PropertyChangedCallback
Listening to changes on DependencyProperties in WPF can be complex. When you register a DependencyProperty you can supply a static PropertyChangedCallback but if you want to listen to changes from elsewhere things can get complicated and error-prone.
In Avalonia, there is no PropertyChangedCallback at the time of registration, instead a class listener is added to the control's static constructor in much the same way that event class listeners are added.
Avalonia XPF - Cross-Platform WPF