[Question] GetValue for abstract AvaloniaProperty<T>

[jp2masa]

I noticed some projects using Avalonia expose property definitions as AvaloniaProperty<T> instead of the concrete type (StyledProperty, DirectProperty,...), so there are some calls to GetValue for those properties which no longer work after #3255 and #3287 (the resolved overload is the one taking AvaloniaProperty, which returns untyped value, i.e. object).

So my question is whether it's incorrect to expose properties as AvaloniaProperty<T>, and if not, if it would make sense to provide a GetValue<T>(AvaloniaProperty<T> property) method in AvaloniaObject in order not to break those usages.

[grokys]

Yes, you're right - as mentioned in the Breaking Changes section of #3255:

If your readonly property fields are declared as of type AvaloniaProperty then you should change them to StyledProperty<> or DirectProperty<,> fields

The reason for this is: because DirectProperty and StyledProperty use different backing stores, they have completely different code paths. Now we have the XAML compiler, setting and binding properties can be strongly typed. so we can avoid the cast to detect which whether the property is styled or direct at runtime. This is preferable because even though a cast is fast, it does have some performance overhead and we set a lot of properties on application startup.

There is still an extension method overload for GetValue which takes an AvaloniaProperty<T>, so things should continue to work (however you may need to use this.GetValue() instead of GetValue() in order for the extension method to work).

If this is causing a lot of problems we can rethink this decision, but note that there will be other breaking changes related to performance coming for 0.10 so it was my judgement that it's best to break things now.

[jp2masa]

That's completely fine, I simply wasn't aware of the breaking change (missed it somehow) nor the extension method.

Thanks for explaining, keep up the good work!