MSDN勘误-关于ICallbackEventHandler接口

最新推荐文章于 2022-08-17 15:33:40 发布

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本文探讨了ASP.NET 2.0中的Callback功能，通过示例代码详细解释了ICallbackEventHandler接口的使用，并分享了作者在实践过程中的调试经验和代码修改心得。

这两天在研究asp.net2.0的callback。在msdn中找到了这段示例代码：
ms-help://MS.VSCC.v80/MS.MSDN.v80/MS.VisualStudio.v80.chs/dv_aspnetcon/html/14b85dbe-1c60-4f66-9c38-f22416fd72b4.htm

' ClientCallback.aspx page

<%@ Page Language="VB" AutoEventWireup="false"

CodeFile="ClientCallback.aspx.vb" Inherits="ClientCallback" %>

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN"

"http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd">

<title>Untitled Page</title>

function LookUpStock()

{

var lb = document.forms[0].ListBox1;

var product = lb.options[lb.selectedIndex].text

CallServer(product, "");

}

function ReceiveServerData(rValue)

{

Results.innerText = rValue;

}

</script>

</head>

<body>

<div>

<asp:ListBox ID="ListBox1" Runat="server"></asp:ListBox>

<br />

<button onclick="LookUpStock()">Look Up Stock</button>

<br />

Items in stock: <span ID="Results"></span>

<br />

</div>

</form>

</body>

</html>

' ClientCallback.aspx.vb code-behind file

Partial Class ClientCallback

Inherits System.Web.UI.Page

Implements System.Web.UI.ICallbackEventHandler

Protected catalog As ListDictionary

Sub Page_Load(ByVal sender As Object, ByVal e As _

System.EventArgs) Handles Me.Load

Dim cbReference As String

cbReference = Page.ClientScript.GetCallbackEventReference(Me, _

"arg", "ReceiveServerData", "context")

Dim callbackScript As String = ""

callbackScript &= "function CallServer(arg, context) { " & _

cbReference & "} ;"

Page.ClientScript.RegisterClientScriptBlock(Me.GetType(), _

"CallServer", callbackScript, True)

' Populate List Dictionary with invented database data

catalog = New ListDictionary()

catalog.Add("monitor", 12)

catalog.Add("laptop", 10)

catalog.Add("keyboard", 23)

catalog.Add("mouse", 17)

ListBox1.DataSource = catalog

ListBox1.DataTextField = "key"

ListBox1.DataBind()

End Sub

Public Function RaiseCallbackEvent(ByVal eventArgument _

As String) As String Implements _

System.Web.UI.ICallbackEventHandler.RaiseCallbackEvent

Dim returnValue As String

If catalog(eventArgument) Is Nothing Then

returnValue = "-1"

Else

returnValue = catalog(eventArgument).ToString()

End If

Return returnValue

End Function

End Class

复制到vs里面却提示错误，检查了一下，发现ICallbackEventHandler要求实现两个方法，一个是sub,另外一个是function:

Public Function GetCallbackResult() As String Implements System.Web.UI.ICallbackEventHandler.GetCallbackResult

End Function

Public Sub RaiseCallbackEvent(ByVal eventArgument As String) Implements System.Web.UI.ICallbackEventHandler.RaiseCallbackEvent

End Sub

这段示例代码给的是一个function，但是名字却是接口规定的那个sub的名称。根据名称，我判断，当callback的时候，首先执行RaiseCallbackEvent，直译为“发生CallBack事件”，然后再执行GetCallbackResult，直译为“得到CallBackResult”。于是我做了一个实验，首先定义了一个全局变量：
Private TestStr as String=""

然后把这两段代码修改一下：

Public Function GetCallbackResult() As String Implements System.Web.UI.ICallbackEventHandler.GetCallbackResult

TestStr=TestStr & "GetCallbackResult"

End Function

Public Sub RaiseCallbackEvent(ByVal eventArgument As String) Implements System.Web.UI.ICallbackEventHandler.RaiseCallbackEvent

TestStr=TestStr & "RaiseCallbackEvent"

End Sub

然后在TestStr=TestStr & "RaiseCallbackEvent"处加上断点。执行到这一句是，检查TestStr的值，结果是空值，也就是说，发生Callback的时候，首先执行的是RaiseCallbackEvent，按F11调试，果然，执行完这个方法后才去执行GetCallbackResult。
于是我就对示例代码作了一些修改：
1、定义一个全局变量CBRE用来存储eventArgument的值：
Private CBRE As String
2、修改代码如下：