dropdownlist jquery ddl.change() 似乎在更改发生之前触发

Question

我有三个级联下拉列表。

ddlIllnessType、ddlIllnessSubType 和 ddlInfectiousAgent

最初，仅显示 ddlIllnessType。更改时，ddlIllnessSubType 会填充 json 数据并使其可见。到目前为止，一切顺利。

接下来，当用户从 ddlIllnessSubType 中选择一个值时，将在 ddlIllnessSutType 更改事件处理程序中为 ddlInfectiousAgent 运行类似的过程。然而，在下面的代码中，$(this).val() 总是显示为“未定义”，即使用户选择了一个现有值：

$("#ddlIllnessSubType").change(function() {

var selection = $(this).val();

// go and get Json based on the value of selection.  
// Doesn't work cos selection == 'undefined'
var url = "/IllnessDetail/CascadedDdlInfectiousAgent/" + selection;

getJSON(url, function(data) {...});
...
});

为什么选择 == 'undefined'?????!

提前致谢

安德鲁

PS：完整的jQuery和HTML如下：

完整的jQuery代码是：

<script type="text/javascript">

    $('document').ready(function() {
        var pEst = $("#pEncephalitisSubType");
        var pIa = $("#pInfectiousAgent");
        var ddlEst = $("#IdEncephalitisSubType");
        var ddlIa = $("#IdInfectiousAgent");

        $('#SubTypeContainer').hide();

        pEst.hide();
        pIa.hide();
//        debugger
        $('select').each(function() {
            $(this).val(""); //alert($(this).val()); 
        });

        // Change Event Handler
        $("#IdEncephalitisType").change(function() {
            var selection = $(this).val();
            pEst.fadeOut('slow');
            pIa.fadeOut('slow');
            ddlEst.val("");
            ddlIa.val("");

            if (selection == 0) {
                $('#SubTypeContainer').fadeOut('slow');
            }
            else {
                var url = "/Members/IllnessDetail/CascadedDdlSubType/" + selection;
                AjaxEncephalitisSubTypes(url);
            }
        });

        // Change Event Handler
        $("#IdEncephalitisSubType").change(function() {
            var selection = $(this).val();
            debugger
            pIa.fadeOut('slow');
            ddlIa.val("");
            if (selection != "") {
                if (($("#IdEncephalitisType").val() == 1) && ((selection == 1) || (selection == 2))) {
                    var url = "/Members/IllnessDetail/CascadedDdlInfectiousAgent/" + selection;
                    AjaxInfectiousAgents(url);
                }
            }
        });

        function AjaxEncephalitisSubTypes(url) {
            $('#SubTypeContainer').fadeOut('slow');
            $.getJSON(url, null, function(json) {
                ddlEst.empty();
                ddlIa.empty();
                PrependDdlDefaults(ddlEst);
                var i = 0;
                $.each(json, function(index, optionData) {
                    ddlEst.append("<option value='"
                        + optionData.EncephalitisSubTypeId
                        + "'>" + optionData.Name
                        + "</option>");
                    i++;
                });
                $('#SubTypeContainer').fadeIn('slow');
                ddlEst.val("");
                pEst.fadeIn('slow');
            });
        }

        function AjaxInfectiousAgents(url) {
            $('#SubTypeContainer').fadeOut('slow');
            $.getJSON(url, null, function(data) {
                var i = 0;
                ddlIa.empty();
                PrependDdlDefaults(ddlIa);
                $.each(data, function(index, optionData) {
                    ddlIa.append(
                    "<option value='"
                    + optionData.InfectiousAgentId
                    + "'>" + optionData.Name
                    + "</option>");
                    i++;
                });
            });
            ddlIa.val("");
            $('#SubTypeContainer').fadeIn('slow');
            pIa.fadeIn('slow');
        }

        function PrependDdlDefaults(ddl) {
            ddl.prepend(
                "<option value='"
                + ""
                + "'><i>" + " --- Please choose... --- "
                + "</i></option>");
        }
    });


</script>


<fieldset>
    <legend>The Illness</legend>
    <p>
        According to your input, different drop down lists will appear, specialised to only
        show the options that apply.
    </p>
    <p>
        <label for="IdEncephalitisType">
            Type Of Encephalitis:</label>
        <%= Html.DropDownList("IdEncephalitisType", Model.EncephalitisTypes)%>
        <%= Html.ValidationMessage("IdEncephalitisType", "*") %>
    </p>
    <div id="SubTypeContainer" style="margin-left:10px;border: solid 1px #ccc; background: #efefef;">
        <p class="highlight" id="lblSubTypeContainer" style="font-weight:bold;color:#c00;">
            Extra Information regarding the Illness:</p>
        <p id="pEncephalitisSubType">
            <label id="lblEncephalitisSubType" for="IdEncephalitisSubType">
                Sub Type of Encephalitis:</label>
            <%= Html.DropDownList("IdEncephalitisSubType", Model.EncephalitisSubTypes)%>
            <%= Html.ValidationMessage("IdEncephalitisSubType", "*") %>
        </p>
        <p id="pInfectiousAgent">
            <label id="lblInfectiousAgent" for="IdInfectiousAgent">
                Infectious Agent:</label>
            <%= Html.DropDownList("IdInfectiousAgent", Model.InfectiousAgents) %>
            <%= Html.ValidationMessage("IdInfectiousAgent", "*") %>
        </p>
    </div>
</fieldset>

控制器不需要显示，因为传递的 json 是正确的。正如我已经测试过的一样，渲染的内容是正确的。

Answer 1

破解了！！！！！！

问题出在 json 回调中：

$.each(json, function(index, optionData) {                    ddlEst.append("<option value='"                        + optionData.Id                        + "'>" + optionData.Name                        + "</option>");                    i++;                });
$.each(json, function(index, optionData) {
                    ddlEst.append("<option value='"
                        + optionData.Id
                        + "'>" + optionData.Name
                        + "</option>");
                    i++;
                });

...其中 optionData.Id 不是该字段的正确名称！哦，**&^%$£"！¬

因此，所有标签都具有以下值：

value="undefined"

AAAAAAAARRRRRGGGGGGJJJHHHHHHH!!!!!!!!!