IsLastItemConverter / IsNotLastItemConverter

[ewerspej]

I would like to propose to add an IsLastItemConverter as well as an IsNotLastItemConverter to the list of converters to determine whether or not an element is the last item in an IEnumerable and then update the view accordingly.

The implementation could be as simple as:

public class IsLastItemConverter : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
        return parameter is IEnumerable<object> enumerable && enumerable.Last().Equals(value);
    }

    public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
    {
        return Binding.DoNothing;
    }
}

public class IsNotLastItemConverter : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
        return parameter is IEnumerable<object> enumerable && !enumerable.Last().Equals(value);
    }

    public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
    {
        return Binding.DoNothing;
    }
}

One major use case would be to allow users to make decisions in their XAML based on whether an item is the last element in an ItemsSource, for example to show a divider or use a different color value, etc:

<ContentPage.Resources>
    <ResourceDictionary>
        <mct:IsNotLastItemConverter x:Key="IsNotLastItemConverter" />
    </ResourceDictionary>
</ContentPage.Resources>

<VerticalStackLayout 
    BindableLayout.ItemsSource="{Binding Items}">
    <BindableLayout.ItemTemplate>
        <DataTemplate x:DataType="model:MyClass">
            <Grid RowDefinitions="40,auto">
                <Label
                    Grid.Row="0"
                    Text="{Binding Name}" />
                <BoxView
                    Grid.Row="1"
                    HeightRequest="1"
                    IsVisible="{Binding ., Converter={StaticResource IsNotLastItemConverter}, ConverterParameter={Binding Items, Source={RelativeSource AncestorType={x:Type viewModel:MyViewModel}}}}" />
            </Grid>
        <DataTemplate>
    </BindableLayout.ItemTemplate>
</VerticalStackLayout>

I can provide a PR with this myself, if this is something that is deemed useful.

Notes:

1. I am aware that this will only work with reference types, but that's because the IValueConverter interface doesn't allow to specify a generic type argument. Alternatively, I would need to use IEnumerable without the type and iterate through the enumerable until I hit the last item and compare that to the value argument.
2. One problem could be that items can appear multiple times in lists and collections which could lead to unexpected behavior

[ewerspej]

I don't think this is possible, because ConverterParameter is just a property of type object and not a BindableProperty.

[pictos]

So, the Converter is inside the Binding (look you snippet), with that everytime that property changes the new value will be send to the Converter to perform the check

[ewerspej]

it still won't work, because the parameter is not passed as the bound value, it's passed as a Binding instance here:

ConverterParameter={Binding Items, Source={RelativeSource AncestorType={x:Type viewModel:MyViewModel}}}

[ewerspej]

@pictos The intended behavior can be achieved using a MultiValueConverter, which I've just done in a project of mine.