Remote Virtualization of Data

[Tony]

Hi guys,

I seem to be having an issue with virtualizing data for my grid. I've initialized it with the following:

    $("#myGrid").kendoGrid({
        dataSource: {
            type: "json",
            serverPaging: true,
            serverSorting: true,
            pageSize: 32,
            transport: { read: "/Query/GetAddresses" }
        },
        height: 300,
        scrollable: { virtual: true },
        columns: cs /* The columns data stored in the cs variable */
    });

The grid initializes fine, and populates with 32 records (shows about 10 with a scrollbar due to height constraint). When i scroll down, i can see that the next ajax call is made and returns the correct json data for take=32 & skip=32 & page=2 & pageSize=32 using FireBug. The problem is that at this point, the data isn't appended to the grid. Also, any further scrolling on the scrollbar calls for page 2 again with the same parameters.

I've tried a few variations of parameters as i thought they may be intefering with each other, but couldn't get the data to come through. Any help or ideas of things to try would be appreciated.

Regards,
Tony

[Rosen]

Hi Tony,

 Could you please send us a small sample in which this behavior can be observed locally.

Regards,
Rosen
the Telerik team

[Tony]

no worries.

I've set up a test page that exhibits the behaviour here:

http://www.moveware.com.au/kendo.html

Specifically:

$(document).ready(function() {
    $("#TestingAddresses_0").kendoGrid({
        dataSource: {
            type: "json",
            serverPaging: true,
            serverSorting: true,
            pageSize: 32,
            transport: {
                read: {
                    url: "<WebServiceAddress>",
                    dataType: "jsonp"
                }
            }
        },
        height: 320,
        width: 500,
        scrollable: { virtual: true },
        selectable: "row",
        sortable: false,
        pageable: false,
        columns: [{"field":"TID","title":"ID"},{"field":"T3","title":"Place"},{"field":"T5","title":"Country"}]
    });
});

However, possibly because of the cross domain call this page isn't initially being loaded with data. But you can see that the query is being made, the response json data is coming back, and nothing happens after that. No visible javascript errors either, but i assume that an onerror event is firing somewhere.

What am i missing here?

Regards,
Tony

[Rosen]

Hi Tony,

I suspect that in the sample data is not loaded due to incorrect format of the result. As you are using JSONP the result should be a javascript function instead of the actual data. For example you may take a look at this JSONP call url.

Regards,
Rosen
the Telerik team

[Tony]

Thanks for your help in pointing that out - The data is now populating again, and correctly repopulating as the scrollbar is used. I do still have one small problem though.

While completing this virtualization of data, i found that unless a total count was specified in the schema of the response, the virtual scrolling would fail to work correctly. I have some extremely large datasets to work with in certain queries that are quite complex, so providing the total count is not really an option. I tried modifying the total count on each subsequent query for a new page to be a greater value i.e. the skip count plus twice the take count (skip + 2*take). Unfortunately the grid doesn't seem to take any notice of the total count figure on subsequent page requests.

After a little fustration i added the following line of code into kendo.data.js to make the success of a subsequent page request respect the new total passed with the jsonp response data:

1779.    data = that.reader.parse(data);
1780.    that._total = that.reader.total(data);
1781.    range.data = that.reader.data(data);
1782.    range.end = range.start + range.data.length;

You can see what's happening here:

http://www.moveware.com.au/kendo.html

And this one shows the fix:

http://www.moveware.com.au/kendo-fix.html

If there is another way to fix this let me know, as obviously my change will be lost as soon as your controls are updated.

Regards,
Tony

[Rosen]

Hi Tony,

I'm afraid that providing a valid total count is essential in order virtual scrolling to work correctly.
The line you have mentioned is in a function which is not directly related to the population of the grid as its purpose is to prefetch data not to rebind the grid component.

Regards,
Rosen
the Telerik team

[Tony]

Hi Rosen,

Yes, the total count that i'm referring to is in the Kendo datasource control and not the grid control. However i've noticed that the grid respects this total count, even when it is updated dynamically on subsequent requests for more data. I can't afford the time and performance hit to run a count on my query at the start of the request, so as more data is requested by the Kendo datasource object, i send back an updated total count variable. But this total count is only updated in the Kendo datasource on the first request, not on subsequent requests for more data, which is why i added the line of code to update the total on each subsequent request (This field was being effectively ignored after the first request had been made).

So what you're telling me is that if i have a query that when run may return 200 results or may return 2,000,000 results, and i only want a page of 50 records, i still need to effectively run a scan of my query on the first request to get the correct count of my result set?

Regards,
Tony

[Rosen]

Hello Tony,

As I have mentioned in my previous post current implementation of the virtual scrolling does require total number of records to be known upfront. You may have noticed that with the suggested change the scrollbar does not behave as intended i.e. in order more records to be shown user should scroll to the bottom as opposite to the original behavior where one could scroll to arbitrary page. 

All the best,
Rosen
the Telerik team

[Chris]

Hi,

Is this still the case as we have a good number of queries that are huge and we'll need to show them in a grid without trying to count them first?

[Atanas Korchev]

Hi Chris,

 Yes, this is still the case. The grid needs to know the total number of records it is going to display. 

Regards,
Atanas Korchev
the Telerik team