Should the generated bindable expose value types through nullable properties?

[dr1rrb]

When we have a property Feed<int> MyValue in the model, we generate a int MyValue { get; set; } .

1. The issue is that if the source feed does not yet have any value, or if the value is None, the getter returns default(int) = 0.
2. Thanks to the nullable annotations, we recently updated the API to make sure that default(T) is considered as None (if type of the feed is flagged as non nullable).

The only option currently is to create a State<int?>, but this impact the user experience in the model and would cause Some(null) while usually user expect a None (just like before we updated the public API to improve nullable support)

Generating a nullable int? MyValue { get; set; } could improve user experience ... but:

1. What if data-bound property is bool? (e.g. ToggleButton)
2. What if data-bound property is bool (e.g. ToggleSwitch)
3. What is the impact for textual representation (e.g. TextBlock)
4. This would alter the public API of generated models, what would be the impact, noticeably for C# markup.

Related unoplatform/uno#15349 (comment)

[dr1rrb]

After validation, generating nullable properties might cause more confusion. For instance a Slider (where the Value is of type double) can be data-bound 2-way to a property of type int, double or double?, but not a int?. This means that if the generated bindable property is of type int?, a user that has a Feed<int> would no longer be able to data-bind to it directly (it would required a Feed<double>). This might be way too confusing and is not an acceptable solution.

Related discussion: #2170