Binding Indexer - xaml

Question

I have a collection of items in my app, and I want to set the Content of a ContentPresenter to one of these items. The item will be randomly defined by an int index. I can bind an item like this:
<ContentPresenter Content={Binding Items[0]}/>
but not like this:
<ContentPresenter Content={Binding Items[{Binding Index}]}/>
I've seen a number of answers suggesting using MultiBinding in WPF, but this isn't available in UWP. Is there an alternative?

Answer 1

You could create a view model property, returning Items[Index]:
public string RandomItem => Items[Index];
For the PropertyChanged notifications to work, you will need to raise the event whenever Index or Items changes, e.g.:
public int Index
{
get { return _index; }
set
{
_index = value;
RaisePropertyChanged();
RaisePropertyChanged(() => RandomItem);
}
}
If you prefer to have the logic in the view and go the multi-binding way, you can use the Cimbalino toolkit. For that to work, first add 2 NuGet packages:
Cimbalino.Toolkit
Microsoft.Xaml.Behaviors.Uwp.Managed
Now you can create a converter:
public class CollectionIndexConverter : MultiValueConverterBase
{
public override object Convert(object[] values, Type targetType, object parameter, CultureInfo culture)
{
var collection = (IList) values[0];
var index = (int?) values[1];
return index.HasValue ? collection[index.Value] : null;
}
public override object[] ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
{
throw new System.NotImplementedException();
}
}
And use it from XAML:
<ContentPresenter>
<interactivity:Interaction.Behaviors>
<behaviors:MultiBindingBehavior PropertyName="Content" Converter="{StaticResource CollectionIndexConverter}">
<behaviors:MultiBindingItem Value="{Binding Items}" />
<behaviors:MultiBindingItem Value="{Binding Index}" />
</behaviors:MultiBindingBehavior>
</interactivity:Interaction.Behaviors>
</ContentPresenter>