Question

I am not aware if there is any “callback” to Listview creation so that i can display very specific type of data from the List i have.
What i know is that only Complete List can be bound to the ListView, but how can i put some checking to it like its done in iOS by the help of Datasource Delegate.

Anyone ?

Answer 1

Filtering and grouping has traditionally be done with CollectionViewSource. Unfortunately, CollectionViewSource no longer has the Filter event or the GroupDescriptions property. It may seem like filtering and grouping are unsupported, but both can still be achieved using LINQ.

In your Xaml, add a CollectionViewSource in the Resources section of your page. Make sure IsSourceGrouped is set to true:

<common:LayoutAwarePage.Resources>

    <!--
        Collection of grouped items displayed by this page, bound to a subset
        of the complete item list because items in groups cannot be virtualized
    -->
    <CollectionViewSource x:Name="GroupsCV" Source="{Binding Groups}" IsSourceGrouped="True" />

</common:LayoutAwarePage.Resources>

Now, the CollectionViewSource (GroupsCV) should be set as the ItemsSource for your GridView:

<GridView ItemsSource="{Binding Source={StaticResource GroupsCV}}" />

Notice that CollectionViewSource is bound to a property called Groups. This property is part of my ViewModel. The value returned by the Groups property will be the result of a LINQ query. This confused me at first because I didn’t know what type the property should return. I settled on an enumerable grouping of comparable items. This pretty much works with any LINQ query of any type.

So, in your ViewModel (or whatever your DataContext is) add the following property:

private IEnumerable<IGrouping<IComparable, TItem>> groups;
public IEnumerable<IGrouping<IComparable, TItem>> Groups
{
    get { return groups; }
    set { SetProperty(ref groups, value); }
}

Now, whenever you want to change the grouping or the filter, just set the Groups property equal to a LINQ query like so:

Groups = from i in musicItems
            group i by i.Genre into g
            orderby g.Key
            select g;

LINQ does great with known property names, but what about letting the user pick from a list of property names and dynamically grouping by that property? Well, the only requirement for LYNQ to be able to create a group is that whatever you pass it must implement IComparable.

Here’s a little extension method that takes the name of a property as a string and returns an IComparable:

static public IComparable GetComparableValue<T>(this T item, string propName) where T : class
{
    return (IComparable)typeof(T).GetTypeInfo().GetDeclaredProperty(propName).GetValue(item, null);
}

With that in place, you can do a dynamic query by property name like this:

string groupByPropertyName = "Artist";

Groups = from i in musicItems
group i by i.GetComparableValue(groupByPropertyName) into g
orderby g.Key
select g;

Hope that helps!