In most cases, Views (Pages) and ViewModels have a one-to-one relationship. However, it is possible for a View (Page) to contain multiple ViewModels or for a ViewModel to be used by multiple Views (Pages). For now, we will simply have a single ViewModel for each Page. Before we create our ViewModels, we will start by creating a base ViewModel class, which will be an abstract class containing the basic functionality that each of our ViewModels will inherit. Initially, the base ViewModel abstract class will only contain a couple of members and wilimplement INotifyPropertyChanged, but we will add to this class as we continue to build upon the TripLog app throughout this book.

In order to create a base ViewModel, perform the following steps:

1. Create a new abstract class named BaseViewModel in the ViewModels folder using the following code:

      public abstract class BaseViewModel
      {
          protected BaseViewModel()
          { 
          } 
      }

2. Update BaseViewModel to implement INotifyPropertyChanged:

      public abstract class BaseViewModel : INotifyPropertyChanged
      {
          protected BaseViewModel() 
          { 
          }

          public event PropertyChangedEventHandler PropertyChanged;

          protected virtual void OnPropertyChanged(
              [CallerMemberName] string propertyName = null)
          {
              PropertyChanged?.Invoke(this, 
                  new PropertyChangedEventArgs(propertyName));
          }
      }

The implementation of INotifyPropertyChanged is key to the behavior and role of the ViewModels and data binding. It allows a Page to be notified when the properties of its ViewModel have changed.

Now that we have created a base ViewModel, we can start adding the actual ViewModels that will serve as the data context for each of our Pages. We will start by creating a ViewModel for MainPage.