Xamarin Forms Subclass with Modified Bindable Property Defaults - xamarin.forms

Question

I am trying to get a subclass of the Xamarin Forms "Label" class. In my subclass, among a lot of other changes, I want to have a different default value for some bindable properties (such as FontSize and FontAttributes). However, if I set these in the constructor, it seems that Style specifiers won't override these, presumably because the bindings are already noticing that they are using non-default values. Is there a way to specify in a subclass that you want to use different default values in a bindable property?
class MyCustomLabel : Label {
public MyCustomLabel() {
FontSize=20;
}
}
<ResourceDictionary>
<Style x:Key="Superbig" TargetType="MyCustomLabel">
<Setter Property="FontSize" Value="3" />
</Style>
</ResourceDictionary>
<MyCustomLabel Style="{StaticResource Superbig}" Text="Hi There!" />
Here, the Superbig style is not being applied because I am setting the new default value in the constructor. Therefore, I was hoping either (a) there was some other way to set a new default value, or (b) there was some other way to set a style so it overrode any value that was already set.

Answer 1

Unfortunately, BindableProperty doesn't seem to support OverrideMetadata like DependencyProperty does. Here's two way to achieve this.
1) Set a default Style for your MyCustomLabel object (XAML)
<ResourceDictionary>
<!--Default style-->
<Style TargetType="local:MyCustomLabel">
<Setter Property="FontSize" Value="10" />
</Style>
<!--Superbig style-->
<Style x:Key="Superbig" TargetType="local:MyCustomLabel">
<Setter Property="FontSize" Value="40" />
</Style>
</ResourceDictionary>
2) Create a new FontSize BindableProperty (C#)
public class MyCustomLabel : Label
{
public MyCustomLabel()
{
base.SetBinding(Label.FontSizeProperty, new Binding(nameof(FontSize))
{
Source = this,
Mode = BindingMode.OneWay
});
}
//Don't forget the "new" keyword
public new double FontSize
{
get { return (double)GetValue(FontSizeProperty); }
set { SetValue(FontSizeProperty, value); }
}
//Don't forget the "new" keyword
public static readonly new BindableProperty FontSizeProperty =
BindableProperty.Create(nameof(FontSize), typeof(double), typeof(MyCustomLabel), 40.0);
}