I've just published the fourth installment of my Calcium for Xamarin.Forms series.
In this article you look at defining a custom AppBar in XAML; binding it to collections of command objects and at directly binding AppBar items to viewmodel properties. You see how to employ platform specific rendering to display an AppBar on iOS, Android, and Windows Phone. You learn how the Calcium AppBar is multi-page aware and can be placed within multiple pages hosted in a CarouselPage or a TabbedPage. You see how to display a custom menu in iOS and Android. Finally, you look at how to register a custom platform-specific view renderer.