Get the results from a callback function in Javascript

Question

I'm writing a Javascript function which will be called by a C# application. I can call the function from C#, but have been unable to retrieve the result of the function.

So I have the following structure:

var B = function() {
    var A = function() {
        var dfd = new $.Deferred();
            // do something and then return the value I need
            return dfd.resolve(x);
        ......
        return dfd.promise();
    }
    $.when(A()).
        then(function(x) {
            // I can get the x I want here.
            alert(x);
            // What to do at this point?
        });
}

Since A() used asynchronous methods, I chose to use jQuery.promise() method to ensure I get the final result of A(). Now I want B() to return the value x to the C# application. Is there any good solution to this?

Possible duplicate of How can I execute Javascript callback function from C# host application — Robert Moskal
– Robert Moskal, Commented Oct 17, 2015 at 23:54
What does "return ... to the C# application" mean? C# has exited the picture long before the JavaScript is executed in the client browser. You will need to make a fresh HTTP request to your application, perhaps with the value of x as a query string parameter. — 76484
– 76484, Commented Oct 18, 2015 at 5:27
@76484 So I'm just invoking the script from C#. My application is synchronous while the API I used isn't, so I want to do something to make function B() appear as synchronous and return the value after everything is done in A(). — dahui
– dahui, Commented Oct 18, 2015 at 5:38
@RobertMoskal Not really. Mine has almost nothing to do with C#, that's just the background. Mine main goal is to know how to let B() return x. — dahui
– dahui, Commented Oct 18, 2015 at 5:45
If I understand your question correctly, it is about JS and has nothing to do with C#. If the value of x is obtained asynchronously, then B cannot simply return it synchronously. Rather, B must return dfd.promise and callers of B must use the $.when(B()).then(...); syntax. Of course, that looks like it would amount to getting rid of the B wrapper and using just A. — 76484
– 76484, Commented Oct 18, 2015 at 5:49

Pavel Durov · Accepted Answer · 2015-10-20 06:24:48Z

I assume that you've called your Javascript function by calling WebView method InvokeScriptAsync with Js function name as a parameter.

The problem here is that your JavaScript function is an asynchronous since it uses promise - so you cannot simply get its return value on C#. The IAsyncOperation ends as the promise object has been returned, not as it has been resolved.

If your javascript function was synchronious:

The return value of the InvokeScriptAsync function is the string result of the script invocation - so if you'll wait for it you'll get the result.

MSDN documentation : https://msdn.microsoft.com/en-us/library/windows.ui.xaml.controls.webview.invokescriptasync.aspx?f=255&MSPPError=-2147217396

However, you can always invoke webView event by calling windows.external.invoke with string parameter from JavaScript and get it on C# by subscribing to webView ScriptNotify event.

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