Create a BindableProperty depending on multiple properties - mvvm

Question

I'm trying to bind a UI element to different model properties A, B and AB. The first two properties A and B are controlled by two sliders. The third property AB is the sum of A and B. For each of the three properties there is a label displaying its value.
Now if I move one of the sliders, the corresponding label updates its Text. But the label for the combined property AB is not updated. Probably no "property changed" event is fired, since there is no setter for AB.
Is there any possibility for binding to such an "aggregated" property?
Here is the bindable object containing the properties A, B and AB:
public class Settings: BindableObject
{
public static readonly BindableProperty AProperty = BindableProperty.Create<Settings, double>(p => p.A, 0);
public static readonly BindableProperty BProperty = BindableProperty.Create<Settings, double>(p => p.B, 0);
public static readonly BindableProperty ABProperty = BindableProperty.Create<Settings, double>(p => p.AB, 0);
public double A {
get{ return (double)GetValue(AProperty); }
set{ SetValue(AProperty, (double)value); }
}
public double B {
get{ return (double)GetValue(BProperty); }
set{ SetValue(BProperty, (double)value); }
}
public double AB {
get{ return A + B; }
}
}
And here is the page containing both sliders and the three labels:
public class App : Application
{
public App()
{
var settings = new Settings();
var sliderA = new Slider();
sliderA.ValueChanged += (sender, e) => settings.A = e.NewValue;
var sliderB = new Slider();
sliderB.ValueChanged += (sender, e) => settings.B = e.NewValue;
var labelA = new Label{ BindingContext = settings };
labelA.SetBinding(Label.TextProperty, "A");
var labelB = new Label{ BindingContext = settings };
labelB.SetBinding(Label.TextProperty, "B");
var labelAB = new Label{ BindingContext = settings };
labelAB.SetBinding(Label.TextProperty, "AB");
MainPage = new ContentPage {
Content = new StackLayout {
VerticalOptions = LayoutOptions.Center,
Children = { sliderA, sliderB, labelA, labelB, labelAB },
},
};
}
}
This is what the running application looks like on iOS:
The last label should display the sum of the first two numbers.
Edit:
I wonder why I can't write
public static readonly BindableProperty ABProperty =
BindableProperty.Create<Settings, double>(p => p.A + p.B, 0);
But this yields the run-time error "System.TypeInitializationException: An exception was thrown by the type initializer for AggregatedBindablePropertyMnml.Settings ---> System.Exception: getter must be a MemberExpression"

Answer 1

Based on the suggestion from Taekahn (updating AB within the setters of A and B) I came up with the following solution.
By overriding the OnPropertyChanged method and setting the ABProperty, the bound label text is updated as well. In contrast to modifying each setter individually, this way we only need to modify the Settings class at one place.
protected override void OnPropertyChanged(string propertyName = null)
{
base.OnPropertyChanged(propertyName);
SetValue(ABProperty, A + B);
}
Now both sliders impact the third label: