How do you set the BindingContext of a nested collection in Maui? - xaml

Question

I am not sure how to refer to another collection object for a BindingContext of a CollectionView inside another CollectionView in Maui/XAML. Take the following example:
XAML:
<ContentPage xmlns="http://schemas.microsoft.com/dotnet/2021/maui"
xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
x:Class="NestedContextExample.MainPage"
xmlns:model="clr-namespace:NestedContextExample"
Title="{Binding Name}">
<ContentPage.BindingContext>
<model:Form />
</ContentPage.BindingContext>
<ScrollView Padding="10">
<CollectionView ItemsSource="{Binding FieldGroups}">
<CollectionView.ItemTemplate>
<DataTemplate>
<StackLayout>
<Label Text="{Binding Name}" />
<!-- something like this -->
<CollectionView ItemsSource="{Binding Source={x:Reference Answers.Where(a => Fields.Contains(a.Field)}}">
<CollectionView.ItemTemplate>
<DataTemplate>
<VerticalStackLayout>
<Label Text="{Binding Field.Name}" />
<Label Text="{Binding Value}" />
</VerticalStackLayout>
</DataTemplate>
</CollectionView.ItemTemplate>
</CollectionView>
</StackLayout>
</DataTemplate>
</CollectionView.ItemTemplate>
</CollectionView>
</ScrollView>
Code Behind:
namespace NestedContextExample;
public partial class MainPage : ContentPage
{
public Form Form { get; set; }
public List<Answer> Answers { get; set; }
public MainPage()
{
InitializeComponent();
Form = new Form
{
Name = "My Form",
FieldGroups = new List<FieldGroup>
{
new FieldGroup
{
Name = "Group A",
Fields = new List<Field> {
new Field
{
Name = "Field 1"
},
new Field
{
Name = "Field 2"
}
}
},
new FieldGroup
{
Name = "Group B",
Fields = new List<Field> {
new Field
{
Name = "Field 3"
},
new Field
{
Name = "Field 4"
}
}
}
}
};
Answers = Form.FieldGroups.SelectMany(g => g.Fields).Select(f => new Answer
{
Field = f
}).ToList();
BindingContext = Form;
}
}
public class Form
{
public string Name { get; set; }
public List<FieldGroup> FieldGroups { get; set; }
}
public class FieldGroup
{
public string Name { get; set; }
public List<Field> Fields { get; set; }
}
public class Field
{
public string Name { get; set; }
}
public class Answer
{
public Field Field { get; set; }
public string Value { get; set; }
}
So, I want the nested collection to be bound to the subset of the list of Answer objects with fields within the current FieldGroup, rather than the Fields of the current FieldGroup. Note the ItemsSource="{Binding Source={x:Reference Answers.Where(a => Fields.Contains(a.Field)}}" - that obviously doesn't work, but that is what I am basically trying to achieve. How would I go about that?