Xamarin Forms are the most exciting library to come along in a long time; certainly for mobile programmers. 

They bring a whole new option to cross platform development: an abstraction layer that lets you write once, and run native on each of the platforms. 

Write Once, Render Natively

This means, for example, when you create a tab page, it is rendered as a native tab page on iOS, Android and Windows Phone.  This is accomplished using pre-defined elements each of which has a hidden renderer for each platform.  Most of the time you don’t know or care how it is done, unless you want to create a custom (or customized) control.

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