[MacOS] make OnElementChanged and OnElementPropertyChanged protected virtual

[MichaelRumpler]

Description of Change

In Xamarin.Forms.Platform.MacOS the PageRenderer and ScrollViewRenderer did not have the usual protected virtual methods OnElementChanged and OnElementPropertyChanged. They were private and partly wrong named. Therefore users could not inherit from those renderers and use them as every other renderer on every other platform.

API Changes

Changed:

• PageRenderer: void OnElementChanged => protected virtual void OnElementChanged
• PageRenderer: void OnHandlePropertyChanged => protected virtual void OnElementPropertyChanged
• ScrollViewRenderer: void OnElementChanged => protected virtual void OnElementChanged
• ScrollViewRenderer: void HandlePropertyChanged => protected virtual void OnElementPropertyChanged

Behavioral Changes

None

PR Checklist

• Has tests (if omitted, state reason in description)
• Rebased on top of master at time of PR
• Changes adhere to coding standard
• Consolidate commits as makes sense

[StephaneDelcroix]

fine for me, minus the comment

[StephaneDelcroix]

we try to not trigger events from virtual methods.

[MichaelRumpler]

Good call, but you do raise ElementChanged yourself from CarouselPageRenderer.OnElementChanged, MasterDetailPageRenderer.OnElementChanged, NavigationPageRenderer.OnElementChanged, TabbedPageRenderer.OnElementChanged and even VisualElementRenderer.OnElementChanged.

I changed all the *PageRenderers and ScrollViewRenderer now. The event is now raised in the private Raise(or Handle-)ElementChanged method and the virtual OnElementChanged method is just an empty template method. Thus the ElementChanged event is still raised even if a derived class does not call base.OnElementChanged.

I did not change the VisualElementRenderer as this is used everywhere (also shared with iOS).

[StephaneDelcroix]

@MichaelRumpler

but you do raise ElementChanged yourself from CarouselPageRenderer.OnElementChanged, MasterDetailPageRenderer.OnElementChanged, NavigationPageRenderer.OnElementChanged, TabbedPageRenderer.OnElementChanged and even VisualElementRenderer.OnElementChanged.

and on BindableObject.PropertyChanged too... and you can't start to imagine how we regret it