Custom view whose layout depends on parent width/height

[daltzctr]

I'm developing a cross-platform control (no handlers) that contains a StackLayout. When I add items to this StackLayout, I set their requested Width and Height to be the width and height of the parent.

I'm unsure of the correct event or override function where Width/Height of the parent I guaranteed to be set. OnSizeAllocated works but WinUI doesn't seem to like that as it throws COMException on any GUI modifications there.

@mattleibow your the layout expert, what's the intent here.