Defining a ViewModel base class

ViewModel is the mediator between the view and the model. We can benefit from it greatly by creating a common base class for all our ViewModel classes to inherit from. To do this, follow these steps:

1. Create a folder called ViewModels in the DoToo .NET Standard project.
2. Create a class called ViewModel in the ViewModels folder.
3. Resolve references to System.ComponentModel and Xamarin.Forms and add the following code: 

public abstract class ViewModel : INotifyPropertyChanged
{
    public event PropertyChangedEventHandler PropertyChanged;
 
    public void RaisePropertyChanged(params string[] propertyNames)
    {
        foreach (var propertyName in propertyNames)
        {
            PropertyChanged?.Invoke(this, new 
            PropertyChangedEventArgs(propertyName));
        }
    }
 
    public INavigation Navigation { get; set; }
} 

The ViewModel class is a base class for all ViewModel objects. It is not meant to be instantiated on its own, so we mark it as abstract. It implements INotifyPropertyChanged, which is an interface defined in System.ComponentModel in the .NET base class libraries. This interface only defines one thing—the PropertyChanged event. Our ViewModel class must raise this event whenever we want the GUI to be aware of any changes to a property. This can be done manually by adding code to a setter in a property or by using an Intermediate Language (IL) weaver, such as PropertyChanged.Fody. We will talk about this in more detail in the next section.

We will also take a little shortcut here by adding an INavigation property to ViewModel. This will help us with navigation later on. This is also something that can (and should) be abstracted since we don't want ViewModel to be dependent on Xamarin.Forms in order to be able to reuse the ViewModel classes on any platform.