActivity.this.webView.loadUrl("javascript:(function prepareVideo() { var el = document.querySelectorAll('div[data-sigil]');for(var i=0;i<el.length; i++){var sigil = el[i].dataset.sigil;if(sigil.indexOf('inlineVideo') > -1){delete el[i].dataset.sigil;console.log(i);var jsonData = JSON.parse(el[i].dataset.store);el[i].setAttribute('onClick', 'FBDownloader.processVideo(\"'+jsonData['src']+'\",\"'+jsonData['videoID']+'\");');}}})()");
        MainActivity.this.webView.loadUrl("javascript:( window.onload=prepareVideo;)()");
    }
});

CookieManager cookieManager = CookieManager.getInstance();
if (Build.VERSION.SDK_INT < Build.VERSION_CODES.LOLLIPOP) {
    CookieSyncManager.createInstance(this);
}
cookieManager.setAcceptCookie(true);
webView.loadUrl("https://www.facebook.com/login/");

processVideo() is a function used to download video when user clicks on any video in webview:

@JavascriptInterface
    public void processVideo(final String vidData, final String vidID)
    {
        try
        {
            Toast.makeText(this, "here", Toast.LENGTH_LONG).show();
            String mBaseFolderPath = android.os.Environment
                .getExternalStorageDirectory()
                + File.separator
                + "FacebookVideos" + File.separator;
            if (!new File(mBaseFolderPath).exists())
            {
                new File(mBaseFolderPath).mkdirs();
            }
            String mFilePath = "file://" + mBaseFolderPath + "/" + vidID + ".mp4";

            Uri downloadUri = Uri.parse(vidData);
            DownloadManager.Request req = new DownloadManager.Request(downloadUri);
            req.setDestinationUri(Uri.parse(mFilePath));
   req.setNotificationVisibility(DownloadManager.Request.VISIBILITY_VISIBLE_NOTIFY_COMPLETED);
            DownloadManager dm = (DownloadManager) getSystemService(getApplicationContext().DOWNLOAD_SERVICE);
            dm.enqueue(req);
            Toast.makeText(this, "Download Started", Toast.LENGTH_LONG).show();
        }
        catch (Exception e)
        {
            Toast.makeText(this, "Download Failed: " + e.toString(), Toast.LENGTH_LONG).show();
        }
    }

I am using this link - https://stackoverflow.com/a/42097873/16829808

Does anyone see something wrong here?