The first step of introducing MVVM into an app is to set up the structure by adding folders that will represent the core tenants of the pattern, such as Models, ViewModels, and Views. Traditionally, the Models and ViewModels live in a core library (usually, a portable class library or .NET standard library), whereas the Views live in a platform-specific library. Thanks to the power of the Xamarin.Forms toolkit and its abstraction of platform-specific UI APIs, the Views in a Xamarin.Forms app can also live in the core library.
Just because the Views can live in the core library with the ViewModels and Models, this doesn't mean that separation between the user interface and the app logic isn't important.
When implementing a specific structure to support a design pattern, it is helpful to have your application namespaces organized in a similar structure...
