问Ajax自动完成无法设置TextBox值

EN

<link href="http://ajax.googleapis.com/ajax/libs/jqueryui/1.8.1/themes/base/jquery-ui.css" rel="stylesheet" type="text/css"/>
<script type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/jquery/1.4.2/jquery.min.js"></script>
<script type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/jqueryui/1.8.1/jquery-ui.min.js"></script>  
<script type="text/javascript">
    $(document).ready(function () {
        SearchText();
    });
    //data: "{'tagName':'" + document.getElementById('txtSearchDir').value + "'}",  here tagName should be same to the variable name in fn
    function SearchText() {
        $("#txtSearchDir").autocomplete({
            source: function (request, response) {
                //alert('inside');
                $.ajax({
                    type: "POST",
                    contentType: "application/json; charset=utf-8",
                    url: "SearchPage.aspx/autoTagSearch",
                    data: "{'tagName':'" + document.getElementById('txtSearchDir').value + "'}",
                    dataType: "json",
                    success: function (data) {
                    response(data.d);
                    autoFocus: true
                    },
                    error: function (result) {
                    document.getElementById('txtSearchDir').value = "";
                       // alert(url);
                    },
                });
            }
        });
    }