One of the key purposes of the MVVM pattern is to isolate an app's presentation layer from its other layers. In doing so, an app's business logic is also isolated. One of the thoughts behind this isolation is to have a user interface that is only concerned with displaying data, and that is completely independent of how that data is stored, acquired, manipulated, or shared with the rest of the app. As explained in Chapter 2, MVVM and Data Binding, this is typically accomplished through data binding. In MVVM, the actions that a user performs on a page are bound to commands on that page's backing ViewModel. It is very common for these actions to result in a transition to another page—either by directly linking to it or by automatically navigating to a previous page after performing a task, such as saving data. Therefore, it makes sense to...