function createXMLHTTP()
{
    if (typeof XMLHttpRequest == "undefined" && window.ActiveXObject)
    {
        var arrSignatures = ["MSXML2.XMLHTTP.5.0", "MSXML2.XMLHTTP.4.0", 
                             "MSXML2.XMLHTTP.3.0", "MSXML2.XMLHTTP",
                             "Microsoft.XMLHTTP"];

        for (var i=0; i < arrSignatures.length; i++)
        {
            try
            {
                var oRequest = new ActiveXObject(arrSignatures[i]);
                return oRequest;
            }
            catch (oError)
            {
                // ignore
            }
        }

        throw new Error("MSXML is not installed on your system.");

    }
    else
    {
        var oRequest = new XMLHttpRequest();
        return oRequest;
    }
}


function checkUsername()
{
    var strUsername = document.getElementById("username").value;

    if (strUsername == "")
    {
        return false;
    }

    var oRequest = createXMLHTTP();
    document.getElementById("check_username_info").innerHTML = "<img src=/images/spinner_grey.gif style='border: none;'>";

    oRequest.onreadystatechange = function showresponse()
    {
        if (oRequest.readyState == 4)
        {
            var str = oRequest.responseText;
            if (str == 1)
            {
                document.getElementById("check_username_info").innerHTML = "<img src=/images/username_error.png style='border: none;'>用戶名已經存在,請選擇其它用戶名";
            }
            else
            {
                document.getElementById("check_username_info").innerHTML = "<img src=/images/username_ok.png style='border: none;'>用戶名可以注冊";
            }
        }
    }

    try
    {
        oRequest.open("get", "/ajax_user_check.php?uname=" + strUsername, true);
        oRequest.send(null);
    }
    catch (oOpenError)
    {
        alert("Open Error");
    }

    return true;
}