MVVM和Windows Phone Hub控制

Question

I have asked some questions about MMVM here recently and have appreciated the responses. I have a further question about filtered views of the same underlying data source and how that should work.

Let's say I have a simple list of "items" and want to show various filtered views of this, all in a Hub control. Each view would be exposed by a separate HubSection. So, I might have a HubSection that shows all items, another that shows items due today, another for items this week and a final one for this month.

. All . Today . This Week . This Month

My plan is to bind each HubSection to a separate property of the viewmodel (let's keep this simple and assume the datacontext for the hub is set and each HubSection binds to a property of that datacontext). So, properties of type ObservableCollection called, say, GetAlltems(), GetTodaysItems(), GetThisWeeksItems() and GetThisMonthsItems().

My question is about how and when I fetch the data from the model. One option would be to declare and populate four ObservableCollection properties when the viewmodel is created (in it's constructor?). It seems that the problem there is that I have four copies of essentially the same data in memory (just with different filters applied), as well as the same data being in the data store itself (SQLite in my case).

Another option would be to NOT populate these properties when the viewmodel is created but instead just bind to properties that fetch the appropriate filted dataset and return it as an ObservableCollection on the fly. But I am unsure what sort of performance that would have, as the user swipes across the HubSections. If a HubSection is "offscreen" (say the 4th HubSection, while the first one is onscreen), is the bound list control populated at that point or does that only happen when it comes into view?

The datasets for each of the filtered views is small (the "all" collection might be a couple of hundred, max). But on a mobile device it still seems unnecessary to pre-populate the four properties upfront.

Anyway, this seems like a reasonable common requirements (multiple HubSection controls, each bound to a filtered set of the same underlying data source). So, interested in the most elegant way to do this, while having reasonable performance and memory usage.

I haven't even got to changes to the collections yet and how notifications might work :)

Thanks.

Answer 1

Take a look at CollectionViewSource it allows to use the same set of objects but define filters. I do have to say that the implementation has changed a bit with winrt wp8.1. So double check the documentation on how to actually set the filter. I think the way to go these days is by using simple Linq queries

The MSDN doc

http://msdn.microsoft.com/library/windows/apps/br209833

From some older sources ( for wp8 SL )

http://www.hanselman.com/blog/CollectionViewSourceIsCrazyUsefulForBindingToFilteredObservableCollectionsOnWindowsPhone8.aspx http://www.spikie.be/blog/post/2012/04/12/Filtering-collections-from-XAML-using-CollectionViewSource.aspx