In order to move the navigation functionality from MainPage to MainViewModel, we will need to add two new Command properties—one for creating a new log entry and another for viewing the details of an existing log entry:

      public class MainViewModel : BaseViewModel
      {
          // ...

          Command<TripLogEntry> _viewCommand;
          public Command<TripLogEntry> ViewCommand 
          {
              get 
              {  
                  return _viewCommand
                      ?? (_viewCommand = new Command<TripLogEntry>(async (entry) => await ExecuteViewCommand(entry))); 
              }
          }

          Command _newCommand;
          public Command NewCommand 
          {
              get 
              {
                  return _newCommand
                      ?? (_newCommand = new Command(async () => await ExecuteNewCommand())); 
              } 
          }

          async Task ExecuteViewCommand(TripLogEntry entry)
          {
              await NavService.NavigateTo<DetailViewModel, TripLogEntry>(entry);
          }

          async Task ExecuteNewCommand()
          {
              await NavService.NavigateTo<NewEntryViewModel>();
          }

          // ...
      }

With the Command properties in place on MainViewModel, we can now update MainPage to use these commands instead of using the Xamarin.Forms navigation service directly from the page:

1. Create a private MainViewModel property named _vm in the MainPage class that simply provides access to the page's BindingContext, but is casted as a MainViewModel:

      public class MainPage : ContentPage
      {
          MainViewModel _vm 
          {
              get { return BindingContext as MainViewModel; }
          }

          // ... 
      }

2. Replace the Navigation.PushAsync method call in the ItemTapped event handler for the entries ListView with a call to the ViewCommand:

      void Trips_ItemTapped(object sender, ItemTappedEventArgs e) 
      {
          var trip = (TripLogEntry)e.Item;
          _vm.ViewCommand.Execute(trip);

          trips.SelectedItem = null;
      };

3. Replace the Clicked attribute on the New ToolbarItem element with a Command attribute whose value is a binding to the NewCommand:

      <ToolbarItem Text="New" Command="{Binding NewCommand}" />