Xamarin.Forms doesn't currently support visual designers for XAML files. Because of this, when trying to open a Forms XAML file in either Visual Studio's XAML UI Designer or XAML UI Designer with Encoding, the following error message is thrown:
"The file cannot be opened with the selected editor. Please choose another editor."
"There is not yet a visual designer for generating XAML in Xamarin.Forms applications, so all XAML must be hand-written."