ASP.NET AJAX - ScriptManagerProxy and handling html button clicks

I am trying to figure out how to accomplish the following:

• Handle client-side click event of button - prevent postback if result of javascript call is false (e.g. onclick="js:return IsThisAllowed()")
• Javascript method will return true or false, depending on result of call to generated web service method using ScriptManagerProxy

The code below is simplified down to the main components that I am referring to in this question. I can alert() the return value no problem, but the part I'm having trouble with is how I can return the result of the web service method call back to the button onclick handler? Since the function Finish() is called in a separate thread upon success of what I assume is the XmlHttp.send() call, how can I send the value of the response as the return value of the javascript method? Am I going about this all wrong?

I've used similar code many times before to update the DOM with the return values, such as within a <div>, but never had to worry about capturing the return value and using it within my script.

What I have so far:

.NET web service method (VB.NET):

<WebMethod()> _
<ScriptMethod()> _
Public Function IsAllowed() As Boolean
    Return True
End Function

.NET code for the page to generate AJAX web service methods, and html button

<asp:ScriptManagerProxy ID="ScriptManagerProxy1" runat="server">
    <Services>
        <asp:ServiceReference Path="~/WebServices/MyWebService.asmx" />
    </Services>
</asp:ScriptManagerProxy>
<asp:Button ID="_btn" runat="server" OnClientClick="js:return IsAllowed();" />

Javascript

<script type="text/javascript">
    function IsAllowed(or开发者_开发技巧derid) {
        if (!processing) {
            processing = true;

            var ws = new MyWebService();
            ws.IsAllowed(Finish, Error)

            // return TRUE if call to IsAllowed returns true!!!

            return false;
        }
    }

    function Finish(result) {
        alert(result)
    }

    function Error() {
    }

</script>

You can't do this: the call to the web service is an asynchronous request to the server that immediately returns control to the client - and your javascript is (almost) always going to finish executing before the request completes (most likely before the request is even delivered).

Since you don't know when you'll receive the response from the call to ws.IsAllowed, you need to handle it in your callbacks (Finish and Error).

(This is true whatever AJAX mechanism you're using.)

I'm not familiar enough with ScriptManager-based AJAX to tell you exactly how to do this, but the general idea is that I'd turn it around: instead of canceling an ASP.NET button's click event, I'd only trigger the ASP.NET button if the AJAX call returned true.

Using jQuery, I'd do it by making the AJAX call from a vanilla HTML button (not an ASP.NET web control). In the success handler (the equivalent of Finish), I'd trigger my ASP.NET button (which I'd probably hide) when the return value is true.

Have you considered using jQuery?

$.ajax({
    type: "POST",
    contentType: "application/json; charset=utf-8",
    dataType: "json",
    url: "~/WebServices/IsAllowed.asmx/{Method Name}",
    data: ('{ whatever data is needed for decision making json serialized }'),
    success: function (msg) {
        alert(msg);
            },
    error: function (request, status, error) {
                    alert(err.Message);
                    alert(err.StackTrace);
    }
});