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up vote 0 down vote favorite

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(orderid) {
        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>
share|improve this question

2 Answers 2

up vote 2 down vote accepted

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.

share|improve this answer
    
Yep that's what I've experienced, but do you have any suggestions to accomplish what I'm trying to do? –  Keith Nov 9 '10 at 21:34
1  
I think I figured out what you're trying to accomplish - canceling the ASP.NET postback based on the result of the web service call. I'm updating my answer with a solution. –  Jeff Sternal Nov 9 '10 at 21:35
    
Exactly -- and I already have the logic in the click event to prevent processing (and throw an error) if the call is not allowed, but I'd rather not have the overhead of a full postback if I can accomplish this Ajax-style. –  Keith Nov 9 '10 at 21:38
    
I'm familiar with jQuery and triggering click() events, but wouldn't handling this in the click event itself cause a stack overflow? I will try it anyway and see what I get. –  Keith Nov 9 '10 at 21:46
1  
Sorry if I wasn't clear: I'm recommending you use two buttons. One visible button initiates the AJAX call. The other button (probably invisible) is an ASP.NET button that actually submits the form (and calls a handler in your code-behind). –  Jeff Sternal Nov 9 '10 at 21:48
up vote 0 down vote

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);
    }
});
share|improve this answer
    
Both will accomplish the exact same result - on success, I will still have no way to return the result of the call back to the onclick handler. –  Keith Nov 9 '10 at 21:33

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