Knockout html binding not working (bound to a function calling a partial view) in ASP .NET MVC

Question

I have an ASP .NET MVC application, additonally I am using Knockout 2.0.0. I created a partial view which I would like to render to the page using knockout. The partial needs to be rendered within a Knockout foreach statement. I am unable to get the knockout HTML binding to work, and so I'm currently using a hack to put the html into the div using JQuery.

There is a lot of html on this page, so it's not possible to post all of the source code, so I will try and post the pertinent code:

<div data-bind="foreach:issues">
    @* SNIP - A lot of other html here*@
    <div id="myPartialDiv" data-bind="html: $parent.getHtml(issueId())">
    </div>  
</div>

Further down I have the following javascript function on my KO View Model (I have commented out my hack and included the code that returns HTML):

   var getHtml = function (issueId) {
              var baseUrl = '@Url.Action("GetHtmlAction","MyController")';
              $.ajax(
                {
                    type: "POST",
                    url: baseUrl,
                    data: "&issueId=" + issueId,
                    success: function (data) {
                        //$('#mypartialDiv').html(data);                          
                        return data;
                    },
                    error: function (req, status, error) {
                        //$('#myPartialDiv').html('Something went wrong.');
                        return 'Something went wrong.'
                    },
                    dataType: "text"
                });
         }

The code above results in no data being rendered to the page. USing Chrome debug tools, I see that there are no javascript errors occuring, and knockout is simply not binding the html of the div to the results returned from the getHtml function.

What am I doing wrong?

Thanks

Are you applying the bindings? Your example retrieves the data, but does nothing with it, as far as I can see. — Christofer Eliasson
Yeah, sorry that code is not included but I am calling ko.applyBindings() on the page, and other bindings on the page work. — King_Nothing
Oh and the function is being called when the HTML binding occurs, but it seems that knockout does not use the results (the 'return data;' line). If I remove my comments above the update is successful (using JQuery to update the div.Html). — King_Nothing

Steve Greatrex Steve Greatrex · Accepted Answer · 2012-06-12T10:10:28

As Miroslav Popovic explains, the problem is that the AJAX request is asynchronous, so the return data is ignored and there is no return value from your call to getHtml.

I would suggest using a custom binding that handles the asynchronous HTML loading (I've put a working example here).

This works by taking 2 parameters to the asyncHtml: a function to call that takes a success callback as it's final parameter (plus any other parameters) and an array of the parameters that need to be passed to that function.

<div id="myPartialDiv" data-bind="asyncHtml: { source: getHtml, params: [123] }">Loading...</div>

The custom binding then grabs these values, concats a custom callback onto the parameters that are passed to it, and calls the specified function:

ko.bindingHandlers.asyncHtml = {
    init: function(element, valueAccessor) {
        var value = ko.utils.unwrapObservable(valueAccessor());
        var parameters = value.params.concat([function(data) {
           $(element).html(data); 
        }]);
        value.source.apply(null, parameters);
    }
};

Finally we can re-implement our view model HTML-retrieving method to make the POST call and invoke the new success handler:

var ViewModel = function() {
    this.getHtml = function(issueId, callback) {
        $.ajax(
            {
                type: "POST",
                url: "/echo/html/",
                data: {
                        html: "<p>server response " + issueId + "</p>",
                        delay: 1
                    },
                success: callback,
                dataType: "text"
            });            
    };
};

Note: for this example I am using the jsFiddle echo to post back a random response

Knockout html binding not working (bound to a function calling a partial view) in ASP .NET MVC

2 Answers