I'm working with ReCaptcha, ASP.NET and Gaia Ajax. It took me some time to use the ReCaptcha AJAX APIs combined with Gaia to retrieve the contents of the recaptcha_response_field text box in AJAX postback throug开发者_StackOverflow中文版h a patch.

This was just to introduce you to the subject. Now I would like to apply another patch to ReCaptcha, without reimplementing it (a comprehensive open source library that works better than current ASP.NET implementation would be desirable, but I have no time for that): this question explains which.

Basically,

I need, after calling ReCaptcha.Create(), which renders the CAPTCHA during an AJAX postback, to hook to the OnKeyDown event of recaptcha_response_field and inject my Javascript snippet that prevents the form from being submitted.

You understand that since I don't render the <input> tag (I don't have control over it), I must hook from the external.

In general,

I think you may actually answer the general question: "how to set JavaScript event handlers programmatically?" because this surely applies to all classes of events.

Thank you

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I wouldn't recommend on-the-fly checking of captcha, because then a program could just brute force it, after figuring out the basic characters of the picture.

I always use jquery for event handlers something like:

  $(function() 
  {
    $("#{TEXTBOXID}").keydown(function(event)
    {
      alert(event.keyCode);
    });
  });

(taken from Why does JQuery keydown work for window but not textbox?)