Manual blocking VirtualDataCollection.ItemsLoading event

[Vitalii]

As mentioned here, i faced some problems with this event.
So, i decided to try workaround. 

As i don't have SelectedIndex during page loading, i decided to use it for blocking requests. Bu default, it should be String.Empty, but once it is set, requests can to proceed.

public void OnNavigatedFrom()
        {
            SelectedIndex = String.Empty; // Empty, when navigating from
            VirtualDataCollection.Clear();

            Cleanup();
        }

public async void OnNavigatedTo()
        {
            var answer = await _dataService.ReadFromCacheAsync(SelectedIndex); // Now, SelectedIndex holds correct id
            if (answer.Status == AnswerDataServiceStatus.Ok)
            {
                lock (locker)
                {
                    var currentIndex = SelectedIndex; // storing
                    SelectedIndex = String.Empty; // emptying

                    VirtualDataCollection.Count = answer.TotalItems;
                    VirtualDataCollection.LoadItems(0, answer.Collection);

                    SelectedIndex = currentIndex; // restoring
                }
            }
        }

Event itself: 

private async void VirtualDataCollectionOnItemsLoading(object sender, VirtualizingDataCollectionItemsLoadingEventArgs e)
        {
            if (String.IsNullOrEmpty(SelectedIndex)) return; // Leaving, if id is empty

            var answer = await _dataService.RequestServerAsync(SelectedIndex, e.StartIndex, e.Count);
            if (answer.Status == AnswerDataServiceStatus.Ok) // && answer.Collection.SequenceEqual(VirtualDataCollection.))
            {
                DispatcherHelper.CheckBeginInvokeOnUI(() =>
                {
                    lock (locker)
                    {
                        var currentIndex = SelectedIndex; // No double rising at LoadItems or Count
                        SelectedIndex = String.Empty;

                        VirtualDataCollection.Count = answer.TotalItems;
                        VirtualDataCollection.LoadItems(e.StartIndex, answer.Collection);

                        SelectedIndex = currentIndex;
                    }
                });
            }
        }

Page constructor

VirtualDataCollection = new VirtualizingDataCollection(1, Constants.PAGE_SIZE);
            VirtualDataCollection.ItemsLoading += VirtualDataCollectionOnItemsLoading;

However, now, sometimes, ItemsLoading is not rising at all, when user scrolls down fast enough.. any suggestions? Maybe locker is not enough?

[Vitalii]

Yep, the locker is the reason. It cannot be used, because VirtualDataCollection.LoadItems() is quite long, and user can scroll down fast enough. I should consider another solution.

However, it is a bit strange: i cancelled loading of page 2 (by just returning), and page 3 is never even tries to be loaded. I guess, you can make paging requests independent of previous result.

[Deyan]

Hi Vitalii,

Thanks for writing.

We would like to kindly request you to use a single thread regarding the VirtualizingDataCollection difficulties you are facing. This will make it easier for us to handle the case and quickly assist you.

For the purpose of faster support, please send us a ZIP containing a sample project that we can use to reproduce the issues you are facing and directly debug them. In this way we will be able to quickly identify the possible ways of overcoming them.

Thanks for your understanding and for the time taken.

Regards,
Deyan
Telerik