/* Tell the client to display a dialog to confirm navigation away from
the current page. This is the result of the onbeforeunload javascript event.
If the client returns true, WebView will assume that the client will handle
the confirm dialog and call the appropriate JsResult method. If the client
returns false, a default value of true will be returned to javascript to accept
navigation away from the current page. The default behavior is to return false.
Setting the JsResult to true will navigate away from the current page, false will
cancel the navigation.
*/
public boolean onJsBeforeUnload (WebView view, String url, String message, JsResult result){
}
от http://developer.android.com/reference/android/webkit/WebChromeClient.html
Я думаю, вы также можете внедрить код в веб-страницу, отображаемую WebView:
<script type="text/javascript">
function OnBeforeUnLoad () {
return "";
}
</script>
с использованием
@Override
public void onPageFinished(WebView view, String url)
{
webview.loadUrl("string with your script");
}