AJAX CascadingDropDown ViewState问题

Question

问题：如何在回发后维护两个下拉列表的内容(来自查询)和选定的值？

源代码：从这个链接下载我的源代码(链接现在工作)。只需添加对AjaxControlToolkit的引用即可

用户操作：从每个下拉列表中选择一个值。单击Submit。

后回发： StatesDrop：(选定值)，CitiesDrop“选择一个城市”

前后：

alt文本http://www.aphio.org.vt.edu/test/beforeandafter.GIF

我认为，当第一个下拉列表获得其选定的值时，第二个下拉列表就会刷新，从而失去其选定的值。

C#的答案也欢迎。

Default.aspx

Active States<br /><asp:DropDownList ID="StatesDrop" runat="server" /><br />
Active Cities<br /><asp:DropDownList ID="CitiesDrop" runat="server" /><br />

<ajax:CascadingDropDown ID="StatesCasc" TargetControlID="StatesDrop"
        ServicePath="WebService1.asmx" ServiceMethod="GetActiveStates"
        Category="States" runat="server"
        PromptText="Select a State" PromptValue="?"  />

<ajax:CascadingDropDown ID="CitiesCasc" TargetControlID="CitiesDrop"
        ServicePath="WebService1.asmx" ServiceMethod="GetActiveCities"
        Category="Cities" runat="server" ParentControlID="StatesDrop"
        PromptText="Select a City"  PromptValue="?"  />

WebService1.asmx.vb

Imports System.Web.Services
Imports System.Web.Services.Protocols
Imports System.ComponentModel
Imports System.Web.Script.Services
Imports AjaxControlToolkit
<System.Web.Script.Services.ScriptService()> _
<System.Web.Services.WebService(Namespace:="http://tempuri.org/")> _
<System.Web.Services.WebServiceBinding _
    (ConformsTo:=WsiProfiles.BasicProfile1_1)> _
<ToolboxItem(False)> _
Public Class WebService1: Inherits System.Web.Services.WebService

   <WebMethod()> _
   Public Function GetActiveStates (ByVal knownCategoryValues As String, _
        ByVal category As String) As CascadingDropDownNameValue()
        Dim values As New List(Of CascadingDropDownNameValue)()
        'Fill values array'
        Return values.ToArray()
    End Function

    <WebMethod()> _
    Public Function GetActiveCities (ByVal knownCategoryValues As String, _
        ByVal category As String) As CascadingDropDownNameValue()
        Dim values As New List(Of CascadingDropDownNameValue)()
        Dim kv As StringDictionary = _
         CascadingDropDown.ParseKnownCategoryValuesString(knownCategoryValues)
        Dim SelState As String = ""
        If kv.ContainsKey("State") Then SelState = kv("State")
        'Fill values array'
        Return values.ToArray()
    End Function
End Class

Default.aspx.vb

Imports System.Web.Services
Imports System.Web.Script.Services
Imports AjaxControlToolkit

Partial Public Class _Default
    Inherits System.Web.UI.Page

    Protected Sub Submit_Click(ByVal sender As Object, _
                               ByVal e As EventArgs) Handles SubmitBtn.Click
        ResultsGrid.DataBind()
    End Sub
End Class

Answer 1

我放弃了CascadingDropDown，转而使用常规回发和UpdatePanel。

Answer 2

因为依赖下拉列表的项是在客户端填充的。服务器没有意识到这一点。您必须在每个回发上填充依赖项下拉列表。因此，在您的page_load中编写以下代码。

if(!IsPostBack) {
 //Some logic
}
else {
    //populate child drop down list on the base of selected value of parent drop down. 
// you can set the selected value of child control by getting the selected value from Request //object for example write following code to set the value of child control

childControl.SelectedValue = Request[childControl.UniqueID];
}

希望这能帮上忙。

Answer 3

要在回发中维护下拉列表的内容，请确保加载代码背后控件的逻辑位于if语句检查中，以查看是否为回发。例如..。

protected void Page_Load(object sender, EventArgs e)
    {
        if (!Page.IsPostBack)
        {
            //Load Controls
        }
    }

保存控件中的数据将由viewstate完成。